CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to International Patent Application PCT/IB2019/056106 filed on July 17th, 2019, the entire contents thereof being herewith incorporated by reference.

FIELD OF THE INVENTION

The invention relates to imaging systems and more particularly to tomographic and interferometric imaging systems with high sensitivity.

STATE OF THE ART

[0001] High contrast imaging, improved low signal detection, enhanced imaging depth and overall high imaging quality, since decades stay as an important endeavor for optical imaging. As an example, in dark-field microscopy these goals were successfully addressed by splitting the illumination channel and the detection channel. However, this method shows its limitation when intending to visualize 3D objects. The 3D imaging based on the confocal principle slices the 3D object into so-called optical sections. Out-of-focus light is strongly suppressed due to the spatial filtering of the pinhole. A recently published darkfield confocal microscope [1 ] showed an improved contrast. However, the limited penetration depth remains and is limiting the use of confocal imaging for 3D imaging.

[0002] An improved penetration depth can be obtained with an imaging technique, like Optical Coherence Tomography (OCT) [2,3] OCT is relying on the coherence properties of light. Due to the interferometric principle of OCT no depth scan is needed. This advantage results in a significant improvement of the imaging speed and the detection sensitivity. Over the last 3 decades OCT has been mainly applied as a diagnostic tool for many medical disciplines. Originally, this field started with the so-called time-domain optical coherence tomography (TD- OCT), where the position of a reference mirror in the interferometer is rapidly scanned in order to extract the scattering amplitude from the interference signal and to generate a depth profile of the sample, the so-called A-scan.

[0003] The tomograms i.e. the cross-sectional images are synthesized from a series of laterally adjacent A-scans. This two-dimensional map or tomogram of a reflectivity profile over depth and lateral extent is the so-called“B-scan”.

[0004] The axial resolution in OCT is given by the width of the ‘coherence gate’, which is determined by the spectrum of the used broadband light source. The transversal resolution of the OCT tomogram is determined by the resolution properties of the sample arm optics, usually given by the numerical aperture of the objective.

[0005] Today, Fourier Domain optical coherence tomography (FD-OCT) largely superseded the classical TD-OCT. In FD-OCT, the reference arm has a fixed arm-length. The scattering amplitude over depth is derived from the optical spectrum of the detected light resulting from the back- reflected sample field and superimposed with the reference field. This light field is spectrally decomposed by a spectrometer and detected by an array detector, such as a CMOS device, allowing the simultaneous registration of the spectrally resolved information. The registered spectrum encodes the complete depth profile and allows therefore a high-speed acquisition of depth profiles.

[0006] In summary, FD-OCT does not need a depth scanning section-by-section and records the full depth information in parallel.

[0007] During the last 2 decades, FDOCT evolved into 2 techniques, the so-called spectral OCT (SOCT) and the swept source OCT (SS-OCT). SOCT acquires the signal based on a simultaneous full spectrum acquisition, whereas SS-OCT sweeps a narrow band light source at high speed over the entire spectral range. The signal is detected by an ultrafast single point detector. The complexity of a spectrometer in SOCT is replaced by the complexity of a fast swept source. However, both techniques share the above-mentioned sensitivity advantage.

[0008] Classical OCT systems are realized in a Michelson or Mach-Zehnder interferometer configurations (Fig. 1 A and Fig. 1 B correspondingly) where the source of light is split into a reference and a sample beam paths. The light reflected from an object is superimposed with the reference beam. This allows a heterodyne detection, where the weak signal from the object is strongly amplified by the reference field. Due to this property, OCT provides a significantly higher penetration depth compared to confocal microscopy [4]

[0009] The lateral resolution of OCT systems is limited by the numerical aperture (NA) as is the case in classical microscopy. Increasing the lateral resolution by using higher numerical aperture (proportional in NA ^-1 ) as in confocal microscopy, will limit strongly the depth of field (proportional in NA ² ). This reduction of the field depth, almost eliminates an essential feature of OCT, the in depth imaging without depth scan.

[0010] Extended focus Fourier domain optical coherence microscopy (xf-OCM) [5][6][7] overcame this limitation by using a Bessel beam. This Bessel beam can be generated by means of refractive or diffractive optical element as well as by a programmable spatial light modulator (SLM) and enables a considerable extent of the focal field, while the transverse resolution remains nearly constant along the fully extended focal range [6] Different methods and apparatus have been proposed for generating a light field having an extended depth of focus. The conception of an OCT interferometer comprising an axicon lens disposed in the sample arm to simultaneously achieve high lateral resolution and a greater depth of focus [8][9] or generating an efficient extended focus light beam with substantially uniform axial intensity [10] represents an optical solution to this problem. However, a detection performance across axicon lens (used as an objective) represents a significant limitation compared to a Gaussian detection. To avoid a double pass through the axicon lens, the detection and illumination paths has been decoupled. This solution was shown in the patent application W02007085992A1 [5]

[0011] However a further disadvantage of an extended focus optical coherence microscopy (xfOCM, as disclosed in [5]) remains the high sensitivity to low angle reflection, where especially the first air-sample interface strongly overrides the weak signal of light scattered from the in-depth structures, overall resulting in a low sensitivity of such an optical instrument [1 1 ]

[0012] Classical dark-field microscopy illuminates thin or thick but transparent samples with a ring illumination of high NA. This concept relies on classical spatial amplitude filtering. High spatial frequencies within the spatial frequency range of the illumination are blocked, i.e. a highly reflective mirror is not seen (and produces in simple wording a dark field). Only light scattered by the sample may enter the detection aperture and results in a high-contrast image even for relatively weak signals.

[0013] With regard to OCT/OCM interferometers, the dark-field illumination can be accomplished by a spatial filter placed in the aperture or in a pupil plane. As a result, undesirable reflected field components of the sample are suppressed and the total sensitivity of such OCT systems is further enhanced.

[0014] These concepts have been proposed as dark-field Optical Coherence Microscopy (df- OCM) [1 1 ][12] and constitutes an extension of the previously introduced xf-OCM concept [5][6] and are realized by a blocking amplitude filter in the detection arm of a Mach-Zehnder interferometer - in both xf-OCM and df-OCM configurations the detection and illumination paths are decoupled. Furthermore, due to its annular intensity distribution in the aperture plane, the optical field generated by an axicon can also be used for coherent dark-field, whereas the detection is conceived as a gaussian mode. This leads to a significant gain in sensitivity for df- OCM when compared to xf-OCM or classical OCM. A system having a filter for suppressing a specular light contribution was disclosed in [13,14]

[0015] All of the former state of the art OCT concepts rely on beamsplitters to split the light into an illumination or the reference arm and even more important, to recombine the sample field with the reference field. However, these classical beamsplitters by design are so-called amplitude splitters and deviate the sample field back to the source. The resulting signal loss is an additional limitation especially for weakly scattered sample field, partly wasted and uselessly back propagated to the source. As an example, for a 50:50 beamsplitter this results in a decrease of the light field directed to the detection part of the interferometer by a factor of two.

[0016] There are also known another or quite similar dark-field illumination concepts adopted not only in OCM systems:

[0017] Dark field endoscopic microscope [15], where an iris in the detection arm is used as a low- pass filter to block specular rays.

[0018] Dark-field polarization-sensitive optical coherence tomography [16], Long-focus deep- dark-field retina OCT system [17] and a method and apparatus disclosed in [18] also do not resolve the aforementioned issues.

[0019] To improve the efficiency of the Bessel beam illumination and to remedy a drawback of illumination affected by shadows, a pierced mirror solution was proposed by Marchand et al. [19] However, the reference and detection light are coupled by additional beam splitter, limiting as mentioned before the overall efficiency of the setup.

[0020] Therefore, there is a need to further improve the interferometer and detection parts of the OCM system to achieve full sensitivity.

[0021] The existing limitations within this prior art OCT instrument are improved and/or resolved by the present disclosure and open a novel and innovative way to imaging and instrument properties for an increased sensitivity. This novelty and innovation apply in particular to imaging using

1) polarization

2) differential phase

3) dispersion

but is not limited hereto.

OBJECTIVES

[0022] An object of the invention of the present disclosure is to solve at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described herewith in the following.

[0023] Another object of the invention is to minimize the signal loss i.e. the backreflected light field from the sample.

[0024] Another object of the invention is to detect higher spatial frequencies with enhanced sensitivity. [0025] Another object of the invention is to provide a general multimode platform.

[0026] Another object of this invention is a high-sensitivity interferometer using Pupil Splitting Elements (PSEs) generating a pupil splitting for devising a set-up with a further reduction of optical surfaces from the sample to detector.

[0027] An improved signal budget allowing to“see” deeper structures and more structural details.

[0028] A PSE concept with a further potential to miniaturize OCT and OCM systems i.e. a further miniaturization of the whole imaging set-up.

[0029] An axilens providing a compact axicon-lens optical means for a further reduction of optical surfaces.

[0030] A multimode platform for combining several imaging modalities.

[0031] A general PSE system concept for confocal systems but not limited thereto.

SUMMARY OF THE INVENTION

[0032] The invention of the present disclosure concerns an optical system according to claim 1 , as well as a darkfield confocal microscope or darkfield full-field microscope according to claim 41 . Other advantageous features may be found in the dependent claims.

The optical system may, for example, comprise or consist of an interferometer, for example, a sensing or imaging interferometer, or may for example comprise or consist of an interferometric sensing or imaging system.

Unlike the system realizations proposed in the prior art, the principle of invention consists in fully exploiting a useful signal of illumination and detection paths. According to the invention, both light paths, ones defined, stay not divided and not obscured. They are combined in the detection arm with full efficiency based on the proposed pupil splitting means.

[0033] To achieve the aforementioned objects, an improved system for optical coherence tomography/microscopy is provided which implements PSEs and hole-mirror subsystems.

[0034] To further achieve the aforementioned objects, a low coherence interferometric imaging device in free space configuration with separated illumination and detection light paths is disclosed.

[0035] To simplify and reduce the number of optical elements, the integration of a so-called “axilens” is proposed.

[0036] To increase and achieve further contrast enhancement appropriate polarization elements can be placed in the ray path in order to detect polarization modulating sample structures.

[0037] To further achieve the aforementioned objects, the polarization can be modulated and used in a heterodyne detection scheme for quantitative polarization tomography. [0038] Additional advantages, objects and features of the invention will be set forth in part in the description and claims which follow and in part will become evident to those having ordinary skill in the art upon examination of the following or may learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.

The above and other objects, features, and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description with reference to the attached drawings showing some preferred embodiments of the invention.

DEFINITIONS, TERMS AND ELEMENTS

[0039] “Source” is used to mean any source of electromagnetic radiation, preferably a coherent source or a partially coherent source, or a coherent source with sufficient stability and/or a coherent source having a (sufficiently) short coherence length, for example, less than 10 microns and down to nanometers (for example, 100nm). The source may, for example, be (but not limited to) an optical source of electromagnetic radiation, or a source having an emission wavelength at a wavelength in a wavelength range spanning the ultra-violet (UV) to the infrared (IR), for example, between 400nm and 1600nm. The emission wavelength is, for example, at a wavelength typically used for medical applications. The source may, for example, comprise or consist of a laser. An emission wavelength range can, for example, be centered at 800nm or around 800nm.

[0040]“Detector” is used herein to mean any device capable of measuring energy in an electromagnetic signal as a function of wavelength. A detector array means a plurality of detectors. In general, the preferred detector arrays used in this disclosure have their optimal sensitivity in the wavelength range of the used source. The detectors can either be one-, multi dimensional or line arrays, depending on the optical setup and the optical scan system. For example, in the mostly used wavelength range of around 800nm, CMOS detectors have currently the best performance with respect to sensitivity and read out speed. However, current detector technology does not provide CMOS detectors that operate beyond the 1000nm region. Standard InGaAs and new detector technologies as for example GeSi detectors allow an extension of the detection range beyond 1000 nm and are, for example, also included as an example of this present disclosure.

[0041]“Reflector” is used herein to mean any device capable of reflecting an electromagnetic signal. Thus,“reflector” can be used to mean a mirror, an abrupt change in an index of refraction, an auto-reflecting prism as well as a periodically spaced array structure such as a Bragg reflector. Applicants note that the terms“signal”,“beam” and“light” are used in a synonymously manner, for including all forms of electromagnetic radiation suitable for use in imaging systems. It is also understood, that for the purposes of this disclosure, the term “optical” is to pertain to all wavelength ranges of electromagnetic radiation, and preferably pertains to the range of 100 nanometers to 30 micrometers.

[0042]“Phase modulator” means any semiconductor or bulk device used to modulate or alter the phase of an electromagnetic field. The term“phase modulator” comprises, for example, also any liquid crystal device or any spatial light modulator which allows in addition a local lateral phase change. This phase modulation can be timely and spatially modulated on purpose and, for example, be linked via an interface to a programming device or an appropriate computer.

[0043] “PSE” (Pupil Splitting Element) can be an optical means or component based on a dome shaped glass element or a concentric diffractive optical element. This PSE is to be understood as an optical element configured to generate a Bessel beam (or, for example, a ring illumination in a corresponding Fourier plane). This PSE is also understood as an optical component or element to generate any dome shaped illumination cone or any hollow illumination cone, whatever optical realization will be used, as for example, diffractive optical elements, optical elements based on liquid crystals, or spatial light modulators or gradient optical elements.

As a further consideration and generalization any optical means allowing the separation of the illumination beam path from the detection beam path using separated parts of the sample objective aperture for illumination in a first subaperture of the objective and a different not overlapping second subaperture for detection of said objective.

The simplest realization of a Bessel beam is a ring illumination in a well-defined aperture or pupil plane. However, this realization has the disadvantage of a low efficiency of the illumination intensity. Any new optical PSE element equally falls within the scope of this present invention.

[0044]“Axilens” is any optical means combining an axicon (like) entry surface and a spherical/aspherical exit surface. The Axilens may, for example, comprise or consist of an axicon and a spherical/aspherical surface. The flipping of the element lens-axicon surfaces also provides an equivalent solution.

[0045] “Scanning optics” means any system configured to sweep an electromagnetic signal across a chosen area. Often this configuration includes optionally appropriate focusing means, appropriately positioned for performing an object-scan with either a diffraction limited focusing spot, a Bessel beam or a plurality of spots, or with a continuous line. [0046] Beamsplitter means an optical element configured to receive an input optical field and to split or divide the received optical field or the amplitude of the received optical field into a plurality of optical fields of lower amplitude.

[0047] The aforementioned embodiments and advantage are exemplary and not shown as a limit of the present invention. The present teaching may be extended to other instrumentations. The detailed description of the present invention is intended to be illustrative, and in no case to limit the scope of this invention. Many alternatives, alterations, modification and variations will be apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Fig 1A, 1 B and 1 C show and compare an exemplary interferometer (FIG. 1 C) of the present disclosure with the classical interferometers as a Michelson (FIG. 1 A) or a Mach-Zehnder (FIG. 1 B).

[0049] Fig.2A discloses an exemplary interferometer or system according to the present disclosure based on pupil splitting means.

[0050] Fig.2B is nearly identical to Fig.2A, but shows a ring focus generation and the advantages when using a pupil splitting means.

[0051 ] Fig. 3 shows and discloses an exemplary interferometer or system according to the present disclosure set-up fully exploiting the pupil splitting mean’s advantages.

[0052] Fig. 4A is an exemplary symmetric realization of an interferometer or system containing pupil splitting means.

[0053] L Fig. 4B is almost identical to Fig. 4A, but introduces so called Axilenses allowing to simplify and to miniaturize the set-up.

[0054] Fig. 5 shows a further exemplary system that exploits the advantages of a pupil splitting means for a darkfield confocal microscope.

[0055] Fig. 6 is similar to Fig. 5, however the scanning system is removed. Therefore, it exploits the advantages of PSE for fullfield or brightfield microscopy.

Herein, identical reference numerals are used, where possible, to designate identical elements that are common to the Figures.

DETAILED DESCRIPTION OF THE INVENTION

[0056] Fig 1A, 1 B and 1 C show and compare an interferometer (FIG. 1 C) of the present disclosure with the classical interferometers as a Michelson (FIG. 1A) or a Mach-Zehnder (FIG. 1 B). The interferometers (100, 101) are splitting the fields coming from the source (110) into 2 distinct but coherent fields. For both cases (100, 101) the field splitting is achieved based on beamsplitters (140), splitting the fields according to the splitting ratio. As is known, by those skilled in the art, the splitter is acting in the direct as well as in the inverse propagation direction. As indicated in FIG. 1 B and for the interferometer 101 , the first beamsplitter 141 is splitting the field into 2 parts: part 1 follows the path with elements 142 and 143, part 2 with element 144 and to the sample 120. The sample field generated via backreflection is deviated towards the detection means or detector indicated by reference 130. As is indicated in system 101 , the weak sample field undergoes an additional field splitting at beamsplitter 144 back to the source 110. The same argumentation is valid for the reference field as indicated by the dashed field propagators having reference numbers 122 and 121.

As is obvious, for those skilled in the art, the same arguments hold for the interferometer 100.

[0057] When looking at the system or interferometer 102 of the present disclosure (Fig.1 C), the situation is quite different. The beamsplitter 140 is splitting the field from the source 110 into the sample field and reference field. The field propagating along beamsplitter 140, the pupil splitting means 160 and towards the sample 120 does not have any spatial overlap with the light field propagating along the reference path towards the detector 130. Therefore, the backreflected light from the sample 120 propagates towards or to the detector 130 without any further splitting contrary to the cases of the systems 100, 101 mentioned before.

In the exemplary embodiment of Fig.1 C, the input optical field-splitting means 140 includes a first and second beam-splitter 140, the first beam-splitter 140 being located upstream of the first pupil splitting or modification means 160, and the first pupil splitting or modification means 160 being located upstream of a first hole-reflector 123, and the second beam-splitter 140 is located upstream of the second pupil splitting or modification means 160, and the second pupil splitting or modification means 160 is located upstream of a second hole-reflector 124.

The first beam-splitter 140 is arranged and configured to provide part of an input optical field to the second beam-splitter 140.

This improved signal collection represents in essence a main advantage of this disclosed set-up assuring an improved signal-to-noise ratio, an improved dynamic range and an intrinsic darkfield mode.

While the optical system of FIG.1 C may include a first and a second beamsplitter, it should be understood that the embodiments described herein may more generally include an optical field or optical field intensity splitting stage or platform (or a pupil splitting stage or platform) for providing split optical fields to the pupil splitting means 160 that may, for example, comprise first and a second beamsplitter or alternatively, for example, an optical fiber containing a fiber optic splitter. The optical system may also include a pupil splitting stage or platform.

[0058] Fig. 2A is an illustration of an exemplary optical system 200 of the present disclosure containing a pupil splitting means 160 and, in particular, a Pupil Splitting Element (PSE) hole- mirror subsystem 201, with an example of a possible PSE realization as shown in subsystem 201 of Fig.2A, but the present disclosure is not limited hereto.

At the entrance 110 of the optical system 200, a collimated light field is provided and split into two paths: an illumination path 230 and a reference path 220. Due to symmetry of those paths and for simplicity, only a reference path is described in further detail.

The cross section of the collimated light field is shown at reference number 221 (reference number 231 for the illumination path 230). This light field is deviated by the PSE 161 component of the pupil splitting means 160 into a cone shaped field (see subsystem 201 detailing pupil splitting means 160) and, by the following PSE component 162 of the pupil splitting means 160, is transformed into a parallel propagating ring-shaped field 222 (the inner part has no light field anymore, the light field propagates spatially in the form of (or defining) a hollow cylinder of annular cross-sectional profile), whose cross-sectional profile 222 (232 for the illumination path 230) is also shown in Fig.2A. This field is completely reflected by, for example, a hole mirror 123. Reflection occurs, for example, from a surface surrounding a hole, or inner passage or section of the hole mirror 123.

The inner part of the hole mirror 123 is configured or allows to be crossed by a secondary field (that passes fully through the hole mirror 123) without any deviation nor interaction. The field shaping of the optical field propagating across the optical means or components 161, 162, 123 is illustrated by the cross sections 221 , 222, 241 in the respective planes. The optical means or components 161, 162, 123 are elements of what the Inventors name an infinite PSE hole-mirror subsystem (Inf-PSEHM).

The component 161 of the pupil splitting means or PSE 160 is, for example, configured to spatially reshape the optical field to delimit the optical field to propagation in a delimited spatial area and further delimit a depleted spatial area devoid of the light field or devoid of light field propagation. The previously mentioned hollow cone shaped field is an example. The component 162 of the pupil splitting means or PSE 160 is configured to channel or direct the spatially reshaped optical field along a reference or illumination path. As previously mentioned, these functions of the component 161 , 162 of the pupil splitting means or PSE 160 may be achieved using diffractive optical elements, optical elements based on liquid crystals, or spatial light modulators or gradient optical elements.

A further pupil splitting means 160 and a further hole mirror 124 are, for example, included in the illumination path 230. The sample field 240, generated via back-reflection of the optical field shaped by the further pupil splitting means 160, propagates through the further hole mirror 124 and the hole mirror 123 to define a central section or disc of the optical field profile 241. The central section or disc propagates in the depleted spatial area defined by the component 161 of the reference arm.

[0059] The system of Fig. 2B is almost identical to Fig. 2A with the PSE component 162 comprising or consisting of, for example, a lens 172, as shown in the subsystem 203 of Fig.2B detailing the pupil splitting means 170. Instead of a collimated ring field, the ring field is now focused by the lens 172 in a“ring focus” 243, whose cross-sectional profile is shown in Fig.2B. Compared to Fig.2A, the ring profile produced along the reference path 220 is brought to a focus (having a focused ring cross-section), for example, between the hole mirror 123 and the detector 130.

The optical means or components 161 , 172, 123 are elements of what the Inventors name a finite PSE hole-mirror subsystem (fin-PSEHM).

Again, the hole mirror 123 allows crossing light fields with no interaction. For those skilled in the art, there are several possibilities to realize such a subsystem. The same finite PSE hole-mirror subsystem (fin-PSEHM) can be applied or used in the illumination path 230. The solution disclosed here should, however, not be considered as a limiting set-up. Any use of an alternative PSE-hole mirror system may be part of the system.

It should be noted that the ring profile is provided as a non-limiting example and other profiles are also possible, such as for example, a slit optical field profile. The hole mirror is shaped with a through hole that allows the optical field profile 240 provided from the sample 120 to pass there through.

The mirror 123 may alternatively include a transparent section that allows the optical field 240 provided from the sample 120 to pass there-through, or may alternatively include a transparent substrate or body upon which a reflective coating is selectively deposited for selective reflection. The hole-reflectors of the systems of the present disclosure may comprises or consists of a hole- mirror, or a slit-mirror or a reflector containing a through-passage or an elongated opening through-passage. In the exemplary systems of Figs. 2A and 2B, an input optical field-splitting means includes a beam-splitter and a split-beam reflector, the split-beam reflector being located upstream of the second pupil splitting or modification means 160, 170. The second pupil splitting or modification means 160, 170 is located upstream of the second hole-reflector 123. The beam-splitter is located upstream of the first pupil splitting or modification means 160, and the first pupil splitting or modification means 160 is located upstream of the first hole-reflector 124.

The beam-splitter is arranged and configured to provide part of an input optical field to the split- beam reflector.

[0060] Fig. 3 discloses another exemplary interferometric imaging system 300. The so-called “illumination arm” starts at the fiber 310 followed by the optical means 311 , 312, 313, 314, 315, 301 , 302 for generating a Bessel field at sample (or sample plane/zone) 303.

These optical means for generating a Bessel field may include, for example, a collimating lens 311 , Pupil Splitting Element PSE 312, beamsplitter 313, a first (focusing) lens 314, a hole mirror 315, a scan unit 301 and a second lens 302 for focusing onto sample 303.

The alignment of optical means for the illumination arm is indicated by the optical axis 341 . Similarly, the optical axis 342 is indicated for the reference arm.

The Pupil Splitting Element PSE 312 and Pupil Splitting Element PSE 324 are used to adjust or define the ring-shaped profile. Both PSEs 312 and 324 act like a telescope and constitute an inf- PSEHM sub-system. For those skilled in the art the optical means 312, 314 and 315 constitute a fin-PSEHM sub-system. The ring-shaped field (see outer ring in cross section 380 (the inner section for example, the disk-shaped inner section in cross section 380, being provided by the back-reflection from the sample 303) is scanned by the scan unit 301 and imaged by objective lens 302 into the sample 303.

The back-reflected light propagates along a detection arm i.e. across the objective lens 302, the scan unit 301 , the hole mirrors 315 and 335 and will be focused by the lens 306 onto the fiber 307 for further signal detection and processing, at for example a detector and a processor configured for such processing located downstream in the system.

The system includes a first optical fiber 310 and first lens 31 1 configured to provide the input optical beam to the first pupil splitting element 312, and a second optical fiber 307 and second lens 306 configured to collect the reference beam and the sample illumination beam reflected from the sample 303 to be investigated.

For interferometric imaging this backscattered field must be superimposed with the coherent reference field. This reference originates from the same light source represented, for example, by the optical fiber 310. The reference arm starts with the beamsplitter 313 and propagates across the optical means (Pupil Splitting Element PSE) 324, the subunit 330 via the splitter 333, then the hole mirror 335 via the splitter 333 and is focused and superimposed by the lens element 306 onto the entrance of output fiber 307.

The subunit 330 contains a mirror 331 and serves to adjust the optical length of the“illumination path - detection path” with the length of the reference arm to meet the matching length condition within the tolerance given by the coherence length of the light source.

In the exemplary systems of Fig.3, an input optical field-splitting means 313,333 includes a first and second beam-splitter, the first beam-splitter 313 being located downstream of a first pupil splitting element 312, and upstream of a lens 314 and of the first hole-reflector 315. The second beam-splitter 333 is located downstream of a second pupil splitting element 324, and upstream of the second hole-reflector 335.

The first beam-splitter 313 is arranged and configured to provide part of an input optical field to the second beam-splitter 333 via the second pupil splitting element 324.

The scanning means 301 is located in an aperture or pupil position and configured to scan the sample 303 to be investigated with a Bessel beam or a depth extended focal optical field. The scanning means 301 is located downstream of the first hole-reflector 315.

The means 330 to match the optical length of the reference and sample arm is located downstream from the second beam-splitter 333 and upstream from the second hole-reflector 335. As disclosed in relation to Fig.lA and Fig.l B, this system 300 minimizes the number of optical surfaces between the sample 303 and the detector (or output fiber) 307. The detected light field is not exposed to classical beamsplitters like dielectric flats or prism based beamsplitters, which would divide the wavefront in such a way, that on the return path to the detector the field is deviated back to the reference or illumination path and lost.

[0061] Figs. 4A and 4B discloses a further exemplary interferometric imaging system 400. While similar to the interferometer 300 of Fig.3, both designs differ in relation to a number of elements. Elements 413 and 4131 of the systems of Fig 4A and 4B respectively comprise or consist of, for example, an axicon.

In Fig. 4B, the so-called“illumination arm” starts at the fiber 410 followed by the optical means 411 , 412, 4131 , 4141 , 415, 402, 403 for generating a Bessel field at the sample (or sample plane/zone) 401.The alignment of optical means for the illumination arm is indicated by the optical axis 441.

These optical means for generating the Bessel field at the sample 401 may include, for example, a collimating lens 411 , beamsplitter 412, axicon 4131 , a focusing means or element 4141 , a hole mirror/reflector 415, a scan unit 402 and a lens 403 for directing the Bessel field onto sample 401. The“axilens” 4136 constitutes or consists of an optical means or component acting as (or that is) an axicon 4131 at the entrance and an optical means or component comprising of consisting of a refractive optical interface 4141 focusing the light into the pupil plane at the scan mirror position 402. It is noted that an equivalent optical plane in a pupil plane not necessarily on the scan mirror 402 would result in an equivalent effect.

The exemplary system 400 of Fig.4B includes an axicon 4131 and a focusing element or means 4141 as an integrated single component or element forming an axilens. In contrast, the exemplary system 400 of Fig.4A includes an axicon 413 and a (focusing) lens 414 as separate elements or components that are not formed as part of a single block but provide an equivalent function and result albeit with an increased number of optical interfaces.

The axilens 4136A comprising or consisting of elements 4131 , 4141 constitute an infinite axilens hole-mirror (inf-AHM) sub-system. In contrast to the former system 300 the system 400 uses an axilens 4136A for generating an inf-AHM subsystem i.e. a ring-shaped field (see cross section 480) which is scanned by the scan unit 402 and imaged by objective 403 into the sample. A second axilens 4136B is also, for example, included in the reference arm or path.

The scanning means 402 is located in an aperture or pupil position and configured to scan the sample to be investigated with a Bessel beam or a depth extended focal optical field. The scanning means 402 is located downstream of the first hole-reflector 435.

The first axilens 4136A is configured to focus the optical light beam onto a pupil plane situated at or on the scanning means 402.

The back-reflected light back-propagates across the objective 403, the scan unit 402, the hole mirrors 415 and 435 and will be focused by the lens 406 onto the fiber 407 for further signal detection and processing, as done in the system of Fig.3.

For interferometric imaging, this backscattered field must be superimposed with the coherent reference field. This reference originates from the same light source represented, for example, by the optical fiber 410. The reference arm starts with the beamsplitters 412, 431 and propagates across the optical means 434 (axilens 4136B in Fig.4B, axicon 413 and lens 434 in Fig.4A), having passed via the subunit 430 and optionally the dispersion compensation element or dispersion compensator 433 to the hole mirror 435, and is focused and superimposed by the lens element 406 onto the output fiber 407.

[0062] The subunit 430 contains a mirror and serves to adjust and match the optical length of the illumination path-detection path with the length of the reference arm to meet the matching length condition within the tolerance given by the coherence length of the light source. An element configured to modulate the optical length of the reference arm may also be included. For example, attaching the mirror 430 to a piezo element allows to use this interferometer for phase modulation in view of a phase resolved imaging with high phase accuracy.

The means 430 to match an optical length of the reference and sample arm is located downstream from the beam-splitter 412 and upstream from the second hole-reflector 435.

In the exemplary systems of Figs. 4A and 4B, an input optical field-splitting means 412 includes the beam-splitter 412 and the first pupil splitting or modification means includes a first axilens 4136A. The beam-splitter 412 is located downstream of the first axilens 4136A and the first hole- reflector 435. The second pupil splitting or modification means includes a second axilens 4136B, the beam-splitter 412 being located downstream of the second axilens 4136B and the second hole-reflector 435.

The scanning means 402 is located in an aperture or pupil position and configured to scan the sample to be investigated with a Bessel beam or a depth extended focal optical field. The scanning means 402 is located downstream of the first hole-reflector 435.

As disclosed in relation to Fig.lA and Fig.l B, this system 400 minimizes the number of optical surfaces between the sample 403 and the detector 407. The detected light field is not exposed to classical beamsplitters like dielectric flats or prism based beamsplitters, which would divide the wavefront in such a way that on the return path to the detector the field is deviated back to the reference or illumination path and lost.

As can be understood from the above exemplary embodiments and explanations, the present disclosure concerns an optical system 102, 200, 202, 300, 400 including an input optical field splitting means 140, 313, 333 configured to receive an input optical field and to provide at least a sample illumination optical field propagating along a first optical path 230, 341 and at least a reference optical field propagating along a second optical path 220, 342.

The optical system further includes a first pupil splitting or modification means 160, 312, 314 and a first hole-reflector 123, 124, 315 arranged to channel or direct a back-reflected optical field to a detection or collection area 130, 307, 407 from a sample (120,303) to be investigated, and also includes a second pupil splitting or modification means 160, 170, 312, 324 and a second hole- reflector 124, 123, 335 arranged to channel or direct the reference optical field to the detection or collection area 130, 307, 407.

The first pupil splitting or modification means and the first hole-reflector are arranged to channel or direct the sample illumination optical field to the detection or collection area without directing the sample illumination optical beam (i) through the input optical field-splitting means or (ii) through a further optical field-splitting means located downstream of the input optical field-splitting means. Alternatively or additionally, the second pupil splitting or modification means and the second hole- reflector being arranged to direct the reference optical field to the detection or collection area without directing the reference optical field (i) through the input optical field-splitting means or (ii) through a further optical field-splitting means located downstream of the input optical field-splitting means.

The first pupil splitting or modification means and the first hole-reflector are, for example, arranged to channel or direct the sample illumination optical field spatially separated from the reference optical field to the detection or collection area. The second pupil splitting or modification means and the second hole-reflector is also, for example, arranged to channel or direct the reference optical field spatially separated from the sample illumination optical field to the detection or collection area 130, 307, 407.

The optical system is configured to assure no spatial overlap, between the reflected reference optical field and the back-reflected optical field, on the first pupil splitting or modification means and the second pupil splitting or modification means. The first pupil splitting or modification means and the first hole-reflector along with the second pupil splitting or modification means and the second hole-reflector are configured to channel or direct the sample illumination optical field and the reference optical field so that there is no spatial overlap between a reflected reference optical field and the back-reflected optical field on said elements.

The second pupil splitting or modification means is, for example, configured to generate a Bessel beam or configured to deviate an optical light field of the reference optical field into a ring-shaped optical light field propagating to the detection or collection area 130, 307, 407. The first pupil splitting or modification means (160, 312, 314) can alos be configured to generate a Bessel beam or configured to deviate an optical light field of the sample illumination optical field into a ring- shaped optical light field propagating to the sample 120, 303 to be investigated.

The first and second pupil splitting or modification means can be, for example, configured to deviate the optical light field into a cone-shaped optical field and into the ring-shaped optical light field 222, 224, 232, 234, 380.

The first hole-reflector and/or the second hole-reflector comprises or consists of a hole-mirror, or a slit-mirror or a reflector containing a through-passage or an elongated opening through- passage.

As for example shown in the optical system of Fig.3, the hole-reflector and the pupil splitting or modification means may, for example, define a telescope element containing at least one or a plurality of pupil splitting components arranged to split or recast the optical field into a predefined shape. [0063] FIG. 5 shows an exemplary darkfield confocal microscope according to the present disclosure using an axilens, identical to that of Fig.4B, for generating the pupil split. In comparison to the former system 300, this confocal imaging setup borrows the same element like the sample arm in the system 300. As in the teaching before, this concept avoids the loss of signal between the sample 401 and the fiber end 407 which is acting as the pinhole.

The scanning means 402 is located in an aperture or pupil position and configured to scan the sample or object to be investigated with a Bessel beam or a depth extended focal optical field. The axilens 4136 is configured to focus the optical light beam onto a pupil plane situated at or on the scanning means 402.

[0064] FIG. 6 shows, similarly to previous figures, an exemplary darkfield fullfield microscope according to the present disclosure, where an axilens is used for generating the pupil split. In the current configuration a fullfield approach is presented, therefore a scanning system is not needed. For those skilled in the art, the PSE can be used in a fullfield or line scanning configuration. Alternatives as e.g. a slit-scanning and/or variations are also part of the disclosed invention. These exemplary microscopes include at least one pupil splitting or modification means 4136, 4131 , 4141 and at least one hole-reflector 435 arranged to generate a dark field illumination and to channel or direct a back-reflected optical field to a detection or collection area 407 from a sample or object 401 to be investigated.

The first pupil splitting or modification means may include an axilens 4136. The axilens 4136 comprises, for example, optical means functioning as an axicon 4131 at a first entrance end and a refractive optical interface 4141 configured to focus an optical light beam at an exit end. The axilens is located downstream of the hole-reflector 435.

The first optical fiber 410 and first lens 41 1 are configured to provide the input optical beam to the axilens, and the second optical fiber407 and second lens 406 are configured to collect the sample or object illumination beam reflected from the sample or object 401 to be investigated.

[0065] The above embodiments are described in relation to free-space propagation of the optical light and optical fields between system elements, however, the systems may alternatively or additionally include optical waveguides to channel the optical fields between system elements.

[0066] While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments, and equivalents thereof, are possible without departing from the sphere and scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments and be given the broadest reasonable interpretation in accordance with the language of the appended claims. Features of one of the above described embodiments may be included in any other embodiment described herein.

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