Optical element

Abstract

An optical element (100) is provided, the optical element (100) comprising a first surface (102) for emitting and / or receiving electromagnetic radiation, the first surface being arranged to be optically coupled to or being optically coupled to a portion of an optical fiber (104) having an axis. The optical element (100) comprises a second surface (106) positioned to emit and / or receive electromagnetic radiation in a direction transverse to the axis of the optical fiber (104), wherein the optical element (100) has a first focal length for electromagnetic radiation emitted and / or received by an inner portion of the second surface (106) and a second focal length for electromagnetic radiation emitted and / or received by an outer portion of the second surface (106), the first focal length and the second focal length being different focal lengths. A method of forming an optical device comprising the optical element (100) and further comprising the optical fiber coupled to the optical element is also provided.

Description

TECHNICAL FIELD

[0001] The present invention relates broadly to an optical element and in particular (but not exclusively) to an optical element for an optical fibre, such as a lens, and which can form part of a biomedical device. BACKGROUND

[0002] Optical imaging and characterisation techniques are of increasing interest for medical device applications. The use of optical fibres and optical fibre components enables the design of small optical probes for such devices which can be inserted into tissue or body cavities for imaging or characterisation purposes with minimal invasiveness compared to conventional larger devices. Such devices can be suitable for characterisation or imaging or simultaneous use of different techniques and can even use a single optical fibre to carry optical signals associated with different techniques. The use of a single fibre to implement different imaging and characterisation techniques enables measurements and imaging at the same location using a miniature device which improves accessibility of fine narrow lumen organs such as coronary arteries and small airways.

[0003] Optical probes for optical coherence tomography (OCT) have recently been developed in which the OCT signal is captured by the core of a double clad optical fibre. The inner cladding of the same double clad optical fibre has been used to simultaneously guide optical signals or intensities associated with other characterisation techniques including spectroscopic techniques such as fluorescence and absorption spectroscopy. Such single fibre based OCT + fluorescence techniques have been successfully used in animal and human blood vessels. The technique is a promising candidate for providing accurate diagnosis of high risk plaques which are the main precursor lesions of heart attacks.

[0004] However, the optical requirements of OCT are in contrast to those of fluorescence techniques. In order to provide high quality OCT data or images, focusing optics with a low numerical aperture are required to achieve a long working distance (and large depth of field) whereas fluorescence techniques benefit from a large numerical aperture in order to improve the collection efficiency of the fluorescence radiation. However, probes with lenses having a large numerical aperture typically have a small depth of field. Thus, a probe having ideal optical properties for fluorescence techniques does not have ideal properties for OCT techniques. Other optical techniques also have specific optimal configurations of their optical systems and these can conflict when multiple techniques are implemented within a single system. These optical techniques include OCT, fluorescence, confocal microscopy, multiphoton microscopy, diffuse optical tomography, total internal reflection fluorescence microscopy, phase contrast microscopy, stimulated emission depletion microscopy, near field scanning optical microscopy, differential interference contrast microscopy, second harmonic imaging microscopy, reflectance spectroscopy, Raman spectroscopy and optical coherence elastography.

[0005] There is a need for technical progress. SUMMARY

[0006] In a first aspect of the application, there is provided an optical element, the optical element comprising:

[0007] a first surface for emitting and / or receiving electromagnetic radiation, the first surface being arranged to be optically coupled to or being optically coupled to a portion of an optical fiber, the optical fiber having an axis; and

[0008] a second surface positioned to emit and / or receive electromagnetic radiation in a direction transverse to the axis of the optical fiber;

[0009] wherein the optical element has a first focal length for electromagnetic radiation emitted and / or received by an inner portion of the second surface and a second focal length for electromagnetic radiation emitted and / or received by an outer portion of the second surface, the first focal length and the second focal length being different focal lengths; and

[0010] wherein the inner portion and the outer portion of the second surface are optimized for use of the optical element to acquire measurements at different acquisition parameters using one or more optical techniques.

[0011] In one embodiment, the inner portion and the outer portion of the second surface are optimized for use of the optical element to acquire measurements at different acquisition parameters using at least two different optical techniques.

[0012] The optical element can be integrally formed.

[0013] The outer portion of the second surface can completely surround the inner portion of the second surface.

[0014] One or each of the first surface and the second surface can comprise one or more contiguous surface portions, which can be smooth surface portions.

[0015] In some embodiments, the focal length can be defined as a function of the wavelength of the electromagnetic radiation. In one embodiment, the first focal length is greater than the second focal length. In this case, the depth of field associated with the inner portion of the second surface is greater than the depth of field associated with the outer portion of the second surface.

[0016] The optical element can be arranged such that at least some or a majority of electromagnetic radiation received within the inner portion of the second surface is directed to a central region of the optical fiber, such as a core region, and at least some or a majority of electromagnetic radiation received within the outer portion of the second surface is directed to a region of the optical fiber surrounding the central region, such as a cladding or inner cladding region of the optical fiber.

[0017] Thus, the optical element according to embodiments of the application provides at least partially separate paths for electromagnetic radiation received by the outer portion of the second surface and electromagnetic radiation received by the inner portion of the second surface.

[0018] An advantage of embodiments of the present application is that the optical properties of each part of the lens can be tailored to different optical measurement techniques. This has advantages over lens systems that only have a single focal length, where different optical requirements of multiple techniques can result in sub-optimal performance. For example, OCT is an optical technique where it can be desirable to have a large depth of field, which can be associated with a large focal length. In contrast, detection of fluorescent radiation emitted in random directions requires a large numerical aperture to achieve high sensitivity measurements. This can be associated with a small focal length. The inner part of the second surface can have a large focal length at each wavelength of electromagnetic radiation, which when used with OCT achieves a large depth of field. The outer part can have a short focal length at each wavelength of electromagnetic radiation, which achieves a large numerical aperture, allowing fluorescent radiation to be collected efficiently by the outer part.

[0019] In one embodiment, the optical fibre can comprise an inner core for OCT measurements and an inner cladding to collect fluorescent radiation.

[0020] In one embodiment, the focal length of the outer part results in a numerical aperture that is at least 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or even 0.8 larger than the numerical aperture of the inner part.

[0021] The optical element can have any suitable diameter in the direction along the axis of the optical fibre, but in particular embodiments of the present application the optical element has a diameter of less than 1 mm, 0.5 mm, 0.2 mm or even 0.1 mm.

[0022] The second surface of the optical element can be confined within a circle and can have a diameter of less than 1 mm, 0.5 mm, 0.2 mm or even 0.1 mm. The inner part of the second surface can be confined within a circle and can have a diameter of less than 1 mm, 0.5 mm, 0.2 mm, 0.1 mm or even less than 0.01 mm. The outer part of the second surface can also be confined within a circle and can have an outer diameter of less than 1 mm, 0.5 mm, 0.2 mm, 0.1 mm or even less than 0.01 mm.

[0023] The optical element can be arranged such that the second surface is positioned to emit and / or receive electromagnetic radiation in a direction substantially perpendicular to the axis of the optical fibre.

[0024] The second surface of the optical element can be a surface of a spherical or aspherical lens, an axicon lens, a Fresnel lens, a total internal reflection lens, a diffractive optical element, a metallic lens, or a combination thereof, and can be arranged to correct chromatic and / or spherical aberrations and / or astigmatism. For example, the inner portion and the outer portion of the optical element can comprise different zones of a Fresnel lens, or can comprise lens portions of different curvatures. Further, the optical element can comprise a grating for correcting chromatic aberrations or for wavelength-specific separation of the radiation.

[0025] The optical element can further comprise at least one reflective surface, which can be a surface with a reflective coating, a dichroic coating, or can be a surface positioned for total internal reflection of electromagnetic radiation. The at least one reflective surface can be arranged to direct electromagnetic radiation between a direction along the axis of the optical fiber and a direction transverse to the axis of the optical fiber.

[0026] In some embodiments, the reflective surface is planar. In alternative embodiments, the reflective surface comprises a non-planar smooth surface portion. The reflective surface can also be piecewise continuous.

[0027] The reflective surface can be shaped to correct optical aberrations, or can have a focusing or defocusing function. One example of an optical aberration can be caused by other surfaces, such as a closed catheter sheath, through which the electromagnetic radiation has to pass in order to make an optical measurement.

[0028] The optical element can be arranged to be directly or indirectly optically and mechanically coupled to the optical fiber.

[0029] By optimizing the ratio of the diameters of the outer portion and the inner portion of the second surface, the noise or crosstalk of the optical signal between the optical signal received by the outer portion of the second surface and the optical signal received by the inner portion of the second surface can be reduced. For example, the ratio of the outer diameters of the outer portion and the inner portion of the second surface can be 3:1.

[0030] In embodiments where the optical element 100 comprises a reflective surface arranged for total internal reflection, by optimizing the angle between the total internal reflection surface and the axis of the optical fiber, the noise or crosstalk of the optical signal between the optical signal received by the outer portion of the second surface and the optical signal received by the inner portion of the second surface 106 can be reduced. For example, the angle of the total internal reflection surface can be the critical angle for the selected wavelengths of electromagnetic radiation, such that some wavelengths will not experience total internal reflection.

[0031] Additionally or alternatively, by using an isolation element on the reflective surface or between the inner portion and the outer portion of the second surface, the noise or crosstalk of the optical signal between the optical signal received by the outer portion of the second surface 106 and the optical signal received by the inner portion of the second surface can be reduced. The isolation element can reflect or absorb electromagnetic radiation in a specific range of wavelengths.

[0032] The optical element can be formed using any suitable method, but in a particular embodiment, it is formed using a 3D printing process (such as multiphoton lithography). In this embodiment, the optical element 100 is formed directly on the end portion of the optical fiber, or it can be formed separately. The optical element can be formed from any suitable material (such as a photosensitive material), and in a particular embodiment, it is formed from "Nanoscribe IP-S".

[0033] In a second aspect of the invention, an optical device is provided comprising an optical element according to a first aspect of the invention and an optical fiber coupled to the optical element.

[0034] The optical fiber can be of any suitable type, but in one embodiment, the optical fiber is a coreless optical fiber. The optical fiber may include a coating, such as a material having a refractive index similar to that of the outer portion of the optical fiber, but selected to absorb electromagnetic radiation in a specific wavelength range of interest in order to reduce the effects of stray electromagnetic radiation in that specific wavelength range.

[0035] In a third aspect of the invention, a method for forming an optical device according to a second aspect of the invention is provided, the method comprising the following steps:

[0036] Provide design services for optical components;

[0037] Provide optical fibers and position the ends of the optical fibers relative to the multiphoton 3D printing system; and

[0038] Based on the provided design, a multiphoton 3D printing system is instructed to form optical elements.

[0039] The present invention will be more fully understood through the following description of specific embodiments thereof. The description is provided with reference to the accompanying drawings. Attached Figure Description

[0040] Figure 1 These are microscope images of optical elements according to embodiments of the present invention;

[0041] Figure 2 yes Figure 1 A schematic side view of the optical element shown;

[0042] Figure 3 yes Figure 1 A schematic top view of the optical element shown;

[0043] Figures 4(a) to 4(d) An optical component according to an embodiment of the present invention is illustrated;

[0044] Figure 5 is a plot showing measurement data obtained using an apparatus according to an embodiment of the application;

[0045] Figures 6(a) to 6(c) is an image obtained using an apparatus according to an embodiment of the application;

[0046] Figure 7 illustrates an optical element according to another embodiment of the application; and

[0047] Figure 8 is a flowchart illustrating a method according to an embodiment of the application. DETAILED DESCRIPTION

[0048] Reference is first made to Figures 1 to 3 An optical element 100 according to an embodiment of the application is now described. The optical element 100 has a first surface 102 for emitting and / or receiving electromagnetic radiation and is optically coupled to an end portion of an optical fiber 104. In this embodiment, the optical fiber 104 is a coreless fiber spliced to a double-clad optical fiber 105. The optical element 100 is formed directly on the end portion of the optical fiber 104.

[0049] The optical element 100 also has a second surface 106 positioned to emit and / or receive electromagnetic radiation in a direction transverse to an axis of the optical fiber 104. In this embodiment, the second surface 106 is positioned to emit and receive electromagnetic radiation in a direction substantially perpendicular to the axis of the optical fiber 104.

[0050] The optical element 100 has a first focal length for electromagnetic radiation emitted and / or received by an inner portion of the second surface 106 and a second focal length for electromagnetic radiation emitted and / or received by an outer portion of the second surface 106. The first focal length and the second focal length are different focal lengths.

[0051] The inner portion of the second surface 106 has a larger focal length and a larger depth of field, which is advantageous for OCT imaging. The outer portion of the second surface 106 has a shorter focal length than the inner portion but has a larger numerical aperture that allows for efficient collection of fluorescent radiation. Other combinations of focal lengths can be selected. The choice of focal lengths can be selected to optimize the optical performance of the optical element 100 with respect to one or more optical techniques. The optical techniques include OCT, fluorescence, confocal microscopy, multiphoton microscopy, diffuse optical tomography, total internal reflection fluorescence microscopy, phase-contrast microscopy, stimulated emission depletion microscopy, near-field scanning optical microscopy, differential interference contrast microscopy, second-harmonic imaging microscopy, reflectance spectroscopy, Raman spectroscopy, and optical coherence elastography.

[0052] The inner portion of the second surface 106 is arranged to primarily direct received radiation, such as OCT signals, to the inner portion of the coreless fiber 104 and subsequently to the core region of the optical fiber 105. The outer portion of the second surface 106, associated with a larger numerical aperture, is arranged to primarily direct radiation, such as fluorescent radiation, to the outer portion of the coreless fiber 104 and subsequently to the inner cladding region of the optical fiber 105. For example, for fluorescent measurements, the achievable signal-to-noise ratio is related to the numerical aperture of the collection cone. In this embodiment, the outer portion of the second surface 106 has a numerical aperture greater than 0.4, resulting in a relatively high signal-to-noise ratio and a corresponding high sensitivity.

[0053] The optical element 100 is shaped such that light is directed from a transverse direction to a direction along the axis of the optical fiber 104 using total internal reflection at the surface 110. In a variant of the described embodiment, the surface 110 can be replaced with a surface having a reflective or dichroic coating, and the surface 110 can be a freeform surface.

[0054] The optical element 100 can be used in a "side-viewing" endoscope or intravascular probe, which is typically used inside a transparent catheter sheath in order to protect tissue during an examination as the probe moves within it, and also to prevent the probe from becoming contaminated. Such a transparent sheath optically corresponds to a negative cylindrical lens and causes astigmatism. The surface 110 can be shaped to optically correct for astigmatism. Further, the optical element 100 can include a grating structure to correct for chromatic aberration or to achieve color separation.

[0055] The optical element 100 can have any suitable dimensions. For example, the second surface 106 of the optical element 100 can have an outer diameter in the range of 1 mm to 0.1 mm or even less. The first surface 102 can have an outer diameter that is consistent with the outer diameter of the optical fiber 104 to which the optical element 100 is coupled. For example, the first surface 102 can have an outer diameter of less than 1.5 mm to 0.1 mm or less.

[0056] The optical element 100 can be configured such that the inner portion of the second surface 106 has a focal length in the range of 0.5 mm to 2 mm over a wavelength range of 500 nm to 1900 nm. The inner portion can have any suitable shape, but in one embodiment, the inner portion is circular and has an outer diameter that is 10%-90% less than the outer diameter of the outer portion of the second surface 106.

[0057] Further, the optical element 100 can be configured such that the outer portion of the second surface 106 has a focal length of 0.1 mm - 0.5 mm in the wavelength range of 400 nm to 1000 nm. The numerical aperture of the outer portion of the optical element 100 can be at least 0.1 to 0.8 or more greater than the numerical aperture of the inner portion. The outer portion can have any suitable shape, but in the present embodiment, the outer portion is annular.

[0058] In one embodiment, the optical element 100 is arranged to reduce or avoid cross-talk between radiation received by the outer portion and the inner portion of the optical element 100. Such cross-talk, which can have an adverse effect on the measurement results or image quality, can be reduced, for example, by optimizing the ratio between the diameter of the aperture of the outer portion of the second surface 106 and the diameter of the aperture of the inner portion or by optimizing the angles of inclination of the inner portion and the outer portion. In one example, the ratio of the angles of inclination is 3:1 and the angles of inclination are chosen such that radiation having the chosen wavelength range will not experience total internal reflection at the reflective surface of the optical element 100.

[0059] Further, an optical isolating element (not shown) can be used, which selectively absorbs or filters radiation in a certain wavelength range. The optical isolating element can be positioned, for example, at a surface at which received electromagnetic radiation is reflected. Additionally or alternatively, a coating can be applied to the outer surface of the fiber 104 or 105. The coating can be arranged to absorb undesired radiation. For example, the coating can be arranged to absorb stray radiation that is totally internally reflected at the interface of the optical fiber 104 or 105. The use of a coating on the outside of the optical fiber 104 or 105, which has a refractive index similar to the refractive index of the optical fiber 104 or 105 on the outside, but which absorbs light in a chosen wavelength range, can minimize or reduce such stray radiation.

[0060] The optical element 100 can be integrally formed from an optically transmissive material using a 3D printing process, which will be described in more detail below.

[0061] Reference is now made to Figures 4(a) to 4(d) Examples of a lens surface that can form the second surface 106 or the optical element 100 are now described. In each example, the lens surface is shaped such that the inner portion has a larger focal length, a larger depth of field, and a smaller numerical aperture, while the outer portion has a shorter focal length, a larger numerical aperture, and a shorter depth of field.

[0062] Figure 4(a) is a schematic cross-sectional view of a conventional lens 402, which is shown to illustrate a possible shape of a surface 403 that can form the second surface 106 of the optical element 100 described above with reference to Figures 1 to 3 The surface 403 has a non-uniform curvature, resulting in the inner portion having a larger focal length than the outer portion.

[0063] Figure 4(b) is a schematic cross-section of a Fresnel lens 404 shown to illustrate a possible shape of a surface 405 which can also form the second surface 106 of the optical element 100 shown above with reference to Figures 1 to 3 The surface 405 has an inner Fresnel zone which results in a larger focal length and an outer Fresnel zone which results in a shorter focal length.

[0064] Figure 4(c) is a schematic cross-section of a superlens 406 having a surface 407 which can also form the second surface 106 of the optical element 100 shown above with reference to Figures 1 to 3 The surface 407 is configured such that the inner portion has a larger focal length than the outer portion.

[0065] As a further alternative, the surface 407 can for example be a surface of a diffractive optical element (DOE) having diffractive properties which result in a larger focal length for the inner portion than for the outer portion.

[0066] Figure 4(d) is a schematic cross-section of a second surface 106 comprising more than one type of lens. In this example, the outer portion 108 is a total internal reflection (TIR) lens and the inner portion 109 is a refractive lens. Each TIR facet of the TIR lens uses TIR to redirect light to the optical element 100 as shown in the magnified inset of Figure 4(d). Light passing through the outer portion of the lens can additionally be refracted at the surface of the lens. The use of a combination of refraction and TIR in the lens has the advantage of enabling a highly compact optical design with a large numerical aperture.

[0067] As a further example, the inner portion of the second surface 106 can be a diffractive optical element and the outer portion of the second surface 106 can be a refractive lens.

[0068] Figure 5 is a graph illustrating the intensity of fluorescent radiation 502 detected with a device comprising an optical element according to the present application compared to the intensity of fluorescent radiation 504 detected using a prior art device. Figure 5 The ability of a device comprising an optical element according to the present application to enable higher sensitivity fluorescence measurements than using a prior art device is a result of the high numerical aperture associated with the outer portion of the second surface of the optical element according to embodiments of the present application. In contrast, the depth of field of the inner portion of the second surface is advantageous for OCT imaging.

[0069] Those skilled in the art will appreciate that the optical element according to embodiments of the present application can be used for various applications and is not limited to use for OCT and fluorescence imaging. For example, the optical element can also be used for autofluorescence imaging, autofluorescence sensing, fluorescence imaging using a contrast agent, fluorescence sensing using a contrast agent, confocal microscopy, multiphoton microscopy, diffuse optical tomography, total internal reflection fluorescence microscopy, phase-contrast microscopy, stimulated emission depletion microscopy, near-field scanning optical microscopy, differential interference contrast microscopy, second-harmonic imaging microscopy, reflectance spectroscopy, Raman spectroscopy, and optical coherence elastography.

[0070] Figure 6(a) is an exemplary merged OCT and autofluorescence image obtained using an apparatus comprising an optical element according to an embodiment of the present application. Figures 6(b) and 6(c) show the corresponding OCT and autofluorescence images, respectively. The images show an atherosclerotic plaque obtained with the example of the present application. As shown in Figures 6(a) and 6(c), the high intensity of the detected fluorescence radiation confirms the plaque at the 6-9 o'clock position. In this embodiment, the inner part of the optical element is designed to optimally transmit radiation with a bandwidth greater than 50 nm, and the outer part of the optical element can be designed to optimally transmit radiation with a bandwidth exceeding 20 nm.

[0071] Figure 7 An optical element 700 according to another embodiment of the present application is illustrated. In this example, the inner part 702 of the second surface 704 of the optical element 700 is a surface of a diffractive optical element (DOE) and has been optimized for chromatic confocal imaging and allows focusing different wavelength ranges at different positions. In this example, the inner part 702 is a surface of a DOE that is designed to focus different wavelength ranges at different positions. In this example, the different wavelength ranges are represented by different gray shades. Thus, a depth-resolved confocal signal can be detected using a spectrometer (not shown) that separates the signals from the various wavelengths to different pixels of a detector (not shown). The outer part 706 of the second surface 704 can also be a surface of a DOE, or for example, can be a Fresnel zone with a shorter focal length and a larger numerical aperture than the inner part 702. Figure 7

[0072] Those skilled in the art will appreciate that the optical element according to embodiments of the present application can be used for examining biological tissue and can be used in vivo and ex vivo. For example, the optical element can be used for intravascular imaging, diagnosis, and treatment, and can also be used for other endoscopic applications, such as for examining the digestive system, the respiratory system, the urinary system, and the reproductive system, as well as the ear, and for diagnosing and treating cancer and other diseases. Alternatively, the optical element can be used for examining any type of object, including, for example, a pipe, a tank, or other structure.

[0073] ​Further, the optical element and at least a portion of the optical fiber optically coupled to the optical element according to embodiments of the present application can be positioned within a metal tube or needle having an entrance that is at least transmissive to electromagnetic radiation.

[0074] Further, the optical element can not necessarily be used for simultaneous measurements associated with different measurement or imaging techniques. The optical element can also be used to acquire multiple measurements or images using a single technique, wherein the inner and outer portions of the optical element are optimized for different acquisition parameters. For example, the inner and outer portions of the second surface of the optical element can be optimized for acquiring fluorescent radiation at different depths within the object or within the tissue of interest.

[0075] A method 800 of forming an optical device according to embodiments of the present application is now described. The optical device comprises an optical fiber, on which in this embodiment an optical element is formed. The optical element can for example be the optical element 100 described above.

[0076] The method 800 comprises an initial step 802 of providing a design for the optical element. Step 802 comprises designing the optical element, for example by using the optical design software “Zemax”, and exporting the resulting design in a computer-aided design (CAD) file format. Further, the design can be further refined using the software “Solidworks” (Dassault Systemes, France).

[0077] The method 800 further comprises a step 804 of providing an optical fiber and positioning an end portion of the optical fiber relative to a multi-photon 3D printing system. Step 804 can comprise providing a length of coreless or step-index optical fiber spliced to a double-clad or single-mode optical fiber.

[0078] Further, the method 800 comprises a step 806 of instructing the multi-photon lithography system to 3D print the optical element according to the provided design.

[0079] The multi-photon lithography system allows the optical element to be printed directly to the end of the optical fiber. After mounting the optical fiber in the multi-photon lithography system using a suitable fiber holder, the system is aligned relative to the end of the optical fiber, which can be facilitated by directing light to the opposite end of the optical fiber. A CCD camera can then be used to identify the other end of the optical fiber. The optical element is then printed to the end of the optical fiber, thereby integrally forming the optical element using a suitable photosensitive material, such as “Nanoscope IP-S”.

Claims

1. An optical element, comprising: A first surface for emitting and / or receiving electromagnetic radiation, the first surface being arranged to be optically coupled to or optically coupled to a portion of an optical fiber having an axis; and The second surface is positioned to emit and / or receive electromagnetic radiation in a direction transverse to the axis of the optical fiber; The optical element has a first focal length and a second focal length, the first focal length being used for electromagnetic radiation emitted and / or received by the inner portion of the second surface, and the second focal length being used for electromagnetic radiation emitted and / or received by the outer portion of the second surface, wherein the first focal length and the second focal length are different focal lengths. The optical element is used to acquire measurement values ​​using at least two different imaging techniques. Each of the first focal length and the second focal length is optimal for use with respect to one of the at least two different imaging techniques, and... The depth of field associated with the inner portion of the second surface is greater than the depth of field associated with the outer portion of the second surface.

2. The optical element according to claim 1, wherein the optical element is integrally formed.

3. The optical element of claim 1, wherein the outer portion of the second surface completely surrounds the inner portion of the second surface.

4. The optical element of claim 1, wherein one or both of the first surface and the second surface comprise one or more continuous smooth surfaces.

5. The optical element according to any one of claims 1-4, wherein the optical element is arranged such that at least a portion or a majority of the electromagnetic radiation received in the inner portion of the second surface is directed to the central region of the optical fiber, and at least a portion or a majority of the electromagnetic radiation received in the outer portion of the second surface is directed to the region of the optical fiber surrounding the central region.

6. The optical element according to any one of claims 1-4, wherein the focal length of the outer portion results in a numerical aperture that is at least 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, or 0.8 larger than the numerical aperture of the inner portion.

7. The optical element according to any one of claims 1-4, wherein the optical element has a diameter of less than 1 mm, 0.5 mm, 0.2 mm or 0.1 mm.

8. The optical element according to any one of claims 1-4, wherein the second surface of the optical element is confined within a circle and has a diameter less than 1 mm, 0.5 mm, 0.2 mm or 0.1 mm.

9. The optical element according to any one of claims 1-4, wherein the inner portion of the second surface is confined within a circle, and has a diameter that is less than 1 mm, 0.5 mm, 0.2 mm, 0.1 mm, or less than 0.01 mm.

10. The optical element according to any one of claims 1-4, wherein the outer portion of the second surface is confined within a circle, and the outer diameter is less than 1 mm, 0.5 mm, 0.2 mm, 0.1 mm or less than 0.01 mm.

11. The optical element according to any one of claims 1-4, wherein the optical element is arranged such that the second surface is positioned to emit and / or receive electromagnetic radiation in a direction substantially perpendicular to the axis of the optical fiber.

12. The optical element according to any one of claims 1-4, wherein the second surface of the optical element is the surface of a spherical or aspherical lens, an axonoconical lens, a Fresnel lens, a total internal reflection lens, a diffractive optical element, a metallic lens, or a combination thereof.

13. The optical element of claim 1, comprising at least one reflective surface having a reflective coating or a dichroic coating, or being positioned for total internal reflection of electromagnetic radiation.

14. The optical element of claim 13, wherein the reflective surface is arranged to guide electromagnetic radiation between a direction along the axis of the optical fiber and a direction transverse to the axis of the optical fiber.

15. The optical element according to claim 13 or 14, wherein the reflective surface is planar.

16. The optical element of claim 13 or 14, wherein the reflective surface comprises a non-planar smooth surface portion.

17. The optical element according to claim 13 or 14, wherein the reflective surface is segmented and continuous.

18. The optical element according to any one of claims 1-4, wherein the optical element is arranged to be directly coupled or indirectly optically and mechanically coupled to the optical fiber.

19. The optical element according to any one of claims 1-4, wherein noise or crosstalk between the optical signal received by the outer portion of the second surface and the optical signal received by the inner portion of the second surface is reduced by optimizing the ratio of the diameters of the outer portion and the inner portion of the second surface.

20. The optical element according to any one of claims 1-4, wherein noise or crosstalk between the optical signal received by the outer portion of the second surface and the optical signal received by the inner portion of the second surface is reduced by using an isolation element or a reflective surface between the inner portion and the outer portion of the second surface.

21. The optical element of claim 20, wherein the isolation element is arranged to reflect or absorb electromagnetic radiation having a specific wavelength range.

22. The optical element according to any one of claims 1-4, wherein the optical element is formed using a 3D printing process.

23. The optical element of claim 22, wherein the optical element is formed using multiphoton lithography.

24. The optical element according to any one of claims 1-4, wherein the optical element is formed directly on the end portion of the optical fiber.

25. The optical element according to claim 5, wherein, The central region is the core region, and the region surrounding the central region of the optical fiber is the cladding or inner cladding region of the optical fiber.

26. An optical device comprising an optical element according to any one of claims 1-25, the optical device further comprising an optical fiber coupled to the optical element.

27. The optical device according to claim 26, wherein the optical fiber is a coreless optical fiber.

28. The optical device of claim 26 or 27, wherein the optical fiber comprises a coating selected to absorb electromagnetic radiation having a specific wavelength range in order to reduce the effect of stray electromagnetic radiation in the specific wavelength range.

29. A method for forming an optical device according to any one of claims 26 to 28, the method comprising the steps of: Provide a design for the optical element; Provide optical fibers and position the end portions of the optical fibers relative to a multiphoton lithography system; and According to the provided design, the multiphoton lithography system is instructed to 3D print the optical element.

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