Relative phase-sensitive optical coherence tomography apparatus, method and article of manufacture

Abstract

The present disclosure relates to a relative phase-sensitive optical coherence tomography apparatus, method and article. The relative phase-sensitive optical coherence tomography apparatus comprises: an imaging system for acquiring a first image and a second image of optical properties of a sample region in three spatial dimensions, each image comprising phase and amplitude information within a certain wavelength range, and acquiring each image in a single exposure, the second image being acquired a predetermined time period after the first image; and a processor for: registering the first image with the second image to determine a spatially resolved phase shift caused by motion or deformation of the sample in any spatial dimension; and determining at least one component of a displacement of the sample associated with the motion or deformation from the phase shift.

Description

[0001] This application is a divisional application of Chinese national phase application 201680029027.0, whose international application date is May 27, 2016, international application number is PCT / AU2016 / 050418, and invention name is “High-resolution 3-D spectral domain optical imaging device and method”, all contents of which are incorporated herein by reference. Technical Field

[0002] The present invention relates to optical imaging devices and methods, and in particular to 3D spectral domain optical coherence tomography (OCT) devices with full range and extended focal depth, which sample complex fields. However, it should be understood that the invention is not limited to this particular field of use. Background Art

[0003] 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.

[0004] Optical coherence tomography (OCT) is a widely used interferometric technique for studying biological specimens, including in vivo tissues such as the human eye, with lateral and depth resolution, using information contained in the amplitude and phase of reflected or scattered light. OCT systems typically utilize a Michelson interferometer configuration, employing two main approaches: time-domain OCT and spectral-domain OCT.

[0005] In time-domain OCT, the coherence properties of a partially coherent light source, such as a superluminescent diode (SLED) with a coherence length of a few micrometers, are exploited by interfering the light reflected from the sample with a reference beam provided by the same source, but with a time-varying path length. At a specific depth in the sample corresponding to the path length delay in the reference arm, the interference envelope of the fringes is detected in the combined back-reflected signal, allowing reconstruction of the reflection profile in the depth dimension. Typically, this is done for only one sampling point at a time, and the corresponding depth scan is called an "A-scan".

[0006] Instead of scanning the delay line, spectral domain OCT techniques analyze the reflected light by interfering it with a reference beam as a time-varying function of wavelength (swept source OCT) or by dispersing the different wavelengths with a grating or other spectral demultiplexer and detecting them simultaneously along an array of detectors. The spectral domain information is the Fourier transform of the spatial (depth) reflectance profile, so the spatial profile can be recovered by a fast Fourier transform (FFT). In general, spectral domain OCT systems are superior to time domain OCT systems because they have a -20 to 30 dB sensitivity advantage.

[0007] OCT techniques can be adapted to provide laterally resolved "B-scans" by scanning the sample beam relative to the sample in one axis, or "C-scans" by scanning in two axes. Regardless of the type of scan, faster acquisition speeds are generally desirable, particularly to reduce artifacts caused by motion of in vivo samples, and have improved greatly over the past 20 to 25 years due to advances in several areas including faster scan source scan rates and photodetector array readout speeds. However, laser safety regulations impose a fundamental limitation on scanning spot schemes, particularly for in vivo applications: shortening the dwell time to increase the scan speed cannot be achieved without increasing the applied power, which inevitably degrades the signal-to-noise ratio.

[0008] Therefore, 'parallel' OCT systems have also been investigated, in which an extended sample area is probed with lateral resolution, or an array of sample points is probed simultaneously. It is relatively simple to parallelize time-domain OCT by utilizing a CCD camera and imaging the optics, for example, as described in U.S. Patent No. 5,465,147, entitled 'Method and apparatus for acquiring images using a CCD detector array and without a lateral scanner'. This provides a two-dimensional (2-D) en face image, with depth resolution provided by scanning a reference mirror as in time-domain OCT.

[0009] Swept source spectral domain OCT can be parallelized in a similar manner as described in 'In vivo Fourier-domain full-field OCT of the human retina with 1.5 million A-lines / s' by Bonin et al., Optics Letters 35(20), 3432-3434 (2010). However, because each frame corresponds to a single wavelength, the acquisition time for each A-scan is equal to the frame period multiplied by the number of k-points (wavelength samples) acquired. Even for ultra-high-speed cameras with frame rates of 100skHz, this can result in A-scan acquisition times of multiple milliseconds, which can lead to motion artifacts, particularly for in vivo samples. PCT Patent Application No PCT / AU2015 / 050788, entitled 'Multichannel optical receivers', discloses an alternative parallelized swept source OCT scheme that enables even faster acquisition. In a specific embodiment, multiple points on the sample are illuminated simultaneously, and the reflected or scattered signal light is mixed with a reference beam to form multiple interference patterns having unique carrier frequencies.

[0010] Spectral domain OCT based on parallel spectrometers is capable of single B-scan acquisition, although existing schemes are limited by the fact that one axis of the two-dimensional photodetector array is occupied by wavelength dispersion. In the configuration described in published US Patent Application No 2014 / 0028974A1 entitled 'Line-field holoscopy', a cylindrical lens is used to generate line illumination on the sample and on a reference mirror. Figure 1 , the combined return sample and reference beams from the line illumination 2 are dispersed with a dispersive element such as a grating 4 and detected with a 2-D sensor array 6. The Fourier transform along the spectral axis 8 provides an A-scan for each position 9 along the illumination line 2. For full three-dimensional (3-D) imaging, the illumination line is mechanically scanned in orthogonal directions and the 2-D sensor array is repeatedly read out.

[0011] Even if a linear B-scan of the sample is sufficient, i.e., no 3-D imaging is required, scanning in an orthogonal direction may still be necessary, e.g., for digital wavefront correction, to correct for lens aberrations etc., or to provide increased depth of field. Furthermore, for these purposes the repeated linear scans must be phase coherent, which is often difficult.

[0012] Spectral domain OCT devices are typically preferably configured to sample an explicit complex field of interference signals rather than just the detected real-valued interference signal, in order to distinguish between positive and negative path length delays and thus enable imaging over a full range of depth. Various methods of capturing complex fields have been described. For example, Jungwirth et al. in 'Extended in vivo anterior eye-segment imaging with full-range complex spectral domain optical coherence tomography' in Journal of Biomedical Optics 14(5), 050501 (2009) describe a solution for a scanning point scheme in which the sample phase is jittered as the sample is scanned. A key disadvantage of this approach is that sample movement can result in a loss of phase coherence during scanning. Line field systems with improved phase stability that do not require jittering the sample phase have been described. For example, in US 2014 / 0028974 A1, the complex field is acquired by sampling the signal in the far field of a linear illumination, while in 'Full-range parallel Fourier-domain optical coherence tomography using aspatial carrier frequency' by Huang et al. in Applied Optics 52(5), 958-965 (2013), the line field is captured in the image plane, with off-axis referencing providing access to the complex field.

[0013] For a given wavelength, the lateral resolution of an OCT device is determined by the numerical aperture of the objective. However, increasing the numerical aperture of the objective inevitably reduces the depth of field, resulting in a trade-off between lateral resolution and depth of field. Various software-based or digital focusing techniques have been proposed to overcome this trade-off to increase the depth of field. These methods generally assume that phase coherence between scattering points is maintained during scanning and sampling, and that the field can be captured in the image plane or the Fourier plane.

[0014] In one example, synthetic aperture techniques are discussed in 'Depth-encoded synthetic aperture optical coherence tomography of biological tissues with extended focal depth' by Mo et al., Optics Express 23(4), 4935-4945 (2015). In another example, the forward model (FM) approach of 'Numerical focusing methods for full field OCT: a comparison based on a common signal model' by Kumar et al., Optics Express 22(13), 16061-16078 (2014) involves sampling the 3D captured interference signal I(x, y, k) in the image plane using a full-field swept source OCT device with a 2-D CMOS camera. By requiring the sample to be only on one side of zero delay, an explicit phase is obtained, and defocus correction is achieved by applying digital phase correction based on the Fresnel wavefront propagation model. This numerical phase correction is achieved by first performing a 1-D FFT on the real-valued signal along the spectral axis, giving the complex field I(x, y, k) → E(x, y, Δz). This is followed by the transverse coordinates of all positive delays E(x, y, Δz) → E(k x , k y , Δz). Then, a Fresnel correction for defocus correction is applied: E(k x , k y ,Δz)→E(k x , k y ,Δz)γ, where Here, wavelength is replaced by the central wavelength λ0, n is the refractive index of the sample, and M is the magnification of the OCT device. A two-dimensional inverse FFT (IFFT) relative to the spatial frequency of the phase-corrected field gives an image focused on the entire volume.

[0015] Digital focusing with a full range line-field OCT system has been demonstrated in 'Full range line-field parallel swept source imaging utilizing digital refocusing' by Fechtig et al., Journal of Modern Optics (2014), DOI: 10.1080 / 09500340.2014.990938. In this case, the sample field is measured in the image plane and the full range measurement is achieved by using an off-axis configuration with a reference arm. This off-axis configuration introduces a lateral carrier frequency that offsets the interference terms in frequency space, enabling the separation of positive and negative frequency components, thereby enabling the measurement of a complex signal. Phase noise in the scan direction limits the digital focusing to one dimension, which is applied to each successive B-scan. The complex signal can be obtained by first taking a 1-D FFT along the spatial axis corresponding to the off-axis reference, and then a filter can be applied to select the positive frequency signal component from its complex conjugate artifacts and non-interfering background. The 1-D IFFT then gives a signal measurement with an explicit phase. Digital focusing is achieved by performing a 1-DFFT along the spectral axis, followed by a 1-D FFT of the transverse coordinate to give E(k x , Δz), where Δz now extends over the entire range. The 1-D phase correction factor multiplied by the 1-D FFT gives a focused B-scan over the entire range.

[0016] In 'Holoscopy-holographic optical coherence tomography' by Hillmann et al. in Optics Letters 36(13), 2390-2392 (2011), a full-field swept-source OCT system with sampling in the far field is described. In this system, a two-dimensional interference pattern for each wavelength is propagated to a specific delay Δz. A 1-D FFT along the spectral axis is then used to reconstruct the focused object for that depth Δz. This process is repeated for a range of delays, and the refocused regions are then stitched together. As described in 'Efficient holoscopy image reconstruction' by Hillmann et al. in Optics Express 20(19), 21247-21263 (2012), full-range imaging of the sampling in the Fourier plane was demonstrated using an off-axis reference beam to obtain an unambiguous phase. This digital post-processing approach, in which the 3-D signal is interpolated onto a non-equally spaced grid, provides a volumetric image with a resolution equal to the focal plane resolution in an extended portion of the entire volume. A final 3-D FFT then gives the focused volumetric image. A similar approach is described using an inverted synthetic aperture microscope (ISAM) in, for example, Ralston et al., 'Interferometric synthetic aperture microscopy', Nature Physics 3(2), 129-134 (2007).

[0017] We note that the above method assumes a simple model of depth-dependent defocus. An alternative approach using subaperture correlation to compensate for unknown optical aberrations is described in Kumar et al., "Subaperture correlation based digital adaptive optics for full field optical coherence tomography", Optics Express 21(9), 10850-10866 (2013).

[0018] An important limitation of full-field OCT systems compared to point scanning systems is the susceptibility to crosstalk from multipath scattering and the resulting reduced sensitivity. In addition, the lack of confocal filtering increases the sensitivity to stray reflections from outside the coherence length of the system. The line-field approach of US 2014 / 0028974 A1 partially mitigates these limitations compared to full-field systems by coaxial gating in one axis. Another approach to mitigate crosstalk is to use spatially incoherent sources.

[0019] Unless the context clearly requires otherwise, throughout the specification and claims, the words 'comprising', 'comprises' and the like should be interpreted in an inclusive sense as opposed to an exclusive or exhaustive sense, that is, in the sense of 'including but not limited to'.

[0020] Purpose of the invention

[0021] It is an object of the present invention to overcome or ameliorate at least one limitation of the prior art, or to provide a useful alternative. It is an object of the present invention to provide, in preferred forms, spectral domain OCT apparatus and methods for acquiring 3-D images of a specimen using a single shot acquisition technique. It is another object of the present invention to provide, in preferred forms, apparatus and methods for acquiring improved high resolution optical images of the retina based on numerical reconstruction of the spectral characteristics of light reflected or scattered from a small volume of the retina, correcting for aberrations present in the specimen's eye. Summary of the invention

[0022] According to a first aspect of the present invention, there is provided a device for retinal imaging, the device comprising:

[0023] (i) Multi-wavelength light source;

[0024] (ii) an angle-variable illumination system for directing at least two portions of light emitted from a light source onto each of two or more volumes of the retina of the subject eye;

[0025] (iii) a measurement system for receiving a signal of light reflected or scattered from each of the two or more volumes, each said signal being a function of the phase and amplitude of the electric field vector of the reflected or scattered light, and for simultaneously making a measurement over a range of wavelengths for each of said signals; and

[0026] (iv) a processor for processing the measurements to generate one or more digital representations of the optical properties of the retina at the two or more volumes, and producing from the one or more digital representations a three-dimensional composite image of a region of the retina that includes at least a portion of the two or more volumes.

[0027] According to a second aspect of the present invention, there is provided a device for imaging a sample, the device comprising:

[0028] (i) Multi-wavelength light source;

[0029] (ii) an illumination system for directing at least two portions of light emitted from a light source onto each of two or more volumes of a sample located at or near a focal plane of an optical power element of the apparatus;

[0030] (iii) a measurement system for receiving a signal of light reflected or scattered from each of the two or more volumes, each of said signals being a function of the phase and amplitude of the electric field vector of the reflected or scattered light, and for simultaneously making a measurement over a range of wavelengths for each signal; and

[0031] (iv) a processor for processing the measurements to generate one or more digital representations of the optical properties of the sample across the two or more volumes and producing from the one or more digital representations a three-dimensional composite image of the sample within a region including at least a portion of the two or more volumes.

[0032] The first and second aspects share many preferences. Preferably, the processor is adapted to generate a three-dimensional composite image using digital refocusing or digital correction of aberrations of the sample eye or sample. In some embodiments, the processor is adapted to generate a digital representation of the optical properties at each volume of the two or more volumes, and to generate the three-dimensional composite image from the digital representation. In other embodiments, the processor is adapted to generate a digital representation of the optical properties at the two or more volumes, and to generate the three-dimensional composite image from the digital representation.

[0033] In some embodiments, the illumination system is adapted to sequentially direct at least two portions of light onto two or more volumes of the retina. In other embodiments, the illumination system is adapted to simultaneously direct at least two portions of light onto two or more volumes of the retina.

[0034] The measurement system preferably comprises a two-dimensional lenslet array for sampling the signal and a wavelength dispersive element for dispersing the sampled signal onto a two-dimensional sensor array, wherein the lenslets of the lenslet array are positioned relative to the wavelength dispersive element such that, in use, each sampled signal is dispersed onto a set of pixels of the sensor array. In certain embodiments, the two-dimensional lenslet array is positioned such that the signal is sampled in the Fourier plane. Preferably, the two-dimensional lenslet array comprises a linear array of lenslets angled relative to a dispersion axis of the wavelength dispersive element.

[0035] Preferably, adjacent pairs of the two or more volumes partially overlap.In certain embodiments, the processor is adapted to reduce the three-dimensional composite image to a high resolution B-scan of the retina.

[0036] In some embodiments, the optical property is selected from the group consisting of phase, reflectivity, refractive index, refractive index change, and attenuation. In some embodiments, the measurement system is adapted to capture phase and amplitude information of at least a first polarization state and a second polarization state of the signal. In these embodiments, the optical property may include birefringence or degree of polarization.

[0037] For each of the two or more volumes, the area of ​​the illuminated surface of the retina is preferably less than or equal to 500 μm×500 μm, more preferably less than or equal to 200 μm×200 μm.

[0038] According to a third aspect of the present invention, there is provided a relative phase-sensitive optical coherence tomography device, comprising:

[0039] (i) an imaging system for acquiring a first image and a second image of an optical property of a sample region in three spatial dimensions, each image comprising phase and amplitude information over a range of wavelengths, and each image being acquired in a single exposure, the second image being acquired a predetermined period of time after the first image; and

[0040] (ii) a processor for:

[0041] (a) registering the first image with the second image to determine spatially resolved phase shifts caused by motion or deformation of the sample in any spatial dimension; and

[0042] (b) determining at least one component of the displacement of the sample associated with the motion or deformation from the phase shift.

[0043] The processor is preferably adapted to determine a displacement rate of the sample associated with the movement or deformation from the phase shift and the predetermined time period.In certain embodiments, the processor is adapted to measure strain associated with the deformation of the sample, or to perform elastography measurements on the sample.

[0044] According to a fourth aspect of the present invention, there is provided a polarization-sensitive optical coherence tomography device, comprising:

[0045] (i) an illumination system comprising a multi-wavelength light source for illuminating a volume of the sample with light of at least a first polarization state;

[0046] (ii) a beam splitter for directing a portion of the light reflected or scattered from the sample away from the light source;

[0047] (iii) a measurement system for making a first set of simultaneous measurements over a range of wavelengths of at least a first polarization state and a second polarization state of a signal of light reflected or scattered from the sample, the signal being a function of the phase and amplitude of the electric field vector of the reflected or scattered light; and

[0048] (iv) a processor for processing the first set of simultaneous measurements to generate a three-dimensional representation of one or more polarization properties of the illuminated volume of the sample.

[0049] Preferably, the one or more polarisation properties include birefringence or degree of polarisation.

[0050] In some embodiments, the illumination system is adapted to subsequently illuminate the volume of the sample with light of a second polarization state different from the first polarization state, and the measurement system is adapted to perform a second set of simultaneous measurements over a range of wavelengths. In these embodiments, the processor is preferably adapted to process the first set of simultaneous measurements and the second set of simultaneous measurements to generate a three-dimensional representation of one or more polarization properties of the illuminated volume of the sample.

[0051] In a preferred embodiment, the optical device comprises a polarization-independent beam splitter. Preferably, the beam splitter comprises an opening reflector having a total internal reflection surface and one or more openings that locally destroy total internal reflection at the surface for allowing transmitted light to illuminate the sample. More preferably, the opening reflector comprises two total internal reflection surfaces separated by one or more local index matching regions forming the one or more openings.

[0052] According to a fifth aspect of the present invention, there is provided an optical coherence tomography apparatus for imaging a sample at an extended depth of field, the apparatus comprising:

[0053] (i) an illumination system adapted to illuminate a volume of a sample to be imaged in three spatial dimensions with light beams incident at two or more angles of incidence and each having at least a first wavelength and a second wavelength;

[0054] (ii) an interferometer adapted to measure, at at least a first wavelength and a second wavelength and at least for each angle of incidence, in a single shot, a two-dimensional grid of sampling points of the phase and amplitude of light reflected or scattered from a volume of a sample illuminated at two or more angles of incidence; and

[0055] (iii) a processor for: registering and stitching together measurements from two or more incident angles in the Fourier domain to produce an extended Fourier field of measurements; and producing a three-dimensional image of the optical properties of the sample by Fourier transform or digital processing of the extended Fourier field measurements.

[0056] In certain embodiments, the illumination system is adapted to sequentially illuminate the sample volume with light beams incident at two or more angles of incidence.In alternative embodiments, the illumination system is adapted to simultaneously illuminate the sample volume with light beams incident at two or more angles of incidence.

[0057] In a preferred embodiment, the interferometer comprises: a two-dimensional lenslet array for providing a two-dimensional grid of sampling points; a two-dimensional sensor array; and a wavelength dispersive element for dispersing light from each sampling point onto the sensor array, wherein the lenslets of the lenslet array are positioned relative to the wavelength dispersive element so that, in use, light from each sampling point is dispersed onto a set of pixels of the sensor array. In certain embodiments, the two-dimensional lenslet array is positioned to sample signals in the Fourier plane. The two-dimensional lenslet array preferably comprises a linear array of lenslets angled relative to a dispersion axis of the wavelength dispersive element. In a preferred embodiment, the lateral resolution of the three-dimensional image is enhanced by extended Fourier field measurement.

[0058] According to a sixth aspect of the present invention, there is provided a high-resolution optical imaging device, comprising:

[0059] (i) an illumination system for illuminating a volume of a sample to be imaged in three spatial dimensions with a multi-wavelength light beam;

[0060] (ii) a sampling system for sampling, in a Fourier plane, light reflected or scattered from the illuminated volume of the sample;

[0061] (iii) a measurement system for simultaneously capturing phase and amplitude information over a range of wavelengths of the sampled reflected or scattered light; and

[0062] (iv) a processor for processing the phase and amplitude information to construct a three-dimensional image of the optical properties of the sample over the illumination volume.

[0063] In a preferred form, the processor is adapted to construct a three dimensional image using digital refocusing or digital correction of aberrations of the sample.

[0064] In a preferred embodiment, the measurement system comprises a wavelength dispersive element for dispersing sampled signals acquired from the sampling system onto a two-dimensional sensor array, wherein the sampling system is positioned relative to the wavelength such that, in use, each sampled signal is dispersed onto a set of pixels of the sensor array. The sampling system preferably comprises a two-dimensional lenslet array for sampling reflected or scattered light to provide a two-dimensional grid of sampling points.

[0065] In some embodiments, the optical property is selected from the group consisting of phase, reflectivity, refractive index, refractive index change, and attenuation. In some embodiments, the measurement system is adapted to capture phase and amplitude information of at least a first polarization state and a second polarization state of the reflected or scattered light. In these embodiments, the optical property may include birefringence or degree of polarization.

[0066] The area of ​​the illuminated surface corresponding to the illuminated volume is preferably less than or equal to 500 μm x 500 μm, more preferably less than or equal to 200 μm x 200 μm. In a preferred embodiment, the three-dimensional image has a spatial resolution of 3 μm or better.

[0067] According to a seventh aspect of the present invention, there is provided a method for imaging the retina of a sample eye, the method comprising the following steps:

[0068] (i) providing a multi-wavelength light beam;

[0069] (ii) directing at least two portions of the multi-wavelength light beam onto each of two or more volumes of the retina of the subject eye using an angle-variable illumination system;

[0070] (iii) receiving a signal for light reflected or scattered from each of the two or more volumes, each signal being a function of a phase and a magnitude of an electric field vector of the reflected or scattered light;

[0071] (iv) measuring each of said signals simultaneously over a range of wavelengths; and

[0072] (v) processing the measurements to generate one or more digital representations of the optical properties of the retina over the two or more volumes, and producing from the one or more digital representations a three-dimensional composite image over a region of the retina that includes at least a portion of the two or more volumes.

[0073] According to an eighth aspect of the present invention, there is provided a method for imaging a sample, the method comprising the following steps:

[0074] (i) providing a multi-wavelength light beam;

[0075] (ii) directing at least two portions of the multi-wavelength light beam sequentially onto each of the two or more volumes of the sample;

[0076] (iii) receiving a signal for light reflected or scattered from each of the two or more volumes, each signal being a function of a phase and a magnitude of an electric field vector of the reflected or scattered light;

[0077] (iv) measuring each signal simultaneously over a range of wavelengths; and

[0078] (v) processing the measurements to generate one or more digital representations of optical properties of the sample across the two or more volumes, and producing from the one or more digital representations a three-dimensional composite image of the sample within a region including at least a portion of the two or more volumes.

[0079] According to a ninth aspect of the present invention, there is provided a method for performing relative phase-sensitive optical coherence tomography measurements of a sample, the method comprising the following steps:

[0080] (i) acquiring a first image and a second image of optical properties of a sample region in three spatial dimensions, each image comprising phase and amplitude information within a certain wavelength range, and each image being acquired in a single exposure, the second image being acquired a predetermined time period after the first image;

[0081] (ii) registering the first image with the second image to determine spatially resolved phase shifts caused by motion or deformation of the sample in any spatial dimension; and

[0082] (iii) determining from the phase shift at least one component of the displacement of the sample associated with the motion or deformation.

[0083] According to a tenth aspect of the present invention, there is provided a method for performing polarization-sensitive optical coherence tomography measurements of a sample, the method comprising the following steps:

[0084] (i) illuminating a volume of the sample with multi-wavelength light of at least a first polarization state;

[0085] (ii) directing a portion of the light reflected or scattered from the sample away from the source of the multi-wavelength light;

[0086] (iii) making a first set of simultaneous measurements over a range of wavelengths of at least a first polarization state and a second polarization state of a signal of light reflected or scattered from the sample, the signal being a function of the phase and amplitude of the electric field vector of the reflected or scattered light; and

[0087] (iv) processing the first set of simultaneous measurements to generate a three-dimensional representation of one or more polarization properties of the illuminated volume of the sample.

[0088] According to an eleventh aspect of the present invention, there is provided a method for performing optical coherence tomography imaging of a sample at an extended depth of field, the method comprising the following steps:

[0089] (i) illuminating a volume of a sample to be imaged in three spatial dimensions with light beams incident at two or more angles of incidence and each having at least a first wavelength and a second wavelength;

[0090] (ii) a two-dimensional grid of sampling points interferometrically measuring, at least in a single shot for each angle of incidence, the phase and amplitude of light reflected or scattered from a volume of the sample illuminated at two or more angles of incidence, at at least the first and second wavelengths; and

[0091] (iii) registering and stitching together measurements from two or more incident angles in the Fourier domain to produce an extended Fourier field of the measurements; and

[0092] (iv) Producing a three-dimensional image of the optical properties of the sample by digital processing of Fourier transform or extended Fourier field measurements.

[0093] According to a twelfth aspect of the present invention, there is provided a method for performing high-resolution optical imaging of a sample, the method comprising the following steps:

[0094] (i) illuminating the volume of a sample to be imaged in three spatial dimensions with a multi-wavelength beam;

[0095] (ii) sampling the light reflected or scattered from the illuminated volume of the sample in the Fourier plane;

[0096] (iii) simultaneously capturing phase and amplitude information over the wavelength range of the sampled reflected or scattered light; and

[0097] (iv) Processing the phase and amplitude information to construct a three-dimensional image of the optical properties of the sample over the illumination volume.

[0098] According to a thirteenth aspect of the present invention, there is provided a product comprising a computer-usable medium having a computer-readable program code, the computer-readable program code being configured to operate a device according to any one of the first to sixth aspects or to implement a method according to any one of the seventh to twelfth aspects.

[0099] According to a fourteenth aspect of the present invention, there is provided an opening reflector, comprising: a total internal reflection surface for reflecting light; and one or more openings, wherein the one or more openings partially destroy the total internal reflection at the surface for transmitting light without reflection.

[0100] Preferably, the opening reflector comprises two total internal reflection surfaces separated by one or more local index matching regions forming one or more openings. More preferably, the opening reflector comprises two prisms having polished optical surfaces forming two total internal reflection surfaces, the two prisms being fixedly connected to each other and separated by local regions of refractive index matching adhesive forming one or more openings. The two total internal reflection surfaces are preferably separated by about 10 μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0101] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0102] Figure 1 The acquisition of B-scan data using a 2-D sensor array in a prior art line-field OCT system is shown in schematic form;

[0103] Figure 2 A general scheme for mapping data from three spatial dimensions (equivalent to two lateral dimensions and one spectral dimension) onto a 2-D sensor array is shown;

[0104] Figure 3 shows the sampling in the Fourier plane Figure 2 Implementation of the solution;

[0105] Figure 4 A spectral domain OCT device configured for Fourier plane sampling of light scattered or reflected from a sample according to an embodiment of the present invention is shown;

[0106] Figure 5 Another spectral domain OCT device configured for Fourier plane sampling of light scattered or reflected from a sample according to an embodiment of the present invention is shown;

[0107] Figure 6 A scheme for reducing sample power losses that occur when analyzing the polarization of a reference beam and a returning sample beam is shown;

[0108] Figure 7 shows a mapping of a 2-D grid of beamlets spread across a 2-D sensor array;

[0109] Figure 8 shows the magnitude of an exemplary 2-D Fourier space transform of an interferogram sampled in the Fourier plane;

[0110] Fig. 9 shows the sampling in the image plane Figure 2 Embodiments of the scheme;

[0111] Fig.10 A spectral domain OCT device configured for image plane sampling of light scattered or reflected from a sample according to an embodiment of the present invention is shown;

[0112] Fig.11 Another spectral domain OCT device configured for image plane sampling of light scattered or reflected from a sample according to an embodiment of the present invention is shown;

[0113] Fig.12 A further spectral domain OCT device configured for image plane sampling of light scattered or reflected from a sample according to an embodiment of the present invention is shown;

[0114] Fig.13A A linear OCT device configured for image plane sampling of light scattered or reflected from a sample according to an embodiment of the present invention is shown;

[0115] Fig. 13B shows that for a given wavelength Fig.13A2-DFFT of the interference pattern obtained by the device;

[0116] Fig. 13C Shown Fig.13A 2-D-FFT of the dispersed wavelengths of the interference pattern obtained by the device;

[0117] Fig.14A A spectral domain OCT device adapted to provide angular structured illumination to an ocular sample to enhance lateral resolution and / or extend focal depth according to one embodiment of the present invention is shown;

[0118] Fig. 14B The illustration shows Fig.14A Changes in equipment for providing angular structured illumination to non-ocular specimens; and

[0119] Fig.15 Shows the application Fig.14A An opening reflector in the device. DETAILED DESCRIPTION

[0120] It is apparent from the foregoing description of the prior art that single-shot acquisition of OCT data is advantageous not only for enhanced speed, especially for reducing motion artifacts of in vivo samples, but also for maintaining phase coherence for digital refocusing or digital wavefront correction. Acquisition schemes for digital reconstruction of complex fields that are not single shots (i.e., requiring multiple readouts of the sensor array) face the difficulty of ensuring phase registration between the data of each of the multiple frames. This difficulty is not insurmountable, but requires additional computation, for example, for stitching together single-shot images acquired from adjacent sample volumes.

[0121] Existing spectrometer-based spectral domain OCT systems (e.g., the system described in US 2014 / 0028974 A1) can acquire B-scans (one lateral dimension) in a single shot, rather than C-scans (two lateral dimensions) in a single shot. This is because one axis of the two-dimensional sensor array is occupied by wavelength dispersion, such as Figure 1 As shown. This limitation can be overcome if the combined return sample and reference wavefront are sampled in two lateral dimensions by a sampling system, which sampling system may, for example, include a 2-D lenslet array, a MEMS mirror array, or a diffractive optical element (DOE), and the resulting sampling points dispersed over separate pixel groups of a 2-D sensor array. The effect of this overall scheme is to compress data from three spatial dimensions (equal to two lateral dimensions and one spectral dimension) onto a two-dimensional sensor array. The mapping of the dispersed sampling points onto separate pixel groups can be ensured by appropriate orientation or positioning of the sampling points (e.g., lenses of a 2-D lenslet array) relative to the wavelength dispersive element. As shown Figure 2, one specific way to implement this general scheme is to sample the combined wavefront with a 2-D lenslet array 10, which includes a linear (X, Y) array of lenslets tilted at an angle θ relative to a dispersion axis 11 of a dispersion element 4, which disperses beamlets 14-1, 4-2, etc. onto a 2-D sensor array 6. If the tilt angle is chosen judiciously and the sensor array has sufficiently fine pixels 12 (as shown in the partial cutaway view), each beamlet 14-1, 14-2, etc. from the lenslet array can be dispersed, for example, by a grating 4, to a unique set of pixels 16-1, 16-2, etc. of the sensor array, thereby achieving a single-shot C-scan acquisition.

[0122] Another way of expressing the general requirement of obtaining a preferably unique mapping is to appropriately angle the projection 13 of the sampled beamlets onto the sensor array 6 relative to the projection onto the sensor array of the dispersion axis 11 of the dispersion element 4. Other solutions will occur to those skilled in the art, such as the use of a 2-D lenslet array with a non-linear arrangement of lenslets.

[0123] Ideally, the wavelength dispersive element 4 and sensor array 6 are arranged so that the projection of the dispersion axis 11 onto the sensor array is parallel to the rows of pixels 12 in the sensor array, i.e. parallel to the axis of the sensor array as shown. However, in practice, the dispersion image formed on the sensor array from each beamlet will typically have a degree of curvature such that the mapping (although known) is unlikely to correspond to a single row of pixels over an extended length.

[0124] The systems to be described below are generally designed to illuminate a small, continuous region of the sample with a multi-wavelength collimated or nearly collimated beam of light of the order of about 100 μm in diameter at the sample, and to capture an image of the interaction volume in a single snapshot with a spatial resolution significantly better than the size of the illuminated region (e.g., about 3 μm or better). In a preferred embodiment, the area of ​​the continuous illuminated region is kept small, less than or equal to 500 μm×500 μm, more preferably less than or equal to 200 μm×200 μm. This is typically necessitated by the available number of sampling points (i.e., the number of lenslets in a commercially available lenslet array), but it also reduces the effects of multiple scattering that can severely degrade the resolution of full-field, wavelength sequential devices. The phase coherence between scatterers in the sample enables accurate volume reconstruction through digital correction of aberrations and extended depth of focus. For example, a greater lateral range is obtained by scanning the illuminated region (e.g., by scanning the beam or sample laterally), and sequentially stitching together the captured volumes (preferably, partially overlapping adjacent volumes) to facilitate accurate phase registration. Importantly, simultaneous illumination of adjacent regions reduces sensitivity to crosstalk from multipath scattering and to parasitic reflections from outside the coherence length.

[0125] In a preferred embodiment, a grating-based spectral OCT system is used to capture 3-D snapshots, where a 2-D lenslet array samples light reflected or scattered from a small continuous illuminated area, and the resulting beamlets are dispersed and imaged on a 2-D sensor. Importantly, the resolution of the sensor (number of pixels) is much greater than the resolution of the lenslet array (number of lenslets), so both lateral and spectral information can be captured on the 2-D sensor in a single snapshot. As discussed above with respect to Figure 2 As described, in a preferred embodiment, the 2-D linear lenslet array is tilted relative to the dispersion axis of the dispersion element to ensure that each beamlet is dispersed onto a unique set of pixels. A significant advantage of grid-based sampling is that a simple imaging system can be used without the need for a high-power "microscope" to match the sample illumination area to the two-dimensional sensor. Such microscopes typically require magnifications on the order of 100, which requires complex imaging optics. The reflected or scattered field can be sampled by the lenslet array in the Fourier plane (i.e., the far field) as a form of holography or in the image plane (i.e., the near field). In either case, the lateral resolution is determined by the numerical aperture of the objective lens, and the lateral area captured in a single snapshot is determined by the product of the lateral resolution and the number of lenslets. Advantageously, sampling in the Fourier plane allows subsequent processing to mathematically regenerate the imaging lens and possibly change the lens for different parts of the sample.

[0126] Full range imaging can be achieved by mixing the signal with an off-axis reference beam to introduce a spatial carrier, enabling unambiguous phase measurements. Considering a phase-coherent signal sampled in both the transverse axis and wavelength, many well-known digital refocusing techniques can be applied. For example, techniques developed for swept source holography can be applied to extend the depth of field or compensate for aberrations.

[0127] We now turn to describing the use of Figure 2 Various 3-D spectral domain OCT systems have been developed that use the tilted lenslet array technology shown in for single-shot C-scan acquisition (e.g., over areas up to 500 μm × 500 μm with lateral resolution of 3 μm or better). These systems are capable of full-range imaging, i.e., the ability to distinguish between positive and negative path length delays, wavefront correction such as digital refocusing and aberration correction, and enhanced resolution. Importantly, the ability to capture the complex domain in a single snapshot ensures that phase coherence is maintained throughout the sample volume, a requirement for accurate wavefront correction.

[0128] In some embodiments, the combined beam is sampled in the far field, i.e., in the Fourier plane. Figure 3As schematically shown in FIG. 1 , light 18 reflected or scattered from a point (x', y') in a continuously illuminated volume 19 of a sample 20 is collected by an objective 22, mixed with an off-axis reference beam 24, and sampled in the Fourier plane by a 2-D lenslet array 10. In this configuration, the sample 20 and preferably also the lenslet array 10 are located approximately at the focal plane of the objective 22, recognizing that a three-dimensional sample cannot be exactly at the focal plane throughout its depth. After passing through an aperture array 25, the focused beamlets 14 are collimated, dispersed, and imaged on the 2-D sensor array 6. The aperture array 25 is optional, but is used to block scattered signals from outside the coherence length that would degrade the sensitivity of the device. As previously explained, in a preferred embodiment, the linear 2-D lenslet array 10 is tilted relative to the dispersion axis of the wavelength dispersive element to provide a mapping of the dispersed beamlets to the displaced pixel groups 16-1, 16-2, etc. of the sensor array 6. Since the reflected or scattered signal 18 is sampled in the Fourier plane, its lateral content is obtained from the spatial (lateral) frequency content of the sampled signal. As is common in spectral domain OCT, the axial reflectivity profile of the interaction volume 19 is encoded in the spectral frequency content. Importantly, the spatial Fourier transform separates the positive and negative components of the signal. Subsequent Fourier transforms along the spectral axis 8 provide a full-range reflectivity profile. Multiple optical properties of the sample can be extracted from the reflectivity profile with spatial resolution, including, for example, phase, reflectivity, refractive index, refractive index change, and attenuation. If the measurement system is polarization-sensitive, that is, suitable for capturing phase and amplitude information for at least the first and second polarization states of the beamlet, one or more polarization-dependent optical properties, such as birefringence or degree of polarization, can be extracted. Many, if not all, of these optical properties are generally wavelength-dependent.

[0129] Figure 4A spectral domain OCT device (with high lateral resolution) configured for Fourier plane sampling of light reflected or scattered from a sample 20 is shown. In the illumination system of the device, light from a fiber-coupled multi-wavelength or broadband light source 26, such as a superluminescent diode (SLED), is split by a 2x2 fiber coupler 28 into a sample arm 30 and a reference arm 32. For example, the split ratio of the 2x2 coupler can be, for example, 90 / 10 sample / reference, or even 99 / 1, since in many practical applications the reflectivity of the sample 20 will be low. The sample beam 34 is collimated with a lens 36 and then directed onto the sample 20 through a converging lens 38 and an objective lens 22. In a preferred embodiment, the objective lens has a relatively high numerical aperture to ensure high lateral spatial resolution. For example, a 0.16NA objective lens typically provides a lateral spatial resolution of 3.0 μm. The purpose of the converging lens 38 is to be able to illuminate an extended continuous volume 19 of the sample, for example, with a diameter of 100 μm. Since the sample 20 is located in the focal plane of the objective 22, if the converging lens is omitted, the illuminated area will be a diffraction limited spot rather than an extended area. It will be appreciated that a single lens may be used in place of the collimating lens 36 and the converging lens 38.

[0130] In the measurement system of the device, reflected or scattered sample light 18 from within the illumination volume 19 is collected by an objective lens 22 and directed to a beam splitter such as a beam splitter cube 40 where it is mixed with an off-axis collimated reference beam 24. The combined beam is sampled in the Fourier plane by an appropriately positioned rectilinear 2-D lenslet array 10, optionally followed by an aperture array (not shown), and the resulting beamlets 14 are collimated with a lens 42, dispersed with a wavelength dispersive element in the form of a reflection grating 43, and focused via a lens 44 onto a 2-D sensor array 6, from which the combined interference pattern can be read out in a single frame for subsequent analysis by a processor 45 equipped with suitable machine-readable program code. Alternatively, the dispersive element may be a transmission grating or a prism. As described above with respect to Figure 2 As described, the lenslet array 10 is preferably tilted relative to the dispersion axis of the grating 43 so that each beamlet is mapped onto a unique set of pixels of the sensor array 6. In one particular embodiment, the lenslet array has 1000 lenslets in a rectilinear 40×25 grid with a 300 μm pitch, and the 2-D sensor array is a 20 megapixel CMOS camera with a pixel size of 5.5 μm.

[0131] The combined interferogram read out from the sensor array 6 represents a wavelength-dependent measurement of the signal of light reflected or scattered from the interaction volume 19, where the signal is a function of the phase and amplitude of the electric field vector of the reflected or scattered light 18. Using the mathematical techniques described below, these wavelength-dependent measurements can be processed to produce a digital representation, or to construct a three-dimensional image of the optical properties of the sample with spatial resolution over at least a portion of the interaction volume 19. Multiple optical properties of the sample can be extracted, including, for example, phase, reflectivity, refractive index, refractive index change, and attenuation. Many, if not all, of these optical properties are generally wavelength-dependent. We note that the measurement system can be made polarization-sensitive, for example, by including a polarization discrete element in front of the 2-D sensor array 6, as described below with reference to Fig.14A In this case, one or more optical properties related to the polarization properties of the sample may also be extracted, for example, birefringence or polarization.

[0132] Figure 5 Another spectral domain OCT device (with high lateral resolution) configured for Fourier plane sampling of light reflected or scattered from a sample 20 is shown. In this device, the sample beam 34 and the reference beam 24 are generated and combined using a polarization beam splitter 41, a quarter wave plate 46, and a polarization analyzer 48. The use of a polarization beam splitter and associated polarization optics instead of, for example, Figure 4 The advantage of the power beam splitter 40 of the illustrated arrangement is that it avoids wasting 50% of the light from the broadband light source 26. As previously mentioned, the reference beam 24 is typically much more powerful than the returning sample beam 18. Figure 6 As shown in , one method for compensating for this is to orient the polarization analyzer so that its transmission axis 49 is nearly parallel to the polarization direction 50 of the low-intensity return sample beam, and therefore nearly orthogonal to the polarization direction 52 of the more intense reference beam. This reduces the loss of sample power from 3 dB to less than 1 dB compared to the common practice of orienting the polarization analyzer at 45° to the polarization directions of both beams. Back to Figure 5 , better equalization of the reference and return sample beam powers may also be achieved with an optional quarter wave plate 54 in the source arm, oriented so that the polarizing beam splitter 41 preferentially directs the source light 60 into the sample arm.

[0133] Similar to Figure 4In the case of the apparatus shown, a combination of a converging lens 38 and a high NA objective 22 is used to illuminate an extended continuous volume 19 (e.g., 100 μm in transverse diameter) of a sample 20. Reflected or scattered sample light 18 from within the illuminated volume 19 is collected by the objective 22, mixed with a reference beam 24, and sampled in the Fourier plane by an appropriately positioned rectilinear 2-D lenslet array 10 (followed by an optional aperture array (not shown)). The beamlets 14 are collimated with a lens 42, dispersed with a wavelength dispersive element in the form of a transmission grating 56, and focused via lens 44 onto the 2-D sensor array 6. Alternatively, the dispersive element may be a reflection grating or a prism. As described above with respect to Figure 2 As described, the linear lenslet array 10 is preferably tilted relative to the dispersion axis of the grating 56 so that each beamlet is mapped onto a unique set of pixels 16 of the sensor array 6. The combined interferogram can be read out from the 2-D sensor array in a single frame for subsequent analysis by a processor 45 equipped with suitable machine-readable program code. Again, the interferogram represents a wavelength-dependent measurement of the signal of light reflected or scattered from the illuminated volume 19, where the signal is a function of the phase and amplitude of the electric field vector of the reflected or scattered light 18. As previously described, these wavelength-dependent measurements can be processed to produce a digital representation, or to construct a three-dimensional image of the optical properties of the sample with spatial resolution over at least a portion of the interaction space 19.

[0134] It is usually preferred to interfere the returning sample beam with a reference beam that is well collimated and covers all lenslets in the array 10. Figure 4 The device is clear, appropriately selected reference arm collimating lens 58, but in Figure 5 The device is more difficult because the source beam 60 entering the beam splitter 41 is intentionally uncollimated to illuminate an extended (rather than diffraction-limited) area of ​​the sample 20. To this end, Figure 5 The reference arm of the device includes an NA converter 62 located between the quarter wave plate 46 and the reference mirror 64 to convert the smaller diameter divergent beam 66 into a larger diameter collimated beam 24. The NA converter 62 includes a larger diameter lens 70 and a smaller diameter lens 72 (e.g., a lenslet) separated by a distance equal to the focal length f1 of the larger lens. These two lenses, in combination with the reference mirror 64, cause the divergent beam 66 to reach a focal point 73 inside the lenslet 72. Therefore, the lenslet has no refractive power on the return path, so that the outgoing beam 74 with increased NA is collimated by the larger lens 70.

[0135] We now turn to describing the analysis of interferometric data acquired when sampling in the Fourier plane. With Fourier plane sampling, each beamlet 14 contains phase and amplitude information from every point in the interaction volume 19, but at different discrete angles. Spatial information is thus encoded as angular information.

[0136] For simplicity, we consider Figure 3 The scattering or reflection of a single point at the position (x', y') shown (and with a depth Δz). Assuming that the scattering or reflection point is close to the focal plane of the objective lens 22, the collimated field incident on the lenslet array 10 will be a plane wave with an incident angle x' / f. The interference signal incident on the lenslet array at position X, Y can therefore be expressed as:

[0137]

[0138] Wherein, R(x', y') is the sample reflectivity, S(k) is the spectral power distribution, f is the focal length of the objective lens 22, and x0 and y0 are angles with the reference beam 24 relative to the axis of the objective lens (or the axis of the lenslet array 10).

[0139] First, the interference signal component at the aperture array 25 of the small lens (circular aperture) can be approximated as:

[0140]

[0141] Among them, X i , Y j describes the axis of the lenslet, D is the pitch of the lenslet array, and for X 2 -Y 2 <(D / 2) 2 , circ(X, D) = 1, otherwise it is 0.

[0142] From the combined interference patterns measured by the two-dimensional sensor array 6 and the knowledge of the wavelength mapping of each lenslet on the 2-D sensor array, a set of interference patterns I can be extracted i,j (k l ), where i, j represent the positions of the small lenses in the small lens array 10, and k l represents the wave number resolved by the spectrometer (i.e., grating), such as Figure 7 As shown. i,j (k l) is conveniently viewed as a sequence of two-dimensional interference patterns, one for each of the M different wave numbers. The dimensions of each two-dimensional interference pattern are equal to the dimensions of the lenslet array, e.g. 25 rows × 40 columns. The analysis is therefore similar to that of full-field swept source holography (Hillmann et al., Optics Express 20(19), 21247-21263 (2012)), with the key difference being that the low sampling resolution of the lenslet array (e.g. 300 μm lenslet pitch compared to 5 μm pixels) limits the field of view achievable in a single snapshot compared to the sampling resolution of the photodetector array. Since the sample is measured in the Fourier plane, the image plane is acquired by applying a 2-D spatial Fourier transform to each interference pattern. Advantageously, by off-axis referencing, the Fourier transform can be used to separate the positive and negative spatial frequency components of the interference pattern, thereby achieving the full axial depth range for a subsequent 1-D FFT along the spectral axis of the positive spatial frequency component. This can be readily seen from the transverse Fourier component of the cosine term in equation (1):

[0143]

[0144] The phase of the corresponding term now depends on the sign of Δz.

[0145] We note that if the samples are only on one side of zero delay, no off-axis reference is needed. A complex signal with an explicit phase is acquired by first 1-D FFT along the spectral axis, and then for positive delays, a subsequent spatial 2-D FFT is taken. Thus, for a given lateral bandwidth, the lateral range is doubled compared to a full-range system.

[0146] As a use Figure 4 As an illustration of the field of view achievable by a Fourier plane sampled spectrum OCT device as shown in or 5, consider the following set of parameters. At wavelength λ = 0.8 μm, the lenslet array 10 has a pitch P = 300 μm, NA = 0.08, and a spot size = 0.61λ / NA = 6.1 μm. For the objective 22, we assume f = 40 mm, and a diameter D = 1 mm (NA = 0.14), giving an expected resolution of 3.6 μm. The maximum lateral range (Δx = x-x0) for the full range system can be estimated from the Nyquist limit (λ·f) / Δx)>2P which gives Δx < 55 μm. The lateral range for the case where the retardation is only on one side of zero retardation is twice this value, Δx < 110 μm. Here, we assume a unity magnification between the objective 22 and the lenslet array 10, and note that a lateral field of 55 μm results in a change in the focal position of the lenslet array that is less than 0.5 times the focal spot size of each lenslet.

[0147] Figure 8 shows the above parameters I i,j (kl ), the reference beam offset is used to ensure that the positive frequency component 76 and the negative frequency component 78 are separated. In this particular example, the reference beam is offset in both the horizontal and vertical axes, and we have subtracted the non-interference terms. The positive frequency component 76 or the negative frequency component 78 can be extracted by filtering, and the frequency offset is removed. It can be seen from equation (3) that for a given sampling position, the spatial frequency is wavelength dependent. Before the spectral components are Fourier transformed, this wavelength dependence is removed by multiplying with a wavelength-dependent phase factor to obtain a full range depth profile.

[0148] In other embodiments, the combined beam is sampled in the image plane. Fig. 9 As schematically shown in Figure 1, light 18 reflected or scattered from a point (x', y') in the illumination volume 19 of the sample 20 is amplified and re-imaged on the lenslet array 10. The complex field is obtained by mixing the return sampling light 18 and an off-axis reference beam 24 with an incidence angle α. As in Fourier plane sampling, the focused beamlets 14 from the lenslet array pass through an optional aperture array 25 and are then collimated, scattered and focused on the 2-D sensor array 6. In this case, the reference beam 24 needs to be off-axis to separate the positive and negative frequency components of the signal. As previously mentioned, in a preferred embodiment, the configuration of the lenslet array is rectilinear and tilted relative to the dispersion axis of the dispersion element to provide a mapping of the dispersed beamlets on unique pixel groups 16-1, 16-2, etc. of the sensor array 6. The composite field is obtained from a first spatial Fourier transform on the sampled field, setting the negative frequency components to zero, and then applying an inverse Fourier transform. As in the Fourier plane case, a subsequent Fourier transform along the spectral axis 8 provides a full range reflectivity profile of the illuminated volume 19 of the sample 20. From this reflectivity profile, a number of optical properties of the sample can be extracted with spatial resolution, including for example phase, reflectivity, refractive index, refractive index variation and attenuation, and if the measurement system is polarization sensitive, also birefringence and degree of polarization.

[0149] Fig.10 A spectral domain OCT device (with high lateral resolution) is shown configured for image plane sampling of light scattered or reflected from a sample 20. In the illumination system of the device, light from a fiber-coupled multi-wavelength or broadband light source 26 is split into a sample arm 30 and a reference arm 32 by a 2x2 fiber coupler 28. Figure 4As with the devices, the split ratio of the 2x2 coupler can be, for example, 90 / 10 or 99 / 1 sample / reference. The sample beam 34 is collimated with a lens 36 and then directed onto the extended continuous volume 19 of the sample 20 via a converging lens 38 and an objective lens 22. In a preferred embodiment, the objective lens has a relatively high numerical aperture to ensure high lateral spatial resolution. For example, a 0.16NA objective lens typically provides a spatial resolution of 3.0 μm.

[0150] In the measurement system of the device, reflected or scattered sample light 18 from within the interaction volume 19 is collected by an objective lens 22 and directed to a beam splitter such as a beam splitter cube 40 where it is mixed with an off-axis collimated reference beam 24. The combined beam is sampled in the Fourier plane by an appropriately positioned rectilinear 2-D lenslet array 10, optionally followed by an aperture array (not shown), and each beamlet 14 is collimated with a lens 42, dispersed with a wavelength dispersive element in the form of a reflection grating 43, and focused via a lens 44 onto a 2-D sensor array 6, from which the combined interference pattern can be read out in a single frame for subsequent analysis by a processor 45 equipped with suitable machine-readable program code. Alternatively, the dispersive element may be a transmission grating or a prism. As described above with respect to Figure 2 As described, the lenslet array 10 is preferably tilted relative to the dispersion axis of the grating 43 so that each beamlet 14 is mapped onto a unique set of pixels of the sensor array 6. In one specific embodiment, the lenslet array has 1000 lenslets in a rectilinear 40×25 grid with a 300 μm pitch, and the 2-D sensor array is a 20 megapixel CMOS camera with a pixel size of 5.5 μm. With this image plane sampling scheme, each beamlet 14 contains phase and amplitude information from a different portion of the interaction volume 19.

[0151] As previously described, the combined interferogram read out from the sensor array 6 represents a wavelength-dependent measurement of the signal of light reflected or scattered from the interaction volume 19, where the signal is a function of the phase and amplitude of the electric field vector of the reflected or scattered light 18. Using the mathematical techniques described below, these wavelength-dependent measurements can be processed to produce a digital representation, or to construct a three-dimensional image of the optical properties of the sample with spatial resolution over at least a portion of the interaction volume. We note that the measurement system can be made polarization-sensitive, for example, by including a polarization discrete element in front of the 2-D sensor array 6, as described below with reference to Fig.14A In this case, the optical property may be related to the polarization properties of the sample, for example, birefringence or polarization.

[0152] Fig.11Another spectral domain OCT device (with high lateral resolution) configured for image plane sampling of light reflected or scattered from a sample 20 is shown. In this device, a sample beam 34 and a reference beam 24 are generated and combined by a polarization beam splitter 41, a quarter wave plate 46, and a polarization analyzer 48. Figure 6 As discussed, the polarization analyzer 48 can be oriented so that its transmission axis 49 is nearly parallel to the polarization direction of the low-intensity return sample beam to reduce the loss of sampling power. An optional quarter-wave plate 54 in the source arm can also be used to achieve better equalization of the reference and return sample beam powers, oriented so that the polarization beam splitter 41 preferentially directs light from the broadband light source 26 into the sample arm.

[0153] Similar to Fig.10 In the case of the apparatus shown, a combination of a converging lens 38 and a high NA objective 22 is used to illuminate an extended continuous volume 19 (e.g., 100 μm in transverse diameter) of the sample 20. Reflected or scattered sample light 18 from within the illuminated volume 19 is collected by the objective 22, mixed with a reference beam 24, noting that the reference mirror 64 is angled on the non-dispersing axis so that the reference beam is off-axis relative to the returning sample beam. The combined beam is sampled in the image plane by an appropriately positioned rectilinear 2-D lenslet array 10, followed by an optional aperture array (not shown), and the resulting beamlets 14 are collimated with a lens 42, dispersed with a wavelength dispersive element in the form of a transmission grating 56, and focused via a lens 44 onto the 2-D sensor array 6. Alternatively, the dispersive element may be a reflection grating or a prism. As described above with respect to Figure 2 As described, the lenslet array 10 is preferably tilted relative to the dispersion axis of the grating 56 so that each beamlet is mapped onto a unique set of pixels 16 of the sensor array 6. The combined interferogram can be read out from the 2-D sensor array in a single frame for subsequent analysis by a processor 45 equipped with suitable machine-readable program code. Fig.10 As with the device, each beamlet 14 contains phase and amplitude information from a different portion of the illumination volume 19 .

[0154] Fig.12Yet another spectral domain OCT device configured for image plane sampling is shown, in this case light reflected or scattered from a 2-D grid of discrete points 79 on a sample 20 rather than from an extended continuous area. In the illumination system of this device, a 2-D lenslet array 81 in the sample arm separates the sample beam into a grid of beamlets 80 that are focused onto the sample via lenses 82 and 84 and a mirror 86. Optionally, the mirror can be scanned in one or two axes to translate the focused beamlets on the sample, for example, to analyze different zones or fill in gaps between beamlets. Alternatively, the sample 20 can be mounted on a translation stage. Lenses 82 and 84 typically form a high magnification system, for example, 100x, and it may be preferred to include additional lenses to perform magnification in two or more stages.

[0155] In the measurement system of the device, the sample light reflected or scattered from the discrete point 79 is collimated by the sample arm lenslet array 81, relayed to the combining arm lenslet array 10 by means of the 4F lens system 88, and mixed with the reference beam 24 deflected off-axis by the reference mirror 64 on the non-dispersive axis. Fig.11 As in the apparatus of FIG. 4 , the combined beam passes through a polarization analyzer 48 and is then sampled in the image plane by an appropriately positioned lenslet array 10 (followed by an optional aperture array (not shown)). The resulting beamlets 14 are collimated by lens 42, dispersed by a wavelength dispersive element in the form of a transmission grating 56, and focused via lens 44 onto the 2-D sensor array 6 to form a combined interferogram. Alternatively, the dispersive element may be a reflective grating or prism. The combined interferogram may be read out from the 2-D sensor array in a single frame for subsequent analysis by a processor 45 equipped with suitable machine-readable program code. The combined interferogram represents a wavelength-dependent measurement of the signal of light reflected or scattered from a grid of discrete points 79, wherein the signal is a function of the phase and amplitude of the electric field vector of the reflected or scattered light.

[0156] If scanning of the mirror 86 or translation of the sample 20 is enabled, this can conveniently be controlled synchronously with the readout of the sensor array by means of the processor 45 when provided with a suitable machine-readable program code.

[0157] Now we turn to Fig.10 and Fig.11 1 is a description of the analysis of interferometric data acquired when light scattered or reflected from an extended continuous volume 19 is sampled in the image plane. Assume that the sample 20 is in the focal plane of the objective lens, as Fig. 9 As shown, the interference signal components incident on the lenslet array 10 at positions X and Y can be expressed as:

[0158]

[0159] Wherein, M is the magnification of the lens system 22, 38 in the sample arm, and α is the incident angle of the reference beam 24 at the microlens array 10, as Fig. 9 As shown. In this case, it is assumed that the reference beam is aligned with the Y axis of the lenslet array. Importantly, for full range imaging, α must be large enough to separate the positive and negative frequency components of the interference signal.

[0160] The analysis follows a similar approach to that used in off-axis swept-wavelength OCT as described, for example, in Huang et al., Applied Optics 52(5), 958-965 (2013) or in Fechtig et al., Journal of Modern Optics 2014 (DOI: 10.1080 / 09500340.2014.990938). The complex interferogram I is obtained by performing a Fourier transform along the T axis, removing negative frequency components and frequency offsets, and then applying an inverse Fourier transform. i,j (k), to obtain the complex field, where i and j represent the position (X i , Y j ) on a small lens.

[0161] Assume similar experimental parameters as in the previous far-field case, i.e. a resolution of 3.6 μm (objective NA = 0.14), a lateral range of 55 μm and a lenslet pitch of P = 300 μm. The large magnification between the sample and the lenslet array converts the high numerical aperture high-resolution sample information into a small numerical aperture spot comparable to the lenslets, which can then optimally interfere with the reference beam after both are focused by the lenslet array. This range of incident angles is approximately Δθ < λ / (2P), i.e., about 1 / 2 Airy radius. A magnification of NA / Δθ ≈ 100 will therefore ensure capture of the light emitted from the sample. We note that this is equivalent to requiring the lenslet array pitch to be smaller than the magnified resolvable spot size.

[0162] In the above discussion, it is assumed that the sample is located in the focal plane of the objective lens. However, in general, for three-dimensional samples, most of the interaction volume will be slightly offset from the focal plane. Scattering from points far from the focal plane produces a curved wavefront at the lenslet array. The capture of partially overlapping continuous 3-D snapshot samples enables accurate phase alignment of the data set before or after the snapshot samples are stitched together to form a three-dimensional composite image, for which digital refocusing techniques can be applied using a processor equipped with suitable machine-readable program code. Alternatively, digital refocusing can be applied directly to the lateral point in the center of the snapshot data set to avoid refocusing the stitched data set. Digital refocusing requires that a signal with a clear phase be measured first, and can therefore be applied to full-range systems with off-axis lateral references and systems with all delays and on-axis lateral references of the same sign. The signal can be digitally refocused by employing one of the many known techniques described for full-field and line-field systems (e.g., Kumar et al., Optics Express 22(13), 16061-16078 (2014)) or models (e.g., as used in Fechtig et al., Journal of Modern Optics 2014 (DOI: 10.1080 / 09500340.2014.990938)). Although these methods are applied to OCT systems that sample the image plane, they can be adapted for sampling in the Fourier plane. The digital focusing techniques described in Hillmann et al., Optics Express 20(19), 21247-21263 (2012) can be directly applied to the case of sampling in the Fourier plane.

[0163] As an alternative embodiment to the various spectral domain OCT devices previously described, Fig.13A A linear OCT device configured for image plane sampling of light scattered or reflected from a sample is shown. In the illumination system of the device, light from a multi-wavelength or broadband light source 26 is separated by a beam splitter 40-1 to form a sample beam 34 and a reference beam 24. As shown in Fig.10As in the device shown, an extended continuous volume region 19 of the sample 20 is illuminated by a collimated sample beam 34. In the measurement system of the device, the reflected or scattered light 18 is collected and amplified by a telescope system including lenses 22 and 38 and sampled in the image plane by a suitably positioned linear 2-D lenslet array 10 (followed by an optional aperture array (not shown)). The resulting beamlets 14 are then collimated with a lens 42 and directed onto the 2-D sensor array 6 via a further beam splitter 40-2. In contrast to the previously described spectral domain approach, the collimated reference beam 24 is dispersed, for example using a reflection grating 43, and then mixed with the sample field directly on the sensor array 6. The linear lenslet array 10 is preferably tilted relative to the dispersion axis of the grating 43 so that the wavelength-dependent spatial frequency components obtained from the 2-D FFT of each beamlet 14 are mapped onto a unique set of pixels of the 2-D sensor array 6. These wavelength-dependent spatial frequency components can be read out from the 2-D sensor array for subsequent analysis by a processor 45 equipped with suitable machine-readable program code. Each component represents a wavelength-dependent measure of the signal of light reflected or scattered from a different portion of the interaction volume 19, where the signal is a function of the phase and amplitude of the electric field vector of the reflected or scattered light 18.

[0164] For a given wavelength, Fig. 13B The 2-D FFT 134 of the corresponding interferogram read out from the sensor array 6 is shown. Each beamlet corresponding to a lenslet in the lenslet array 10 is represented by a distinct spatial frequency component 136 having both amplitude and phase. The dispersion grating 43 in the reference arm enables the amplitude and phase of each wavelength component of each beamlet to be measured. Because the lenslet array 10 is angled relative to the dispersion axis of the grating 43, the spectral content of the beamlets remains distinct in the frequency domain, allowing us to obtain Fig. 13C A different 2-D-FFT 138 for each beamlet is shown. A one-dimensional Fourier transform on the spectral components of the positive spatial frequency content provides a full range of three-dimensional depth profiles. For simplicity, the dispersed beamlets 140 are shown in Fig. 13C 140 are depicted as parallel, although in reality their slope will generally vary with the angle of incidence of the beamlets 14 on the sensor array 6, and therefore will not be identically parallel. Nevertheless, with proper system design, the slope variation between the dispersed beamlets 140 will be small enough so that they do not overlap in spectral content.

[0165] The advantage of this method over spectral domain OCT methods is that expensive and difficult to align spectrometers can be avoided. As with the spectral domain method, the reference beam is tilted on an axis perpendicular to the dispersion axis, allowing the positive frequency term 142 and the negative frequency term 144 to be separated, as shown in FIG. Fig. 13BAs shown. Therefore, a 1-D FFT along the spectral axis enables full range axial measurements. Similarly, digital wavefront correction and measurements can be implemented similarly to spectral domain methods. However, linear OCT suffers from the well-known time domain sensitivity loss compared to spectral domain OCT. For applications where discrete wavelength combs can be used, i.e., sparse axial reflectivity profiles, this sensitivity loss can be reduced.

[0166] As mentioned previously, OCT imaging has a tradeoff between lateral resolution and depth of field. Fundamentally, this tradeoff is because higher NA lenses enable smaller spot sizes, so lateral resolution increases, but at the expense of reduced depth of field.

[0167] Fig.14A An apparatus for retinal imaging is shown in which the retina 89 of an eye 90 is illuminated at a plurality of different angles through a lens system having a finite numerical aperture. For ocular specimens, the numerical aperture is limited by the size of the pupil 92, whereas in other microscopic optical systems or for other specimens, the numerical aperture of the imaging system may be limited by the size of the optical elements. Importantly, the angularly structured illumination provided by the apparatus, i.e., illumination of a volume 114 of the retina at two or more angles of incidence, enables higher lateral resolution to be obtained than would be possible with an imaging system having a finite numerical aperture, while maintaining a greater depth of field than would be possible with a lower numerical aperture. Note that this angularly structured illumination is distinct from the different angles required to illuminate different volumes of the retina 89 incident on the cornea 94 as controlled by the mirror 112 (as explained below).

[0168] Light from a superluminescent diode (SLED) 26 or some other broadband or multi-wavelength source is used to probe the retina 89 and interferometrically measure properties of the retina via reflected or scattered light. In the most general form, the light source 26 should emit light having at least a first wavelength and a second wavelength. When polarization-sensitive detection as described below is used, the light source should be polarized, i.e., emit light of a given polarization state. In the illumination system of the device, the SLED output is formed into a beam by a collimating element 36 and is separated by a polarization-insensitive beam splitting element 40 (e.g., a conventional polarization-independent beam splitting cube, such as Figure 4The sample path beam is divided into a sample path 30 and a reference path 32 by a beam reducer 96 (e.g., an inverse Gaussian beam expander) and directed to a beam control element 98, such as a dual-axis MEMS mirror. The beam control element is adapted to direct the sample path beam in a plurality of paths that become parallel and spatially separated after passing through a parallelizing element 100. In the illustrated embodiment, the parallelizing element is a lens positioned one focal length from the MEMS mirror 98, although in other embodiments, it may be a prism or a mirror. In certain embodiments, the parallelizing element also adjusts the size of the sample beam. The diameter of each synthetic beamlet 101 should be significantly smaller than the diameter of the pupil 92 of the sample eye so that it will illuminate a small area of ​​the retina 89, preferably having a diameter in the range of 50 to 500 μm in the present case. For a normal relaxed eye, a parallel set of beamlets 101 will illuminate the same point of the retina, although for highly myopic eyes, the position of the parallelizing element 100 may need to be adjusted to provide a non-parallel set of beamlets that overlap primarily on the retina. In an alternative embodiment, beamlets 101 are generated simultaneously using a diffractive optical element (DOE) rather than sequentially using MEMS mirror 98, in which case the illumination on the retina is in the form of simultaneous angled structured illumination. In general, the subsequent embodiments are more analytically explicit, as long as the analysis is not unduly affected by sample movement between frames.

[0169] The sequential or simultaneous array of beamlets 101 passes through a beam splitting element which, in a preferred embodiment, includes an open reflector 102 wherein light can pass through a plurality of discrete holes 104 without significant loss at locations where the different angles of the MEMS mirror 98 (for sequential beamlets) or the structure of the DOE (for simultaneous beamlets) can be resolved. Typically, the open reflector has a surface for reflecting light (preferably a total internal reflection surface 110) and one or more openings 104 that locally disrupt total internal reflection at the surface for transmitting light without reflection. In one embodiment, the open reflector includes a prism having a polished optical surface for total internal reflection and one or more openings in the form of drilled holes that disrupt total internal reflection. In Fig.15In the preferred embodiment shown, the opening reflector 102 comprises a pair of prisms 106 which provide total internal reflection 107 at one or the other of the polished optical surfaces 110 except for the opening 104, which is defined by a droplet 108 of index matching adhesive fixedly attached and separating the reflective surfaces. The spacing between the prisms 106 imparted by the droplet of adhesive is not a critical parameter, but can be, for example, in the range of 5 to 500 μm, for example, about 10 μm. As shown, preferably, the prisms 106 are right angle prisms. The choice of an opening beam splitter element 102 can be particularly advantageous over a conventional power beam splitter because it is possible to illuminate the sample and capture the back reflected light which is polarization independent and has low inherent power loss. Obviously, some signal light will be lost in the return path through the aperture 104, but this can be small when the illumination beamlet 101 is small because the total area of ​​each reflective surface 110 is significantly larger than the aperture 104. In any form, the beam splitter element 102 is preferably polarization independent and any residual polarization sensitivity can be calibrated if necessary.

[0170] return Fig.14A , the beamlets passing through the opening reflector 102 are then incident on an angle variable element in the form of a steerable mirror 112, optionally in combination with an optical relay system (not shown) which directs the beamlets onto a common interaction volume 114 of the retina 89. It will be appreciated that the beamlets 101 are directed onto the common interaction volume 114 at different angles of incidence determined by the beam control element 98 and the optical power of the eye 90. The position of the common interaction volume 114 on the retina can be varied by angling the mirror 112, for example, to position 114', so that a larger area of ​​the retina can be imaged by collecting multiple images that can be stitched together to produce a composite image. Light 18 scattered or reflected from the interaction volume 114 for a given position of the mirror 112 is then captured across the pupil 92, approximately collimated by the optical power of the eye, and then directed by the mirror 112 to have an equivalent direction of propagation independent of the point at which the retina is illuminated, for the case of an ideal relaxed eye focused at infinity. After being directed away from the light source 26 by the open reflector 102, the roughly collimated return beam 116 is focused downwardly by the lens 118 to form an image of the retina. Preferably, the lens 118 is variable focus to provide some overall aberration correction to accommodate the large variations in myopia and astigmatism typically found in a clinical setting. Variable focus lenses are provided, for example, by Varioptic.

[0171] We note that, with e.g. Figure 4 Different devices, Fig.14A The device shown in does not require an objective lens 22 to illuminate the common interaction volume 114 and collect the scattered or reflected light 18, because these functions are performed by the cornea and lens of the sample eye 90. Fig. 14BAs shown, Fig.14A The sample-related portion of the apparatus may be adapted to provide angular structured illumination to other types of samples 20 by including an objective lens 22 or other optical power element, wherein the sample is located at or near the focal plane of the lens. The objective lens directs the beamlets 101 onto a common interaction volume 114 of the sample 20 and collects scattered or reflected light 18. In lieu of a variable angle mirror 112, an X, Y translation stage 119 may be used to move the sample for imaging an additional volume 114'. It should be appreciated that there are many other schemes for providing angular structured illumination to one or more volumes of non-ocular samples and collecting scattered or reflected signal light, for example involving reverse illumination, beam splitters or combiners, and rotating the sample.

[0172] The image of the retina passes through the beam combiner 120 and enters the measurement system of the device including the interferometer. In the embodiment shown, the beam combiner 120 is an open reflector similar to the element 102, but has a single opening 104 for transmitting the retinal image. This allows the returning sample beam to be combined with the reference beam 24 and sampled in the Fourier plane by the appropriately positioned 2-D lenslet array 10.

[0173] We now turn to a description of the path of the reference beam 24, which is reflected at the beam splitter 40 and passes through a delay line including a pair of right angle prisms 106A, 106B that approximately maps the group delay of the sample arm light traveling to and from the sample eye 90. The delay line can be combined with a dispersion equalizing element 124 to ensure that the dispersion in the reference arm 32 is similar to the dispersion in the sample arm. Optionally, the reference arm can include a polarization modifying element 126 such as a half-wave plate or polarizer to produce a given polarization state for the reference beam. After the delay line, the reference beam 24 is focused onto the total internal reflection surface 110 of the opening reflector 120 to form a focus close to the position of the retinal image (i.e., close to the opening 104). The reflected reference beam and the transmitted sample beam are then approximately collimated by a lens 122 (the lens 122 converts the far-field angular distribution into a spatial distribution), which can be sampled on a planar 2-D surface with the lenslet array 10. A co-registered aperture array 25 is preferably included to exclude stray light that would compromise the resolution of the 2-D dispersive optical system 128 described below.

[0174] In this configuration, the use of an open reflector of the beam combiner 120 is particularly advantageous, since both the sample beam and the reference beam focused at the beam combiner 120 can be delivered to the interferometer portion of the device without significant losses. If the lateral displacement between the sample and reference foci is small, the effect of the displacement on the fringe contrast in the interferometer can be minimized. Thus, for a given illumination power on the sample, the device is able to provide a very high signal-to-noise ratio, which obviously has to be limited for eye samples.

[0175] The two-dimensional dispersive optical system 128 includes a first collimating lens 42 located approximately one focal length from the aperture array 25, a wavelength dispersive element in the form of a transmission grating 56 that provides dispersion along one axis, and a second lens 44 for focusing the dispersed grid point array onto a 2-D sensor array 6 such as a CMOS camera or other focal plane array. In alternative embodiments, the dispersive element may be a reflective grating or a prism. Note that for simplicity of illustration, in Fig.14A Representative ray paths through the dispersion engine 128 are not shown in FIG. As previously described, the orientation of the lenslet array 10 and the dispersion axis of the grating 56 are selected so that Figure 2 As shown, the spectral dispersion lines produced in each beamlet emerging from the opening are slightly laterally offset. A polarisation walk-off element 130, such as a YVO4 plate, is provided to separate the polarisation states of the signal and reference beamlets, thereby providing two wavelength dispersion lines 132 for each sampling point (opening or lenslet) as shown. In an alternative embodiment without polarisation sensitive detection, the walk-off element 130 is omitted.

[0176] First describe Fig.14A The imaging device is shown operating in its normal lateral resolution mode (i.e., illumination without angular structure), which is limited by the NA of the sample eye 90. During a single frame acquisition cycle of the CMOS camera 6, which is preferably operated in global shutter mode, the SLED 26 is pulsed for a sufficiently short time to avoid significant motion artifacts in the eye. The sample beam is directed through a single opening 104 of the opening reflector 102 to illuminate a volume 114 of the retina 89 through a specific area of ​​the pupil 92. The scattered interference image of the back-reflected or scattered light from the illuminated volume with or without polarization separation imparted by the discrete elements 130 is then digitally read from the CMOS camera 6 and processed with a processor 45 equipped with appropriate machine-readable program code to provide a pixel-to-wavelength mapping for each sampling point defined by the lenslet array 10. As is well understood in the art of spectral domain OCT, the wavelength can be converted to a linearized k-vector by interpolation to construct a phase and amplitude map on the surface of the illuminated volume 114 for a regularly spaced set of k-vectors. Since this set of sampling points corresponds to sampling of the electric field vector in the Fourier plane, a Fourier transform can be used to construct or generate a 3-D image or representation of the illuminated sample volume 114. The use of digital aberration correction and / or digital refocusing can be applied to maintain the lateral resolution of the image over an enhanced depth of field compared to what can be achieved with a single scanning beam with an equivalent numerical aperture limitation.

[0177] The constructed or generated 3-D image or representation may be a single polarization amplitude or phase measurement of the reflected or scattered light, suitable for extracting optical properties of the sample, such as phase, reflectivity, refractive index, refractive index change, or attenuation. Alternatively, if the detection system is polarization sensitive, for example, by means of a polarization discrete element 130, the image or representation may be a polarization property of the sample, such as birefringence or degree of polarization, which may indicate the type of material being sampled. These measurement techniques are well understood for OCT systems (e.g., scanning, time domain, or full-field OCT systems) and can now be directly applied.

[0178] In a preferred embodiment, the polarization-sensitive detection system is supplemented with a polarization control element 134, such as a voltage-controlled liquid crystal element, in the sample arm to enable the sample volume 114 to be illuminated with light of a different second polarization state. In this case, the measurement system makes an additional second set of simultaneous measurements of the reflected or scattered light 18, and the processor 45 processes the two sets of measurements to construct or generate a three-dimensional image or representation of one or more polarization properties of the sample. For example, this modification to the device avoids the situation where the sample birefringence cannot be measured if it is exactly parallel to the input polarization state. In general, while illumination with a single polarization and subsequent polarization-sensitive detection can often provide a clinically useful contrast mechanism, the ability to make two or more separate measurements of a sample with different polarization illumination states allows a more complete description or representation of the polarization properties of the sample to be obtained.

[0179] To image additional portions of the retina 89, the angular position of the mirror 112 is adjusted to illuminate a second volume 114' that is preferably adjacent to the first volume 114 and has a small overlap to facilitate registration of the composite stitched image. This process can be repeated for multiple angles of the mirror 112 to construct a three-dimensional composite image of increasingly larger areas of the retina. Each individual image can be considered a digital representation of the optical properties of the retina over the corresponding volume. Of particular interest, digital refocusing and / or aberration correction can be performed for each individual volumetric image so that off-axis aberrations or eye length variations depending on retinal position can be post-processed after acquisition to provide a clearer image across the entire field of view. Alternatively, digital refocusing and / or aberration correction can be applied after the individual volumetric images have been stitched together to form a single digital representation of the optical properties of the retina over the combined volume. Significant eye length variations are a common feature of myopic patients, for example, which typically limits the resolution of the image in the absence of adaptive optics that can track acquisition. This information on the level of digital refocusing of off-axis aberrations is also clinically meaningful for assessing myopia progression, as it provides a quantitative measure of some axial aberrations of the eye being tested. In visualizing layers of the retina or cornea or non-ocular specimens, it is often useful to do as a B-scan, where a sample slice is imaged. To enable visualization of high-resolution details in a B-scan, multiple contiguous volumes containing the slice of interest may be processed together and the resulting three-dimensional composite image reduced to a high-resolution B-scan by sampling or weighted averaging of the area surrounding the slice.

[0180] It should be understood that Fig.14A The various elements in the device should operate in a coordinated manner. These include, for example, the SLED 26, the sample arm polarization control element 134, the beam control element 98, the angle variable mirror 112 (or the translation stage 119), and the CMOS camera 6. For example, the processor 45 can provide this overall level of control when equipped with appropriate machine-readable program code.

[0181] To enhance the lateral resolution and depth of field achievable with an imaging system having a given numerical aperture, consider now the following case: for each set angle of the mirror 112, the same volume 114 of the retina 89 is measured two or more times, in each case illuminated with a different angle of incidence through a different path through the pupil 92 and the opening 104 in the opening reflector 102. In one specific example, the opening reflector has four openings, two of which are separated by the ends of the pupil on the vertical axis, and the other two are separated on the horizontal axis. In some embodiments, the different illumination trajectories are established by angular adjustment of the 2-axis MEMS mirror 98, so that the sample beam propagates sequentially through the specified openings. In other embodiments, the different illumination trajectories are established simultaneously, for example, by a diffractive optical element (DOE) as described above. In the former case, the multiple measurements of the volume 114 are performed sequentially, i.e., a single shot acquires each illumination trajectory (incident angle). In the latter case, the multiple measurements are performed simultaneously, i.e., a single shot acquires all illumination trajectories. In either case, the far field captured for each different illumination trajectory thus corresponds to an angular shift in the far field. In this way, different regions of the high frequency spatial content of the image (which would otherwise fall outside the NA of the system) have their frequency content shifted or 'mixed' down to the baseband. Since each illumination trajectory captures a different high frequency region, the combined spatial content may be doubled compared to a single illumination capture, thereby achieving a super-resolution of half the Rayleigh criterion. This approach is an improvement over Fourier ptychography (described, for example, in 'Aperture-scanning Fourier pychography for 3D refocusing and super-resolution macroscopic imaging' by Dong et al. in Optics Express 22(11), 13586-13599 (2014)) because the field is now captured interferometrically in a single snapshot rather than having to be iteratively reconstructed to be consistent with the intensity image. With our approach, the passband of the Fourier field can be extended by registering and stitching together different partial far fields (parts of the far field acquired in a single shot) to produce a stitched Fourier plane. Fourier transforms or other digital processing of the extended Fourier field measurements result in enhanced lateral resolution. We do not have to rely on iterative methods to infer what the field should be based on measurements of intensity alone.

[0182] Doppler-like measurements of relative phase are valuable in many situations, for example, for measuring capillary blood flow or for strain or elastography measurements in the presence of mechanical, acoustic, thermoacoustic or ultrasonic interferences. Figure 4 or Fig.10The device shown may be adapted to provide a relatively phase sensitive OCT device that can be used for Doppler-like measurements of motion or deformation and related quantities (eg strain) over a three-dimensional volume of a sample. It will be appreciated that other previously described devices may be adapted in a similar manner.

[0183] In this 'Doppler' embodiment, a multi-wavelength light source 26 is triggered to generate at least a first light pulse and a second light pulse, each light pulse having a sufficiently short duration to allow phase measurement in the presence of motion or deformation to be measured within the interaction volume 19. As previously described, the reflected or scattered light 18 from the first pulse is captured and analyzed in a single exposure or frame of the 2-D sensor array 6. After a predetermined period of time, a second pulse is generated and its reflected or scattered light 18 is subsequently analyzed in a second exposure of the two-dimensional sensor array. After readout and analysis, each exposure provides a composite image including phase and amplitude information from the interaction volume over a range of wavelengths. In some embodiments, the timing between pulses is less than the frame rate of the sensor array, which can be achieved by appropriately timing the pulse illumination relative to the exposure of the sensor array. That is, the first pulse may occur near the end of one frame and the second pulse near the beginning of the next frame. Obviously, the pulsing of the light source 26 needs to be coordinated with the operation of the sensor array 6. In some embodiments, the light source 26 is triggered by the same processor 45 that reads out and analyzes the data from the sensor array.

[0184] When using Figure 4When sampling the far field or Fourier plane as shown, the sample reflectance spectrum is analyzed in the Fourier plane at an array of positions corresponding to the sampling of the 2-D lenslet array 10. It is noteworthy that for a given depth within the interaction volume 19 (corresponding to the frequency components of the spectral interference pattern) located approximately at the focal plane of the lens 22, there is a 2-D far-field spatial frequency component for each lateral sampling position. In addition, a given specular reflection or speckle frequency component will have a phase associated with it. The lateral phase of a particular reflection is detected on the two-dimensional sensor array 6 and can be retrieved by taking a complex FFT (i.e., a composite image) of the phase and amplitude of the returned reflection at spatial resolution during numerical processing by the processor 45 at the interaction volume 19. In the area where the amplitude is strong enough to provide a meaningful phase measurement, a set of points can be established where the phase can be mapped. In some embodiments, the mapping constitutes only the axial component of the phase obtained by the FFT of the spectral components. In other embodiments, the mapping also includes a lateral phase component corresponding to the phase of the spatial far-field component. The lateral phase needs to be scaled according to the focal length of the lens 22 and will generally be a less sensitive measure of displacement. That is, depending on the numerical aperture of the captured light 18, a given phase shift will represent a displacement that is around 3 to 10 times larger than the axial phase shift. Even allowing for reduced sensitivity for sub-micron displacements, this is still a valuable measurement that is difficult to accurately achieve using conventional raster scanning OCT systems. Thus, the true velocity vector, i.e., displacement rate, can be measured more accurately, particularly for capillary flow or other motion to be measured where much of the motion is in the transverse plane and therefore lacks an axial component.

[0185] If there is no overall motion or deformation of the sample 20 between the illumination pulses, i.e., a gross shift much greater than the intra-sample motion or deformation of interest in the Doppler measurement, then the relative phases of the data sets of points are simply subtracted. However, this is not always the case. For many measurements, it is particularly important to be able to properly align the two information frames corresponding to the two measurement sets. This overall alignment between frames can be achieved by optimizing the cross-correlation function in the presence of a grid transformation that provides a given displacement and deformation (e.g., linear compression) of the grid of the sample mapped between one frame and the next. Accurate alignment of the two frames involves taking into account the phase shift associated with the mapping to identify a basis for determining the relative phase shift caused by displacement associated with intra-sample motion (e.g., capillary flow) or deformation (e.g., mechanical perturbation). Knowledge of the predetermined time period between frames enables the rate of displacement to be determined.

[0186] Elasticity imaging determines the local elasticity or stiffness of a sample (e.g., biological tissue) by displacement measurements. For example, the local displacement caused by compression of a sample can be accurately determined from relative phase measurements before and after compression. Local elasticity is inferred from the measured displacements as a function of depth. Alternatively, elasticity can be determined by using impulsive perturbations to generate low-amplitude shear waves whose velocity and dispersion are sensitive to the mechanical properties of the sample. Measuring low-amplitude sample displacements caused by wave propagation requires the resolution provided by phase-sensitive measurements.

[0187] It will be appreciated that the illustrated spectral domain OCT and linear OCT embodiments (wherein the returned sample and reference beams are sampled using a rectilinear 2-D lenslet array angled relative to the dispersion axis of a wavelength dispersive element) are capable of acquiring a three-dimensional image of a sample in a single shot. Specifically, the illustrated embodiments provide apparatus and methods for acquiring improved high-resolution optical images of the retina based on numerical reconstruction of spectral characteristics of light reflected from a small volume of the retina, correcting for aberrations present in the sample eye.

[0188] In each of the illustrated embodiments, focusing of the light beam is performed using an optical power element in the form of a lens. However, it will be appreciated that other forms of optical power elements may be used, for example an off-axis parabolic or ellipsoidal reflector.

[0189] 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

1. A relative phase-sensitive optical coherence tomography device, comprising: (i) an imaging system for acquiring a first image and a second image of an optical property of a sample region in three spatial dimensions, each image comprising phase and amplitude information over a range of wavelengths, and each image being acquired in a single exposure of a two-dimensional sensor array, the second image being acquired a predetermined period of time after the first image; as well as (ii) a processor for: (a) registering the first image with the second image to determine spatially resolved phase shifts caused by motion or deformation of the sample in any spatial dimension; and (b) determining from the phase shift at least one component of the displacement of the sample associated with the movement or deformation, The imaging system comprises: a two-dimensional lenslet array for sampling light reflected or scattered from the sample to produce a plurality of sampling points; and a wavelength dispersive element for dispersing the sampling points onto the two-dimensional sensor array, wherein the lenslets of the lenslet array are positioned relative to the wavelength dispersive element so that, in use, each of the sampling points is dispersed onto a group of pixels of the two-dimensional sensor array.

2. The device according to claim 1, wherein The processor is adapted to determine a displacement rate of the sample associated with the motion or the deformation from the phase shift and the predetermined time period.

3. The device according to claim 1, wherein: The processor is adapted to measure strain associated with the deformation of the sample.

4. The device according to claim 1, wherein: The processor is adapted to perform an elastography measurement on the sample.

5. The device according to claim 1, wherein: The movement comprises capillary flow in the sample.

6. The device according to claim 1, wherein: The two-dimensional lenslet array is positioned to sample reflected or scattered light in the Fourier plane.

7. The device according to claim 1, wherein: The two-dimensional lenslet array is positioned to sample reflected or scattered light in an image plane.

8. The device according to claim 1, wherein: The predetermined time period is less than a frame rate of the two-dimensional sensor array.

9. The device according to claim 1, wherein: The imaging system is configured to acquire an image of optical properties of a region of a sample including a retina of an eye.

10. An article of manufacture comprising a computer usable medium having computer readable program code configured to operate the apparatus of any one of claims 1 to 9.

11. A method for performing relative phase-sensitive optical coherence tomography measurements of a sample, the method comprising the steps of: (i) acquiring a first image and a second image of an optical property of a sample region in three spatial dimensions, each image comprising phase and amplitude information within a range of wavelengths, and acquiring each image in a single exposure of a two-dimensional sensor array, the second image being acquired a predetermined time period after the first image; (ii) registering the first image with the second image to determine spatially resolved phase shifts caused by motion or deformation of the sample in any spatial dimension; as well as (iii) determining from said phase shift at least one component of a displacement of said sample associated with said movement or deformation, The step of acquiring the first image and the second image includes: sampling the light reflected or scattered from the sample to generate a plurality of sampling signals; and dispersing the sampling signals onto pixel groups of the two-dimensional sensor array using a wavelength dispersive element.

12. The method according to claim 11, further comprising: The step of determining a displacement rate of the sample associated with the motion or deformation from the phase shift and the predetermined time period.

13. The method according to claim 11, further comprising: The step of measuring strain associated with said deformation of said sample.

14. The method according to claim 11, further comprising: The step of performing an elastography measurement on the sample.

15. The method according to claim 11, wherein: The movement comprises capillary flow in the sample.

16. The method according to claim 11, wherein: The reflected or scattered light is sampled in the Fourier plane.

17. The method according to claim 11, wherein: The reflected or scattered light is sampled in the image plane.

18. The method according to claim 11, wherein: The predetermined time period is less than a frame rate of the two-dimensional sensor array.

19. The method according to claim 11, wherein: The sample includes the retina of an eye.

20. An article of manufacture comprising a computer usable medium having computer readable program code configured to operate the method of any one of claims 11 to 19.

Images