Laser system and method for modifying a sample

Abstract

A laser system for modifying a sample 300 (e.g. marking a diamond / gemstone) has one or more scanning mirrors 180a / b to adjust the angle of the laser beam at a rear pupil plane of an objective lens 190

Description

FIELD OF THE DISCLOSURE The present disclosure relates to a laser system and method for modifying a sample to form a modified region such as a mark, at a target location in a sample, in particular a system for forming a plurality of modified regions in a sample. BACKGROUND OF THE DISCLOSURE Laser modification or marking of gemstones at small length scales can be achieved using pulsed lasers with pulse durations in the range of hundreds of femtoseconds to a few picoseconds and focussing through high numerical aperture lenses. The combination of energy confinement in the time and spatial dimensions lead to high instantaneous focal intensities that create material modifications through nonlinear optical processes. Processing effects may therefore be confined to focal regions. Light focussed from an objective lens into the body of a sample may be aberrated from its ideal focus. It is known to improve focussing efficiency by using an adaptive optical element to correct for spherical aberrations and coma aberrations. Spherical aberrations may arise, for example, by the mismatch in refractive index of the sample and the immersion medium of the focusing objective lens. If uncorrected, spherical aberration leads to a loss of resolution and power efficiency of fabrication, and increases with focussing depth. This problem is particularly pronounced for fine-scale fabrication in diamond due to the material’s high refractive index (2.4) and the need to use high numerical aperture optics to obtain the highest resolution. Coma aberrations may arise, for example, due to a tilt in the surface of the sample with respect to the focussing objective lens. Laser systems for modification of a sample, including laser marking of gemstones, are disclosed in WO 2019 / 030520 A1 and WO 2020 / 161462 A1. SUMMARY OF THE DISCLOSURE Aspects of the disclosure are set out in the accompanying claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims. According to an aspect of the disclosure, there is provided a laser system for modifying a sample to form a modified region at a target location in the sample, the laser system comprising: a sample receiving area for receiving the sample; a laser light source configured to provide a laser beam; and a plurality of optical elements configured to direct the laser beam from the laser light source into the sample when the sample is received in the sample receiving area, the plurality of optical elements comprising: an objective lens for focussing the laser beam at the target location; one or more scan mirror, wherein an orientation of the or each scan mirror is adjustable for adjusting an angle of the laser beam at a rear pupil plane of the objective lens to adjust a location of a focus of the laser beam in the sample; and an adaptive optical element, adjustably configurable to correct for aberrations at the target location; wherein the adaptive optical element is optically conjugate to the rear pupil plane of the objective lens; wherein the or each scan mirror is in a respective optical plane which is optically conjugate with the adaptive optical element. By providing at least one scan mirror as disclosed above, wherein the or each scan mirror is located in a respective optical plane which is optically conjugate with the adaptive optical element and with the rear pupil plane of the objective lens, it is possible to increase the speed at which samples are modified, in particular when forming extended marks or patterns of marks. This is because steering or scanning the laser beam can be much faster than moving the sample while marking. Importantly, by ensuring that the or each scan mirror is located in a respective optical plane which is optically conjugate with the adaptive optical element, and the adaptive optical element is optically conjugate to the rear pupil plane of the objective lens, an image of a spatially-varying phase and / or amplitude correction applied to the laser beam by the adaptive optical element may remain stationary at the back focal plane of the objective lens when scanning the position of the laser beam across the sample, enabling accurate correction of aberrations to be maintained at all marking locations. A suitable energy density for laser modification, for example laser marking, may thereby be achieved when scanning the laser beam across the full field of view of the system. A further advantage is that the or each scan mirror may be used in conjunction with the adaptive optical element to determine the position and orientation of the sample, and then to correct for a residual error in position or orientation of the surface of a sample relative to a focal plane of the objective lens. Thus, laser modification of samples may be achieved using the system of the present disclosure without requiring a high-resolution motion system for moving the sample relative the objective lens, despite variations in the position and orientation of samples presented to the system for modification or marking. Accordingly, the system and method of the present disclosure enable higher throughput of the laser modification process, which is particularly important for efficient laser modification, for example for marking ultra-fine patterns in a plethora of gemstones, for example small gemstones such as melee stones. Another advantage is that the system and method of the present disclosure may allow safer handling of samples having non-uniform but optically transparent surfaces, for example diamond or other samples in a “rough” state (as-grown natural, part blocked natural, as-grown synthetic etc). Using one or more scan mirrors to steer the laser beam may avoid the risk of moving a non-planar surface within the working distance of the objective lens. In the present disclosure, a modified region may be any region in the sample (i.e., at the surface of the sample or within the sample) that has undergone change due to exposure to the laser. Typically, a modified region will comprise different properties and may comprise different optical, structural, mechanical, electrical, etc., properties from the bulk material of the sample surrounding it. In some embodiments, laser modification of the sample to form a modified region may comprise, for example, formation of a mark or a pattern of marks. An extended mark may be formed by producing a plurality of closely-spaced or overlapping marks. Each mark may, for example, be a micron-scale mark or may be formed from a plurality of micron-scale marks. In some embodiments, an individual modification or mark may appear as dark or opaque against a transparent background. For example, in diamond, laser modification may primarily cause local conversion of sp3 bonds to sp2 bonds (i.e., conversion of diamond to graphite). The mark may then appear dark due in part to absorption from the graphitised regions, but also scatter from the non-uniformity in the structure. However, in some embodiments, an individual modification or mark may be transparent in nature, within a transparent material. Such marks may only be visible with suitable lighting at high enough optical magnification and may be invisible with low magnification optics. Transparent marks may be formed by inducing strain in the sample. In diamond, this may still involve some local conversion of sp3 bonds to sp2 bonds, but at a sufficiently low density that the mark is not visible by conventional means. In some embodiments, modification of the sample to form a modified region may include generating a defect in a crystalline material, for example a point defect such as an isolated vacancy or a colour centre, for example an NV centre in diamond. The defect may be detectable as a fluorescent feature rather than an optically apparent feature. The laser beam is suitable for modifying the sample to form a modified region at the target location. The result of modifying the material, in some embodiments, may also lead to a change in the local refractive index of the crystal. The target location in the sample may be at the surface of the sample or below the surface of the sample, i.e., sub-surface. The or each scan mirror may comprise a galvanometric scanning mirror. Advantageously, galvanometric scanning mirrors allow high-speed scanning of the laser beam, helping to speed up the time required to produce an extended mark or pattern in a sample without loss of fidelity. Said one or more scan mirror may comprise two galvanometric scanning mirrors. Advantageously, the inclusion of two scan mirrors enables one scan mirror to be used for scanning the laser beam in each transverse direction orthogonal to the optical axis. This may further speed up the overall laser marking process. The plurality of optical elements may further comprise a plurality of optical relay elements arranged such that the adaptive optical element and the or each scan mirror is each optically conjugate with the rear pupil plane of the objective lens. The optical relay elements may thereby ensure that rotating the or each scan mirror over a limited range to scan the laser beam across the sample does not change significantly the position of the laser beam at the objective lens. In particular, the plurality of optical relay elements may be configured to reproduce both phase and intensity of the optical field of the laser beam at the adaptive optical element at the rear pupil plane of the objective lens. In some embodiments, each optical relay element of the plurality of optical relay elements may be configured to relay both intensity and phase of the optical field of the laser beam. In some embodiments, at least one optical relay element of the plurality of optical relay elements comprises a pair of lenses. For example, at least one optical relay element may comprise a pair of lenses in the “4f” configuration, such that the optical field on the object side of the pair of lenses is reproduced at the image side. Such a 4f configuration allows for accurate relay of both intensity and phase. At least one optical relay element comprising a pair of lenses in the “4f” configuration may be located between the adaptive optical element and the objective lens. In other embodiments, at least one optical relay element of the plurality of optical relay elements comprises further scan mirrors. In some embodiments, the plurality of optical relay elements comprises two optical relay elements, positioned between the laser light source and the objective lens. The adaptive optical element and said one or more scan mirror may be arranged, in any order, at a first optical plane between the laser light source and said one or more optical relay element, and at a second optical plane between said one or more optical relay element, wherein the first, and second optical planes are both optically conjugate with the rear pupil plane of the objective lens. In some embodiments, said one or more scan mirror comprises two scan mirrors located adjacent to each other. In such embodiments, there may be no intervening optical elements between the two scan mirrors. In some embodiments, said one or more scan mirror comprises a first scan mirror and a second scan mirror, and the plurality of optical relay elements comprises three optical relay elements, positioned between the laser light source and the objective lens. The adaptive optical element, the first scan mirror and the second scan mirror may be arranged, in any order, at a first optical plane between the laser light source and the first optical relay element, at a second optical plane between the first and second optical relay elements, and at a third optical plane between the second and third optical relay elements, wherein the first, second and third optical planes are all optically conjugate with the rear pupil plane of the objective lens. Said aberrations may include at least one of: aberrations due to components of the optical system, spherical aberration caused by a mismatch in refractive index at the surface of the sample through which the laser light enters the sample, and coma aberrations due to a tilt of the sample relative to a normal to the optical axis. In some embodiments, the adaptive optical element is further adjustable to vary an axial position of the focus of the laser beam. This feature may be helpful in avoiding any need for a high-resolution motion system, for example for positioning or moving the sample in the axial direction or for changing an orientation of the sample to remove a residual tilt. By adjusting the adaptive optical element to vary an axial position of the focus, it is possible to correct for an error in positioning of the sample in the sample receiving area, for example to account for an axial displacement between the surface of the sample and an ideal focal plane of the objective lens or some other reference plane, or to control a depth of a modified region relative to the sample surface. In particular, when a sample is tilted relative to a focal plane of the objective lens, the adaptive optical element may be adjusted to vary the axial position of the focus of the laser beam as a function of scan angle to track the surface of the sample (when surface marking), or to control the depth of marking within a sample when producing sub-surface marks. In addition, the feature of the adaptive optical element being further adjustable to vary an axial position of the focus of the laser beam may be used when forming three-dimensional (3D) modifications or patterns of modifications, for example three-dimensional extended marks or patterns of marks. For example, the adaptive optical element may be adjusted to change the axial position of the focus of the laser beam after forming a first modified region at a first target location at a first depth relative to the surface of the sample and before forming a second modified region at second target location at a second depth relative to the surface of the sample. In some embodiments, the adaptive optical element is further configurable to correct for edge effects when producing marks close to an edge of a sample. In some embodiments, the system further comprises a control system configured to: control the laser light source to form a first modified region at a first target location in the sample; adjust an orientation of the or each scan mirror to adjust a location of the focus of the laser beam in the sample from the first target location to a second target location in the sample; control the laser light source to form a second modified region at the second target location. During formation of the first modified region, the adaptive optical element may be configured to apply a first correction for correcting for aberrations at the first target location. During formation of the second modified region, the adaptive optical element may be configured to apply a second correction, different from the first correction, for correcting for aberrations at the second target location. The correction applied at each target location may depend on the angle of the laser beam (determined by the respective orientation or scan angle of the or each scan mirror), the position and / or tilt of the sample, and / or the desired depth of the modified region within the sample. In some embodiments, the first target location is located within a first region of a first field of view of the objective lens and the first correction is an average correction for correcting aberrations at locations within the first region; and the second target location is located within a second region of the first field of view of the objective lens and the second correction is an average correction for correcting aberrations at locations within the second region. By dividing the field of view of the objective lens into regions, and determining and applying a fixed correction for target locations within each region, the modification (e.g., marking) of each sample may be achieved in a shorter time. The first and second regions of the first field of view may correspond to rings of the field of view at different radii from the optical axis of the laser system, or to layers of a pattern at different depths, or to some other subdivision of the field of view. In some embodiments, the system is further configured to: form a plurality of first modified regions in the sample within the first region of the first field of view, using the first correction; and produce a plurality of second modified regions in the sample within the second region of the first field of view, using the second correction. In some embodiments, the system further comprises a motion device for moving the sample and / or the objective lens relative to one another, wherein the system is further configured to: move the sample and / or the objective lens relative to one another; and form a third modified region at a third target location in the sample, wherein during formation of the third modified region, the adaptive optical element is configured to apply a third correction for correcting for aberrations at the third target location. This may be useful for forming modified regions or patterns of modified regions over an area which is larger than the field of view of the objective lens, requiring the field of view to be moved relative to the sample, or for modifying the sample over a range of depth which is beyond the range of adjustment configurable using the adaptive optical element. In some embodiments, the motion device may comprise a high-resolution 2D or 3D translation stage for moving the sample relative to the objective lens. In some embodiments, the motion device may alternatively or additionally comprise an objective scanning device (for example an objective stepper actuator or a piezo objective focus scanner) for scanning the objective lens relative to the sample in a direction parallel to the optical axis. Provided that the movement of the objective lens is small relative to the depth of focus of the optical elements relaying the optical field of the laser beam at the adaptive optical element to the rear pupil plane of the objective lens, any loss of phase information during the translation of the objective lens would be minimal. Effectively, the rear pupil of the objective lens would remain optically conjugate with the adaptive optical element. As an example, an achromatic doublet lens has a depth of focus on the order of millimetres, for example between 0.1 and 5 mm, or more typically around 2 to 3 mm. Therefore, a movement of the objective lens of less than 0.5 mm, for example up to 0.2 mm, incurs minimal loss of phase information. The third target location may be located outside the first field of view of the objective lens. In some embodiments, the control system may be further configured to: obtain a pattern of modifications to be formed in the sample; divide the pattern of modifications into a plurality of regions; and determine, for each region of the plurality of regions, a respective correction to be applied to the adaptive optical element for correcting aberrations when forming parts of the pattern comprised within said region. In some embodiments, the control system may be further configured to, for each region of the plurality of regions: configure the adaptive optical element to apply the respective correction to the laser beam; and while the respective correction is applied to the laser beam, modify the sample to form the modifications corresponding to parts of the pattern comprised within said region. In some embodiments, the control system is further configured to: adjust an axial position of the focus of the laser beam until the focus is at a first location on the surface of the sample to thereby determine an axial distance of the sample relative to the objective lens. The axial distance of the sample may be determined relative to the objective lens, relative to an ideal focal plane of the objective lens, or relative to some other reference plane associated with the objective lens. In this way, the position of the surface of the sample may be determined without requiring a high-resolution motion system for moving the sample in the axial direction. In some embodiments, the control system is further configured to: adjust an orientation of the or each scan mirror to adjust a transverse position of the laser beam to a second location at the surface of the sample; and adjust an axial position of the focus of the laser beam until the focus is at the second location on the surface of the sample to thereby determine a tilt of the surface of the sample relative to a focal plane of the objective lens. In this way, the orientation or tilt of the surface of the sample may be determined relative to the optical system. Measurement of the axial position of the surface of the sample at two different locations is sufficient to determine a tilt of the sample in one dimension. By repeating the process of adjusting an orientation (i.e., a scan angle) of the or each scan mirror and adjusting the axial position of the focus to determine the axial position of the surface at a third location on the surface of the sample, it is possible to determine the local tilt of the surface of the sample in two dimensions. Advantageously, the tilt may be determined without requiring a high-resolution motion system for moving the sample relative to the objective lens. Instead, the or each scan mirror and the adaptive optical element may be used to determine the residual axial offset and tilt of the sample and the adaptive optical element may be used to correct for any residual axial offset or tilt. In some embodiments, the laser system may be configured to: adjust a respective orientation of the or each scan mirror to form a plurality of modified regions at a plurality of different respective target locations in the sample; and at each target location, adjust a configuration of the adaptive optical element prior to forming the respective modified region, to vary the axial position of the focus of the laser beam dependent on at least an orientation of the at least one scan mirror and a determined tilt of the sample. In this way, a residual tilt of a sample relative to the focal plane of the objective lens may be taken into account by varying the axial position of the focus of the laser beam. For example, this may be used to track a surface of the sample when marking the surface, to track a plane of constant depth when marking sub-surface, or to control the depth of the mark within the sample to produce patterns of marks having a range of different depths. The required correction to be applied by the adaptive optical element may be determined based on knowledge of the objective lens, the position and (local) tilt of the sample, the angle of the laser beam (due to the orientation of the scan mirror(s)) and the desired depth of the mark relative to the surface of the sample. The system may further comprise an imaging system for imaging a modified region produced in the sample by the laser beam, and / or for imaging the laser beam at the surface of the sample. The system may further comprise a delivery system configured to deliver a sample to the sample receiving area. The delivery system may be a high-speed delivery system, for delivering a plurality of samples in turn to the sample receiving area for laser modification. The delivery system may comprise a robot, motion arm or other motion system. A high-resolution motion system is not necessarily required, because it may be possible to determine residual errors in position and tilt of each sample and to adjust the position (both transverse and axial) of the focus of the laser beam within the sample by adjusting only the orientation of at least one scan mirror and the configuration of the adaptive optical element. According to another aspect of this disclosure, there is provided a method for modifying a sample to form a modified region at a target location in the sample using a laser system as disclosed above. The laser system may comprise: a sample receiving area for receiving the sample; a laser light source configured to provide a laser beam; a plurality of optical elements configured to direct the laser beam from the laser light source into the sample when the sample is received in the sample receiving area, the plurality of optical elements comprising: an objective lens for focussing the laser beam at the target location in the sample; one or more scan mirror, wherein an orientation of the or each scan mirror is adjustable for adjusting an angle of the laser beam at the rear pupil plane of the objective lens for adjusting a location of a focus of the laser beam in the sample; an adaptive optical element, adjustably configurable to correct for aberrations at the target location; wherein the adaptive optical element is optically conjugate to a rear pupil plane of the objective lens; wherein the or each scan mirror is in a respective optical plane which is optically conjugate with the adaptive optical element. The method may comprise: forming a first modified region at a first target location in the sample; adjusting an orientation of the or each scan mirror to adjust a location of the focus of the laser beam in the sample from the first target location to a second target location in the sample; and forming a second modified region at the second target location. The respective modified regions (e.g., marks) may each be produced by controlling the laser light source to modify the sample at the respective target location. During formation of the first modified region, the adaptive optical element may be configured to apply a first correction for correcting for aberrations at the first target location, and during formation of the second modified region, the adaptive optical element may be configured to apply a second correction, different from the first correction, for correcting for aberrations at the second target location. The first target location may be located within a first region of a field of view of the objective lens and the first correction may be an average correction for correcting aberrations at locations within the first region. The second target location may be located within a second region of the field of view of the objective lens and the second correction may be an average correction for correcting aberrations at locations within the second region. The method may further comprise: forming a plurality of first modified regions in the sample within the first region of the field of view, using the first correction; and producing a plurality of second modified regions in the sample within the second region of the field of view, using the second correction. The method may further comprise: moving the sample and / or the objective lens relative to one another; forming a third modified region at a third target location in the sample, wherein during formation of the third modified region, the adaptive optical element is configured to apply a third correction for correcting for aberrations at the third target location. The third target location may be located outside the first field of view of the objective lens. The third correction may be an average correction for correcting aberrations at locations within the third region. The method may further comprise: adjusting a configuration of the adaptive optical element to adjust an axial position of the focus of the laser beam until the focus is at a first location on the surface of the sample to thereby determine an axial distance of the sample relative to the objective lens. The method may further comprise: adjusting an orientation of the or each scan mirror to adjust a transverse position of the laser beam to a second location at the surface of the sample; adjusting a configuration of the adaptive optical element to adjust an axial position of the focus of the laser beam until the focus is at the second location on the surface of the sample to thereby determine a tilt of the surface of the sample relative to a focal plane of the objective lens. The method may further comprise: adjusting a respective orientation of the or each scan mirror to produce a plurality of modified regions at a respective plurality of different target locations in the sample; and at each target location, adjusting a configuration of the adaptive optical element, prior to producing the respective modified region, to adjust the axial position of the focus of the laser beam dependent on an angle of the or each scan mirror and a determined tilt of the sample. The sample may comprise a transparent material, that is, a colourless or coloured transparent material, for example a transparent crystalline material or transparent glass material. The sample may comprise at least one of: a diamond, a gemstone, a melee diamond, a melee gemstone, a coloured gemstone, a diamond plate, a rough diamond (as-grown natural diamond, part-blocked natural diamond, as-grown synthetic diamond, as-grown synthetic diamond, etc.), SiC (silicon carbide), hBN (hexagonal boron nitride), or silicate glass. BRIEF DESCRIPTION OF THE DRAWINGS Example embodiments of the present disclosure will be described, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which: Figure 1A schematically illustrates a system for laser modification of a sample according to an example embodiment of the present disclosure; Figure 1B schematically illustrates a system for laser modification of a sample according to another example embodiment of the present disclosure; Figure 2 schematically illustrates the effect of adjusting an orientation (scan angle) of the scan mirrors in the system of Figure 1A; Figure 3 schematically illustrates correction of aberrations due to the imperfections of the objective lens of the system of Figure 1A or 1B; Figure 4 schematically illustrates correcting for a residual tilt in sample relative to the optical system by varying the depth or axial position of the focus when scanning across the field of view; Figure 5 illustrates a method for creating a mark or a pattern of marks in a sample according to an embodiment of the present disclosure; Figure 6 illustrates a method for creating a mark or a pattern of marks in a sample according to an embodiment of the present disclosure; Figure 7 illustrates an example pattern of marks to which the method of Figure 6 may be applied; Figure 8 illustrates a method for creating a mark of a pattern of marks in a sample, according to an embodiment of the present disclosure; and Figure 9 illustrates an example pattern of marks to which the method of Figure 8 may be applied. DETAILED DESCRIPTION Figure 1A schematically illustrates a system 100A for laser modification of a sample 300 according to an example embodiment of the present disclosure. Figure 1B schematically illustrates a system 100B for laser modification of a sample 300 according to another example embodiment of the present disclosure. In Figures 1A and 1B, the same reference numerals are used to indicate elements which are the same or similar in both embodiments. The system 100A, 100B may each be used for laser modification of a material sample 300, in particular gemstones, for example diamonds. Focusing an ultrashort pulsed laser in a diamond, either at the surface or below the surface of the gemstone, it is possible to make isolated visible marks or other modifications without any modification to the surrounding regions of the sample. Typically, the laser pulses are shorter than 10 ps, preferably of the order of a picosecond. For example, pulses of between 0.25 ps and 2.5 ps are typically used for forming sub-surface marks with an air objective. The combination of tight focusing and short pulse length leads to an electric field at focus which is sufficiently high for non-linear absorption of light, either through a multi-photon absorption or a tunnelling ionisation, followed by an avalanche ionization. The non-linear nature of the process means that the structural modification is confined to the focal volume of the laser, without damage to the surrounding regions. The ultrashort pulse lengths means that all energy is delivered to the modification before any can diffuse via heat, making very small, sub-micron marks and other modifications possible. Some modifications may correspond to modifications of the refractive index of the sample. Such marks, formed within a transparent material, may only be visible with suitable lighting at high enough optical magnification and in the example of a gemstone may be invisible in a conventional 10x jewellers loupe. Other modifications of a sample may include generation of one or more defects in a crystalline material, for example a point defect such as an isolated vacancy or a colour centre, for example an NV centre in diamond. Such defects may be detectable as a fluorescent feature rather than optically apparent features. Modified regions of the sample in the form of laser-written marks may be used as permanent features for the identification or hall marking of gemstones, using a QR code or other type of code, pattern, serial number, or other unique design. Other modifications such as formation of fluorescent features, point defects like isolated vacancies or colour centres, may have applications in quantum devices. For highly repeatable deterministic modification or marking of a sample, the laser should be focussed at high numerical aperture (NA) at the target location in the sample, either at the surface of the sample or below the surface of the sample. This could be with an air, water or oil immersion objective lens. Typically the numerical aperture should be above 0.7, preferably between 0.7 and 1.4. Numerical apertures are typically less than 2.0. Aberrations lead to distortion of the laser focus, and the resulting loss of resolution counteracts the effect of using a high NA lens. When aberrated, the laser marking may become erratic and uncontrolled and it may become impossible to deterministically create submicron marks (sub-micron in the transverse direction perpendicular to the optic axis of the system) at a given pulse energy and dose. Spherical aberrations may result from the optical system itself, for example the objective lens, and may be determined for the optical system. Spherical aberrations also result from refraction of the laser beam at the surface of the sample due to the difference in refractive index between the sample and the immersion medium chosen for the objective lens. The amount of spherical aberration may be calculated analytically and increases with the depth inside the sample, the NA of the objective lens and the refractive index (RI) mismatch. Another important aberration is first order coma, which is related to a tilt of the interface of the sample through which the laser is focused, such that it is not perfectly perpendicular to the optical axis of the system. Coma aberrations, which include wavefront tilt causing a lateral focal shift, are proportional to both the focussing depth and the angle of surface tilt. The combination of spherical aberration and coma cause reductions in the focal intensity and distortions of the intensity distribution that affect the fabrication efficiency and precision of laser marking. Spherical aberrations and coma can be corrected using an adaptive optical element to impose an equal and opposite phase distribution to the laser beam prior to the objective lens, restoring diffraction limited performance at the focus and enabling highly repeatable deterministic marking of sub-micron features at a given pulse energy and dose. The Strehl ratio may be defined as the peak focal intensity relative to that when there are no aberrations present in the system. When a focus simply limited by diffraction has a peak intensity Io and a real system has peak intensity lp the Strehl ratio is defined as the ratio lp / lo. If aberrations are present in a system, wavefront errors diffract light away from the focus and reduce the actual peak intensity lp from the theoretical maximum Io, thus reducing the Strehl ratio. If the wavefront error is characterised as an aberration phase function qj, the Strehl ratio S is given by: S = lp / Io = | <ei’' >|2, where the angled <...> brackets denote an average over a particular aperture in the optical system, taken here as the pupil aperture of the focussing objective lens. For a perfect system, the wavefront error is zero (flat phase) and the Strehl ratio is 1. If the wavefront error increases due to aberration the Strehl ratio decreases. A Strehl ratio of about 0.1 is typical for a laser focussed at e.g., a depth of 100 micrometers in a diamond sample for a high NA lens. The Strehl ratio may therefore be understood as a measure of the effect of aberration on a laser focus, with ratios closer to 1 being less affected by aberrations and flaws in the system. It may also be understood as a measure of the efficiency of a given focus. The aberration function describes all phase errors related to the propagation of light toward the focus, and may be written as a sum of Zernike polynomials such as primary coma, primary spherical and primary astigmatism. The aberration function may alternatively be composed as a sum of another set of basis functions, or as a direct numerical computation / analytical solution for a particular focussing location inside a known sample. Using an adaptive optical element to correct spherical aberrations and coma, a laser focus within the sample having a Strehl ratio of greater than 0.5 can be achieved. The system and method of the present disclosure may be used to modify a sample using a laser focus with a Strehl ratio of greater than 0.5. The focus may have a Strehl ratio of greater than 0.6, greater than 0.7, greater than 0.8, or greater than 0.9. The systems 100A and 100B, illustrated in Figures 1A and 1B respectively, each includes a sample receiving area 110 for receiving the sample 300 for laser marking. The sample 300 may be supported in a sample holder on a motion system 120. The motion system may be part of a delivery system for delivering samples to the sample receiving area for marking, for example a robot, a motion arm or other motion system. The system 100A, 100B comprises a laser light source 130, configured to provide a laser beam 140, and a plurality of optical elements 142, 144, 150, 160, 170, 180, 190, 200 configured to direct the laser beam 140 from the laser light source 130 into the sample 300 when the sample 300 is received in the sample receiving area 110 such that the laser beam 140 is focussed at a target location 303 in the sample 300. References to a target location in the sample 300 may include target locations on the surface of the sample, for surface marking of a sample, and target locations below the surface of the sample, for sub-surface marking of a sample. In each of Figures 1A and 1B, the laser beam 140 is shown focussed at a target location 303 on the surface 302 of the sample 300. The laser light source 130 may be a femtosecond infra-red (IR) fabrication laser, but the present disclosure may also be applied to fabrication systems of any wavelength or any pulse length. For example, ultraviolet (UV) and continuous wave (CW) laser sources can be used. The laser beam 140 first passes through a mechanical laser safety shutter system 142 with built-in power meter for remote measurement of laser power. Polarisation of the laser beam 140 is then controlled by a half-waveplate 144 so that the electric field vector of the laser beam 140 is properly conditioned for the adaptive optical element, 160. The plurality of optical elements for beam delivery further includes a beam expander 150, an adaptive optical element 160, a first optical relay device 170a, a first scan mirror 180a, a second optical relay device 170b, a second scan mirror 180b, a third optical relay device 170c and an objective lens 190. The objective lens 190 focusses the laser beam at the target location in the sample, either at the surface of the sample or within the sample. The objective lens 190 may be a simple or compound objective lens. As an example, the objective lens 190 may be an infinity corrected lens suitable for air immersion and transmissivity for the laser light source, with a numerical aperture higher than 0.7 and lower than 2.0. The beam expander 150, in the form of a pair of lenses 150a, 150b, expands the laser beam 140 output from the laser light source 130 so that it fills the active area of the adaptive optical element 160. However, the beam expander 150 may not be required if the laser beam 140 provided by the laser light source 130 is of a suitable diameter. The adaptive optical element 160 is used to spatially vary the phase and / or amplitude of the wavefront of the laser beam 140 in order to correct aberrations at the target location, and optionally, to adjust the axial position of the focus of the laser beam. The adaptive optical element 160 may be any element capable of dynamically modifying the properties of light incident thereon. For example, liquid crystal spatial light modulator (SLMs), deformable mirrors (or micro-deformable mirrors), digital micromirror devices, and adaptive lenses are all examples of adaptive optical elements which may be used to dynamically impose spatially varying modulations on a laser beam’s profile to thereby control, for example, its phase and / or propagation properties. In each of Figures 1A and 1B, the adaptive optical element 160 is illustrated as a reflective element. However, in other embodiments, the adaptive optical element may be a transmissive element. The first and second scan mirrors 180a, 180b provide for two-axis scanning or steering of the beam relative to the sample 300, for example in the two transverse directions orthogonal to the optical axis of the system 100A, 100B. The first and second scan mirrors 180a, 180b are mirrors capable of being rotated about at least one axis in order to steer or scan the laser beam relative to the sample 300. In the example embodiments illustrated in Figures 1A and 1B, the first scan mirror 180a may be rotated about a (single) first axis, for example an axis orthogonal to the plane of the respective Figure, while the second scan mirror 180b may be rotated about a (single) second axis, orthogonal to the first axis. Accordingly, the first and second scan mirrors 180a, 180b may be used to provide two-axis scanning or steering of the laser beam 140 relative to the sample 300. In some embodiments, it may only be necessary to steer the laser beam in one direction, in which case the second scan mirror and second pair of relay lenses may be omitted. In yet further embodiments, a single scan mirror in the form of a two-axis scanning mirror may be used to steer the laser beam in two orthogonal directions. Preferably, the first and / or second scan mirrors 180a, 180b are galvanometric mirrors which provide the advantage of high-speed scanning. In the embodiment shown in Figure 1A, the adaptive optical element 160 receives the laser beam from the beam expander 150 and reflects the laser beam to the first scan mirror 180a via the first optical relay device 170a. The first scan mirror 180a reflects the laser beam to the second scan mirror 180b via the second optical relay device 170b. The second scan mirror 180b reflects the laser beam to the objective lens 190 via the third optical relay device 170c and a beam splitter or dichroic mirror 200. The first, second and third optical relay devices 170a, 170b, 170c are arranged such that the adaptive optical element 160, the first and second scan mirrors 180a, 180b and the rear pupil of the objective lens 190 are in optically conjugate planes. That is, the first optical relay device 170a is arranged to relay an image of the optical field at the active surface of the adaptive optical element 160 onto the first scan mirror 180a. The second optical relay device 170b is arranged to relay an image of the first scan mirror 180a onto the second scan mirror 180b. The third optical relay device 170c is arranged to relay an image of the second scan mirror 180b onto the rear pupil plane of the objective lens 190. Accordingly, adjustment of the orientation of the first and second scan mirrors 180a, 180b results in a change of the angle of the laser beam at the rear pupil of the objective lens 190, but not the position of the laser beam relative to the objective lens 190. As a result, the intensity of the laser beam at its focus in the sample 300 may remain constant as the beam is scanned across the sample 300. This enables the system to be used for creating extending marks or patterns in the sample across the field of view of the objective lens by adjusting the angle of the scan mirrors 180a, 180b to move the focus of the laser beam from one target location in the sample to another. In addition, since the adaptive optical element 160 is conjugated to the rear pupil of the objective lens 190, the spatially varying phase / amplitude modulation pattern imposed on the laser beam by the adaptive optical element 160 remains stationary at the rear pupil or rear focal plane of the objective lens 190 while scanning the laser beam 140 relative to the sample 300. Each of the first, second and third optical relay devices 170a, 170b, 170c may be formed from a pair of lenses in the so-called “4f configuration” as shown in Figure 1A. Using the first optical relay device 170a as an example, a “4f’ pair of lenses comprises lenses 170a’ and 170a” having respective focal lengths fi and f2, where lenses 170a’, 170a” are separated by the sum of their focal lengths fi+f2. The first optical plane located one focal length h in front of the first lens 170a’ is imaged onto a second optical plane located one focal length f2 after the second lens 170a”. The first and second optical planes are therefore optically conjugate. Magnification of the “4f” pair of lenses 170a’, 170a” is given by f2 / fi, so a “4f” pair of lenses having identical focal lengths fi=f2 gives 1:1 magnification, while a “4f” pair of lenses comprising lenses of two different focal lengths fi#2 may be used to expand or reduce the size of the laser beam. An advantage of using “4f” pairs of lenses for the first, second and third optical relay devices 170a, 170b, 170c is that this arrangement preserves the phase of the image at the back of the objective lens 190. This may be important in applications in which the adaptive optical element 160 is used to modify the phase, as, for example, when using an adaptive optical element 160 in the form of an SLM. In implementations in which only the amplitude (and not the phase) of the laser beam is spatially modulated by the adaptive optical element 160, the first, second and third optical relay devices 170a, 170b, 170c may be implemented by, for example, a single lens. The positions of the adaptive optical element 160, the first scan mirror 180a, and the second scan mirror 180b are interchangeable relative to their respective positions shown in Figure 1A. For example, in other embodiments, the adaptive optical element 160 may be positioned between the first and second optical relay devices 170a, 170b, with the first scan mirror 180a positioned before the first optical relay device 170a. In yet other embodiments, for example as shown in Figure 1B, the adaptive optical element 160 may be positioned between the second and third optical relay device 170b, 170c, with the second scan mirror 180b placed before the second optical relay device 170b. These alternative arrangements may also be useful in that one of the first and second pairs of relay lenses may serve as beam expander for expanding (or reducing) the size of the laser beam so that the laser beam is efficiently translated across the scanning angles. In these alternative configurations, the optical field phase imparted by the adaptive optical element 160 is still retained at the rear pupil of the objective lens 190, by implementing any optical relay devices located between the adaptive optical element 160 and the objective lens 190 by pairs of lenses in the “4f” configuration. In the system 100B shown in Figure 1B, the optical elements are arranged such that the laser beam 140 propagates from the beam expander 150 towards the first scan mirror 180a. The first scan mirror 180a reflects the laser beam to the second scan mirror 180b via the first optical relay device 170a. The second scan mirror 180b reflects the laser beam to the adaptive optical element 160 via the second optical relay device 170b. Following reflection by the adaptive optical element 160, the laser beam is reflected at a beam splitter 210’ (in the form of a beamsplitter cube 210’) and then by the dichroic mirror 200 to the objective lens 190. In the system 100B of Figure 1B, the first, second and third optical relay devices 170a, 170b, 170c are arranged such that the first and second scan mirrors 180a, 180b, the adaptive optical element 160, and the rear pupil of the objective lens 190 are in optically conjugate planes. That is, the first optical relay device 170a is arranged to relay an image of the first scan mirror 180a onto the second scan mirror 180b. The second optical relay device 170b is arranged to relay an image of the second scan mirror 180b onto the adaptive optical element 160. The third optical relay device 170c is arranged to relay an image of the optical field at the active surface of the adaptive optical element 160 onto the rear pupil plane of the objective lens 190. Accordingly, adjustment of the orientation of the first and second scan mirrors 180a, 180b results in a change of the angle of the laser beam at the rear pupil of the objective lens 190, but not the position of the laser beam relative to the objective lens 190. As a result, the intensity of the laser beam at its focus in the sample 300 may remain constant as the beam is scanned across the sample 300. In addition, since the adaptive optical element 160 is conjugated to the rear pupil of the objective lens 190, the spatially varying phase / amplitude modulation pattern imposed on the laser beam by the adaptive optical element 160 remains stationary at the rear pupil or rear focal plane of the objective lens 190 while scanning the laser beam 140 relative to the sample 300. In Figure 1B, an optical field phase imparted by the adaptive optical element 160 may be replicated at the rear pupil of the objective lens 190, by implementing the third optical relay device 170c, located between the adaptive optical element 160 and the objective lens 190, by a pair of lenses in the “4f” configuration. This may be important in applications in which the adaptive optical element 160 is used to modify the phase, as, for example, when using an adaptive optical element 160 in the form of an SLM. However, in the arrangement shown in Figure 1B, the first and second optical relay devices 170a, 170b, both of which are located between the laser source 130 and the adaptive optical element 160, do not need to be in a “4f” configuration since no phase information needs to be passed before the adaptive optical element 160. Thus one or both of the optical relay devices 170a and 170b may be used as a beam expander, for example to ensure that the laser beam is large enough to fill the adaptive optical element 160, and the beam expander 150 may optionally be omitted. The arrangement shown in Figure 1B may be further modified such that the first optical relay element 170a is omitted and the first and second scan mirrors 180a, 180b are located adjacent to each other. As an example, the separation between the first and second scan mirrors 180a, 180b may be less than 20 mm. The depth of focus of the second optical relay element 170b may be just outside the midpoint between the two scan mirrors 180a, 180b. As another example, the first and second scan mirrors 180a, 180b may be replaced by a single mirror, optionally with 2-axis scanning. In yet further alternative embodiments (not shown), the first, second and third optical relay devices 170a, 170b, 170c may be implemented using further scan mirrors in the path of the laser beam instead of the lens pairs shown in Figure 1 A. When tilting one or both of the first and second scan mirrors 180a, 180b to change an angle of the laser beam incident at the objective lens 190, the further scan mirrors would be adjusted so that only the angle of the laser beam at the rear pupil plan of the objective lens 190 was changed, while keeping both the position of the laser beam and the image of the adaptive optical element 160 stationary at the objective lens 190. The system 100A, 100B further includes additional components for beam alignment diagnosis. Firstly, a beamsplitter 210, 210’, tube lens 212 and CCD camera 214 are arranged for diagnosing the alignment of the image of the adaptive optical element 160 at the rear pupil plane of the objective lens 190. In both embodiments of the system 100A, 100B, the beam splitter 210, 210’ is positioned between the two lenses 170c’, 170c” of the third optical relay device 170c so that some laser light reflected by the objective lens 190 is directed to the CCD camera 214 via the tube lens 212. The beam splitter 210, 210’ is chosen such that only a low percentage of light (e.g., up to 20%, optionally around 10%) is lost from the main path of the laser beam towards the objective lens 190, but sufficient reflected light from the objective 190 is directed to the camera 214. For example, a 10:90 R:T beamsplitter may be used for beamsplitter 210 in the system 100A of Figure 1A. For the beam splitter 210’ in the system 100B of Figure 1B, which diverts the path of the main laser beam 140, a 90:10 R:T beamsplitter may be used. The tube lens 212, together with lens 170c” of the third optical relay device 170, forms an additional ‘4f’ system for collecting the light sampled by the beam splitter 210 and forming an image of the rear pupil plane of the objective lens 190 at the CCD camera 214. It is thereby possible to obtain information as to the quality of the alignment of the laser beam 140 at the rear pupil plane of the objective lens 190, in particular to assess the alignment of the image of the adaptive optical element 160 at the rear pupil plane of the objective lens 190. Secondly, a lens 220 and detector 222 are arranged to monitor laser power incident on the dichroic mirror 200. The dichroic mirror 200 transmits a small amount of the laser beam 140, which is then focussed by the lens 220 onto the detector 220 as shown in Figures 1A and 1B. This is sensitive to laser position and yields two advantages: the positional drift of the laser can be monitored, and the overall power efficiency of the system can be recorded by correlating the power measurement at this point with respect to the power output at the laser head. Any change in position and / or power will indicate a potential misalignment in the system. With further reference to Figures 1A and 1B, the laser system 100A, 100B further includes an imaging system comprising a first imaging light source 250, a second imaging light source 260, a pair of lenses 262a, 262b, a beam splitter 264, 264’, an imaging lens 270 (e.g., a tube lens), and a camera 280. The first imaging light source 250, in the form of an apertured LED located below the sample 300, provides illumination of the sample 300 for transmission imaging and may have a different wavelength from the laser light output by the laser light source 130. The dichroic mirror 200 is arranged to reflect substantially all the light of the laser beam 140 towards the microscope objective lens 190, but to transmit imaging light from the second imaging light source 250 so that it passes via imaging lens 270 to the camera 280. The second imaging light source 260 provides illumination of the sample for reflection imaging of the sample. Light from the second imaging light source 260 passes through the pair of lenses 262a, 262b and is then delivered to the sample via the beam splitter 264, 264’, the imaging lens 170, the dichroic mirror 200 and the objective lens 190. The first lens 262a of the lens pair tends to collimate the divergent light from the second imaging light source 260, and the second lens 262b then transforms the beam such that the imaging lens 270 produces an approximately collimated beam at the objective lens 190 that overfills the pupil. The beam splitter 264, 264’ may be a 50:50 beam splitter such that 50% of the light from the second imaging light source 260 is transmitted to the sample, which would therefore imply 50% of the reflected light from the sample is incident upon the imaging camera 280. Under transmission illumination from the first imaging light source 250, a darkfield image is produced at the camera 280 and due to scattering events within the material any imperfection or marks (either natural or made through this process) will therefore appear bright in the image. Under reflection illumination from the second imaging light source 260, information about the surface 303 of the sample 300 can be obtained in a brightfield image, where surface marks typically appear dark in the image. The imaging system can also be used to view the laser beam on the surface 302 of the sample 300. Typical dichroic mirrors will achieve a reflectance percentage greater than 90% and lower than 100% with the remainder as transmission. Therefore, up to 10% of the laser light reflected from the sample surface 302 and incident upon the dichroic mirror 200 may be incident upon the camera sensor 220. In this way, the laser beam at the sample surface may be imaged using the camera 220. This image of the laser spot on the sample surface 302 is only in focus when the sample surface is at the correct height. The system 100A, 100B also comprises a control system in the form of a computer processing system 290 for controlling the adaptive optical element 160, the orientation of the scan mirrors 180a, 180b, the motion system 120, the laser light source 130 and the camera 280. In some embodiments, the system may comprise multiple control systems for controlling various parts of the system. For example, a first computer system may be used to control the motion system 120, while a second computer system may be used to control the scan mirrors 180a, 180b, the imaging system and optionally the adaptive optical element 160. In some embodiments, a further computer system may be used for communication with the adaptive optical element 160. Figure 2 schematically illustrates the effect of adjusting an orientation of the scan mirrors 180a, 180b to adjust an angle of the laser beam 140 at the rear pupil of the objective lens 190. For simplicity, Figure 2 only shows the portion of the system 100A (see Figure 1A) including the laser beam path from immediately before the second scan mirror 180b to the sample 300. The laser beam 140 is reflected by the second scan mirror 180b and directed, via the third optical relay element 170c and the beam splitter 200, to the objective lens 190. When the second scan mirror 180b is adjusted to a first orientation, as indicated by the solid lines in Figure 2, the laser beam 140 is directed on-axis through the objective lens 190 and focused by the objective 190 lens to a central location 303 on the surface 302 of the sample 300. By adjusting the second scan mirror 180b to second orientation, as indicated by the dotted lines in Figure 2, the laser beam 140’ is incident at the same position at the back aperture of the objective lens 190 but at an angle to the axis of the objective lens 190. The laser beam 140’ is therefore focussed to a different location 304 on the surface of the sample 300, transversely displaced from the central location 303. Figure 2 shows that both the position of the laser beam 140, 140’ and the intensity / phase pattern imposed by the adaptive optical element 160 remain stationary at the rear aperture of the objective lens 190, when adjusting an orientation (or scan angle) of the scan mirrors to vary the position of the focus of the laser beam relative to the sample 300. As a result, the intensity of the laser beam at its focus may remain constant at each location 303, 304 on the sample. Although Figure 2 is based on the arrangement of scan mirrors 180a, 180b and adaptive optical element 160 used in the system 100A of Figure 1A, similar considerations apply to the arrangement used in the system 100B of Figure 1B. That is, the position of the laser beam on the adaptive optical element 160 remains stationary (like at the back of the objective lens 190 in Figure 2). In turn, the light beam from the adaptive optical element 160 remains in the same position at the objective lens 190 for all scan angles. The configuration of the adaptive optical element 160 may be adjusted to correct for aberrations at the target location and to adjust the axial position of the focus of the laser beam. The settings of the adaptive optical element 160 are adjusted, for example under control of the processing system 290, to apply the required correction by spatially modulating the phase and / or amplitude of the laser beam 140. By correcting the focal spot morphology and axial position in this way, a suitable energy density (laser fluence) for laser marking may be achieved for target locations across the entire field of view of the objective lens 190, despite the presence of aberrations and residual errors or non-repeatability in the positioning and orientation of samples in the sample receiving area 110. The laser power may also be adjusted to further modify the laser fluence. Aberrations may include those arising from the optical system, spherical aberrations due to focussing within the sample when creating sub-surface marks, and coma aberrations due to a local tilt in the surface 302 of the sample through which the beam passes when focussing below the surface. The required axial position of the focus of the laser beam will depend on the axial position of the surface 302 of the sample 300 relative to the objective lens 190 and the desired depth of marking in the sample. The required corrections to be applied by the adaptive optical element 160 may therefore depend on the characteristics of the optical system 100A, 100B, in particular the objective lens 190, the position and orientation of the sample 300 relative to the optical system 100A, 100B, and the depth of the desired target location relative to the surface 302 of the sample 300. The corrections applied to the adaptive optical element 160 may include a combination of the following: (a) correcting for the aberrations due to the imperfections of the objective lens 190; (b) correcting for spherical aberration due to focusing within the sample 300; (c) correcting for coma aberrations due to a residual tilt of the surface 302 of the sample 300 relative to the laser beam (mainly for subsurface marking); (d) correcting the depth of focus (i.e., the axial position of the focus) of the laser beam to adjust for a residual error in the axial location of the sample from the expected marking position; (e) correcting for a residual tilt in sample relative to the optical system by varying the depth of focus when scanning across the field of view (in particular, for surface marking); (f) correcting for edge effects when focusing near the edge of a sample, e.g., a small sample such as a melee diamond. In addition, the laser power may be adjusted to correct for loss of energy density based on, for example, the determined position or tilt of the sample, in particular taking account of edge effects. The adjustment applied to the laser power may take into account the effect of any corrections applied to the adaptive optical element 160. The required correction may be calculated based on a characterisation of the optical system 100A, 100B (in particular the objective lens 190), the determined position and tilt of the surface of the sample 300, the desired depth of the laser mark within the sample, and / or the position of the target location within the field of view of the objective lens. Accordingly, the processing system 290 may pre-calculate the amplitude and / or phase profile to be applied to the optical field of the laser beam by the adaptive optical element 160 in advance of modifying the sample 300 using the laser system 100A, 100B. The correction may be defined by an analytic solution e.g., for a particular geometry, or a numerical solution for a particular focussing geometry. The correction may be determined through an iterative optimisation using a method of focal feedback. For example, if the surface of the sample is normal to the optical axis, the spherical aberration may be described by the following expression: ^sa(p) ^^nom A (NA p)2 — n2 (NA p)2 This equation is an analytic description of the spherical aberration phase ipsA for light of wavelength A when focussing to a depth dnOm inside a material of refractive index n2 using an objective lens with numerical aperture NA and immersion medium of refractive index rp. The coordinate p is the normalised radius in the pupil of the objective lens. The aberration includes a refocusing effect (a focal shift along to optical axis) in addition to spherical aberration. The amplitude of these effects is proportional to the focussing depth. If the surface normal is tilted with respect to the optical axis, then other aberrations such as coma (which includes wavefront tilt, causing a lateral focal shift) are introduced. These effects are proportional to both the focussing depth and the angle of surface tilt. The additional aberration due to a small surface tilt is given by: t d„nm ^tut(p, G) = —5+ * p3) cos(0 - e) This equation is an analytic description of the aberration component due to surface tilt, where the tilt is at a small angle t. 6 is the azimuthal coordinate in the pupil and £ represents the orientation of the tilt. Factors a and b are scalar coefficients whose values depend upon the refractive indices and NA. Aberration correction may be implemented using an adaptive optical element such as a liquid crystal spatial light modulator (SLM) or a deformable mirror. A phase pattern calculated from the above equations can be imparted upon the adaptive optical element in order to correct for the aberration induced by the sample or objective lens. As SLMs typically have a phase modulation range limited to one wavelength (or a small number of wavelengths) the phase is usually wrapped so that it lies within the accessible range. For example, if only a single wavelength is accessible, then the phase function applied will be 14JSA modulo 2tt, as 277 radians of phase corresponds to one wavelength. Instead of using the analytic expressions directly, the aberrations can be considered as a series of basis functions. Commonly, the Zernike polynomials are used for this purpose. Hence, an aberration may be described a sum of aberration modes. For example, spherical aberration may be expressed as an expansion in terms of Zernike polynomials. Using functions such as these aids in the design of feedback systems for the measurement and correction of unknown aberrations. Accordingly, the correction may be defined by an expansion of orthogonal modes. Each mode may define an aberration mode. The correction may be a pure mode, or the correction may be a superposition of modes. The correction may be defined by a Zernike polynomial, or may be defined by an expansion of Zernike polynomials. The correction may be defined as a superposition of modes using an orthogonal basis other than the Zernike polynomials. A fixed correction to the focal spot morphology and / or the laser power may be applied for marking a particular sample. Alternatively, the correction may be varied with the position of the mark being made within the field of view of the optical system and / or the location of the mark within the sample. The required pulse energy of the laser light source 130 for marking at the target location may also be determined based on the correction. The laser light source 130 is then set to the required pulse energy for modifying the sample and the laser marking is carried out. Following modification of the sample, transmission microscopy may be used to measure the modified region of the sample. In some embodiments, the determined correction may be refined based on the feedback obtained by this further measurement. Figure 3 schematically illustrates correction of aberrations due to the imperfections of the objective lens 190. For example, a field curvature of the objective lens 190 may result in a curved focal plane 192 as illustrated in Figure 3(a). That is, the axial position or depth of focus of the laser beam may depend on the angle of the laser beam entering the objective lens 190 (or the angle of the laser beam at the sample 300), as determined by the orientation of the scan mirrors 180a, 180b. As an example, in Figure 3(a), the depth of focus d of the laser beam 140 propagating on axis to the sample is greater than the depth of focus d’ of the laser beam 140’ propagating at an angle to the axis. By adjusting the settings of the adaptive optical element 160 dependent on the angle of the laser beam, this aberration may be corrected to effectively achieve a flat focal plane 194, for example to maintain the focus of the laser beam 140, 140’ on the flat surface 302 of the sample 300 as shown in Figure 3(b). This type of correction may allow access to more of the field of view of the objective lens 190. Figure 4 schematically illustrates correcting for a residual tilt in a sample 300 relative to the optical system 100A, 100B by varying the depth or axial position of the focus when scanning across the field of view. Figure 4(a) illustrates laser marking on a surface of the sample 300, when surface 302 of the sample 300 is oriented with a residual tilt with respect to the objective lens 190. The scan mirrors 180a, 180b are adjusted to move the laser beam 140, 140’ transversely across the sample 300 to mark the desired pattern on the surface. At the same time, the adaptive optical element 160 is used to vary the axial position of the focus dependent on the transverse position or scan angle of the beam so that the focus of the laser beam always coincides with the surface of the sample 300. That is, for each angle of the scan mirrors, the correction applied to the adaptive optical element 160 is adjusted to correct the axial position of the focus so that it coincides with the surface of the sample 300. When surface marking the sample 300 shown in Figure 3(a), the laser beam focus must be scanned from a target location 305 to a target location 306 on the surface 302 of the sample 300. Due to the tilt, the target location 305 is further away from the objective lens 190 than the target location 306. Accordingly, the adaptive optical element 160 is adjusted to vary the axial position of the focus as a function of beam angle so that the focus of the laser beam tracks a plane 196 coincident with the surface 302 of the sample 300. In order to accurately track the surface of the sample 300 for surface marking, an accuracy of the order of 1 micrometres, preferably less than 0.2 micrometres, is required in the axial position of the laser focus. Figure 4(b) illustrates sub-surface laser marking in a sample 300, when the sample 300 is oriented with a residual tilt with respect to the objective lens 190. In this case, the settings of the adaptive optical element 160 are used to adjust the axial position of the focus of the laser beam in dependence on the transverse position of the laser beam in order to maintain the axial position of the focus at a fixed depth within the sample 300. Accordingly, the adaptive optical element 160 is adjusted to vary the axial position of the focus as a function of beam angle so that the focus of the laser beam tracks a plane 198 at a fixed distance below the surface 302 of the sample 300. The corrections applied to the adaptive optical element 160 for subsurface marking as shown in Figure 3(b) additionally correct for spherical aberrations and coma which arise due to focussing below the surface of the sample 300. Correcting for edge effects when focusing near the edge of a sample may also be important when marking small samples such as melee diamonds. The combination of large beam diameter and high numerical aperture of the objective lens 190 may result in parts of the laser beam being incident on the sample 300 at a large angle, for example approaching 90 degrees. This can result in edge effects when marking close to the edge of a sample 300, as some parts of the beam may pass through different facets of a sample 300. The required correction may be calculated based on the topography of the sample 300. For example, a correction may include operating the laser at a higher power. The adaptive optical element 160 may be used to spatially modulate the phase and / or amplitude of the laser beam 140 to take account of different paths through the sample close to its edge. The axial position and inclination or tilt of the surface 302 of the sample 300 may be determined using the system 100A, 100B of Figure 1A or 1B. The tilt of the surface 302 may be determined relative to a transverse plane perpendicular to the primary propagation direction of the laser. Using the laser with low pulse energy (for example, for diamond, significantly below the bulk graphitisation threshold, e.g., less than 30nJ), the axial position of the sample 300 may be measured by imaging the laser beam at the surface 302 of the sample 300. If the sample is mounted on a 3D motion stage, the sample surface may be found by translating the sample axially, for example in 100 nm steps until the surface coincides with the focus of the laser beam. Alternatively, the adaptive optical element 160 may be used to apply corrections to the phase / amplitude profile of the beam to axially shift the location of the focus of the laser beam so that it coincides with the surface 302 of the sample 300. When the laser beam is focussed on the surface 302, the size of the laser spot on the surface 302 is minimised. The position of the surface 302 relative to a reference plane (for example, an ideal focal plane of the objective lens 190) may therefore be determined from the corrections applied to the adaptive optical element 160 to focus the laser beam on the surface 302 of the sample 300. To determine the tilt of the surface of the sample 300, the measurement of the axial location of the surface is repeated at one or more further locations (for example locations 305, 306 in Figure 4(a)), preferably at two further locations to measure the tilt with respect to both transverse directions. If the sample 300 is mounted on a 3D motion stage, the sample surface 302 may be found by translating the sample in each transverse directions, for example by 0.2 mm in the x-direction and then by 0.2 mm in the y-direction. Alternatively, the scan mirrors 180a, 180b may be used to scan the laser beam across the sample 300 to each of the further locations in turn. At each of the further locations, the adaptive optical element 160 may be used to apply corrections to the phase / amplitude profile of the beam to axially shift the location of the focus of the laser beam so that it coincides with the surface of the sample. These measurements cover an area of the surface 302 of the sample 300 and can be used to determine the local surface tilt. The separation of the measurement points 305, 306 on the surface 302 may be large enough to make an accurate measurement of the tilt, given the range of tilts which are expected to be important. For example, for diamond, a tilt in the range greater than 0.5 degrees may have significant effects on the quality of a focus within the sample. The tilt of the surface of the sample 300 is used to determine the expected coma aberration which will be caused thereby on the laser focus at the target location. The tilt may be corrected for by mechanically removing the tilt angle by adjusting the angle of the sample 300. Alternatively, the expected coma aberration at the target location may be characterised based in terms of a Zernike mode and communicated to the adaptive optical element 160 which is modified to display the required phase and / or amplitude correction to modify the laser beam to counteract the aberration. Figure 5 illustrates a method 400 for creating a mark or a pattern of marks in a sample according to an embodiment of the present disclosure. In a first step 402, the sample 300 is picked up by or loaded into a delivery system, for example a robot, a motion arm or other motion system 120. In a subsequent step 404, the sample 300 is moved by the delivery system 120 so that it is positioned in the sample receiving area 110 under the objective lens 190. In some embodiments, the delivery system 120 may be a low-accuracy or low-resolution motion system. In the absence of a high-accuracy or high-resolution motion system, corrections may be applied to the adaptive optical element 160 to compensate for any residual tilt of the sample 300 or any error in the position (e.g., depth or axial position) of the sample 300 relative to the objective lens 190. In a subsequent step 406, the position and tilt of the sample surface 302 is determined, by adjusting the angle of the or each scan mirror to steer the laser beam to three different positions on the sample surface 302, and, at each position of the laser beam on the sample surface 302, determining the distance of the sample surface 302 from the objective lens 190. The distance of the sample surface from the objective lens may be determined relative to a predetermined or optimum distance, for example an ideal focus of the objective lens 190. To determine the distance, corrections are applied to the adaptive optical element 160 while imaging the changes in the laser spot on the surface 302 using the camera 280 of the imaging system. The correction required to focus the laser beam on the surface 302 yields the distance. Repeating this for each of the three points enables the local tilt of the sample surface 302 to be determined. Further measurements (i.e., at more than three points) may allow more precise determination of the local tilt or curvature of the sample surface 302. In a subsequent step 408, a correction to be applied to the adaptive optical element 160 is determined, taking into account the determined local position and tilt of the sample surface 302, and the desired location of the mark in the sample 300. The parameters applied to the adaptive optical element 160 also correct for aberrations at the desired marking position. For small marks or patterns, it may be sufficient to apply a single correction to the adaptive optical element for marking the entire mark or pattern. The correction may correspond to an average correction determined for the volume of the sample 300 in which the pattern is located. In a subsequent step 410, the determined correction is applied to the adaptive optical element 160 and laser modification of the sample 302 is carried out to create the desired mark in the sample 300. The laser modification of the sample 300 may be at the surface 302 of the sample for surface marking or patterning of the sample, or below the surface for sub-surface marking or patterning of the sample 300. The scan mirrors 180a, 180b are used to move the laser beam focus across the sample 300 from one target location to the next to create all the marks required. If the sample is tilted, the correction should be adjusted as a function of scan angle to maintain a constant marking depth, or to track the surface, as required. In an optional step 412, following laser modification of the sample 300, the mark may be imaged using the camera 280 of the imaging system as described above. Figure 6 illustrates a method 500 for creating a larger mark or pattern of marks in a sample, for example marks or patterns extending across a significant part of the field of view of the objective lens 190. Figure 7 illustrates an example pattern 600 to which the method of Figure 6 may be applied. The entire pattern 600 is located within the area of the field of view 700 of the objective lens 190. Steps 502, 504 and 506 of the method 500 correspond to steps 402 404 and 406 of the method 400 described above. In a step 508, instead of determining a single correction to be applied to the adaptive optical element 160 for marking the entire pattern 600, the desired mark or pattern of marks 600 is divided into multiple regions or “tiles”, 602, 604, 606. The desired mark or pattern 600 may comprise a plurality of laser marking spots or lines, e.g., the line 602. “Tiles” 602, 604, 606 are different regions within the same field of view 700, which are addressable by adjusting the angle of the or each scan mirror 180a, 180b. The tiles 602, 604, 606 indicated in Figure 7 correspond to regions of the pattern 600 located at different respective radii from the centre of the field of view 700. For clarity of Figure 7, only three of the marks of each tile 602, 604, 606 are indicated. In other embodiments, the “tiles” may be square regions, but yet other configurations of tiles may be used, depending on the characteristics of the optical system 100A, 100B, the sample 300, and / or the pattern 600. A respective set of corrections to be applied to the adaptive optical element 160 is determined for each tile 602, 604,606, to correct for the tilt and / or curvature of the sample surface for the respective tile 602, 604, 606, the average distance of the sample surface 302 from the objective lens for the respective tile 602, 604, 606, the depth of the desired marking location for the respective tile 602, 604, 606 within the sample, and for aberrations of the objective lens. For very small samples, the corrections applied to the adaptive optical element 160 may also take into account the distance of the marking positions in the “tile” 602, 604, 606 from the physical edges of the sample 300. A suitable laser power may also be determined for each tile 602, 604, 606. In a subsequent step 510, for each tile 602, 604, 606, the respective set of corrections is applied to the adaptive optical element 160 and laser modification of the sample 300 is carried out to create the desired pattern of marks 600 in the respective tile 602, 604, 606. As in step 410 of the method 400, the scan mirrors 180a, 180b are used to move the laser beam focus across the sample 300 from one target location to the next to create all the marks required. If the sample is tilted, the correction should be adjusted as a function of scan angle to maintain a constant marking depth, or to track the surface, as required. In a subsequent optional step 512, following laser modification of the sample 300, the pattern 600 of marks may be imaged using the camera 280 of the imaging system as described above. Figure 8 illustrates a method 700 for creating larger marks or patterns of marks in a sample, for example marks or patterns extending across a significant part of the field of view of the objective lens. Figure 9 illustrates an example pattern 800 to which the method 700 of Figure 8 may be applied. The pattern 900 covers an area greater than a single field of view 902, 904 of the objective lens 190. To mark the pattern 900 on a sample 300, it is necessary to move the sample 300 relative to the objective lens 190. Steps 702, 704 and 706 of the method 700 correspond to steps 402 404 and 406 of the method 400 described above. In a step 708, the desired mark or pattern is divided into multiple field of view (FOV) regions 902, 904, each having an area corresponding to that of the field of view of the objective lens 190, and further divided into multiple “tiles” within each FOV region. To create marks in different fields of view, it is necessary to move the sample 300 relative to the objective lens 190, for example using the motion system 120, so that the current field of view of the objective lens corresponds to the respective FOV region of the pattern. To move the sample 300 relative to the objective lens 190, the objective lens may be scanned by an objective scanning device, for example an objective stepper actuator or a piezo objective focus scanner, instead of or in addition to moving the sample using the motion system 120. For each tile, a respective set of corrections to be applied to the adaptive optical element 160 is determined as described above, but based on the location of the tile within its respective FOV region 902, 904. For example, the correction to be applied for making the marks located within a single tile may be based on aberrations that would be experienced at the (average) position of that tile within its respective field of view region, i.e. its location within the field of view of the objective lens at the time of marking. In some implementations, tiles at corresponding locations within different FOV regions may use the same set of corrections. In a step 710, for each tile within the current field of view, the respective set of corrections is applied to the adaptive optical element 160 and laser modification of the sample 300 is carried out to create the desired mark in the respective tile. Once all the tiles addressable in a single field of view have been marked, the sample is moved so that the actual field of view of the objective lens 190 corresponds to a different FOV region 902, 904 of the pattern 800. For each tile within the new field of view, the respective set of corrections is applied to the adaptive optical element 160 and laser modification of the sample 300 is carried out to create the desired mark in the respective tile. This is repeated until all the tiles of the pattern 800 in all the FOV regions 902, 904 have been marked. In an optional step 712, following laser modification of the sample 300, the pattern 800 as marked on the sample 300 may be imaged using the camera 280 of the imaging system as described above. In this case, a composite image may be obtained by moving the sample to different locations corresponding to each FOV region used for marking the sample, taking an image at each location, and combining the images. The skilled person will appreciate that the system and method disclosed herein may be applied to laser modification and / or laser marking of various materials, not limited to gemstones, or indeed to diamond. Although particular example embodiments of the disclosure have been described above, it will be appreciated than many modifications, including additions and / or substitutions, may be made within the scope of the appended claims.

Claims

1. A laser system for modifying a sample to form a modified region at a target location in the sample, the laser system comprising:a sample receiving area for receiving a sample;a laser light source configured to provide a laser beam; anda plurality of optical elements configured to direct the laser beam from the laser light source into the sample when the sample is received in the sample receiving area, the plurality of optical elements comprising:an objective lens for focussing the laser beam at the target location;one or more scan mirror, wherein an orientation of the or each scan mirror is adjustable for adjusting an angle of the laser beam at a rear pupil plane of the objective lens to adjust a location of a focus of the laser beam in the sample; andan adaptive optical element, adjustably configurable to correct for aberrations at the target location;wherein the adaptive optical element is optically conjugate to the rear pupil plane of the objective lens;wherein the or each scan mirror is in a respective optical plane which is optically conjugate with the adaptive optical element.

2. A system according to claim 1, wherein the or each scan mirror comprises a galvanometric scanning mirror.

3. A system according to claim 1 or 2, wherein said one or more scan mirror comprises two galvanometric scanning mirrors.

4. A system according to any one of the preceding claims, wherein the plurality of optical elements further comprises a plurality of optical relay elements arranged such that the adaptive optical element and the or each scan mirror is each optically conjugate with the rear pupil plane of the objective lens, wherein the plurality of optical relay elements are configured to reproduce, at the rear pupil plane of the objective lens, both the phase and intensity of the optical field of the laser beam at the adaptive optical element.

5. A system according to claim 4, wherein at least one optical relay element of the plurality of optical relay elements comprises a pair of lenses arranged in a 4f configuration.

6. A system according to any one of the preceding claims, wherein said aberrations include at least one of:aberrations due to components of the optical system,spherical aberration caused by a mismatch in refractive index at the surface of the sample through which the laser light enters the sample, andcoma due to a tilt of the sample relative to a plane orthogonal to the optical axis.

7. A system according to any one of the preceding claims, wherein the adaptive optical element is further adjustable to vary an axial position of the focus of the laser beam in the sample.

8. A system according to any one of the preceding claims, the system further comprising a control system configured to control the laser light source, the adaptive optical element and the or each scan mirror, the system being configured to:form a first modified region at a first target location in the sample;adjust an orientation of said or each scan mirror to adjust a location of the focus of the laser beam in the sample from the first target location to a second target location in the sample; andform a second modified region at the second target location.

9. A system according to claim 8, wherein:during formation of the first modified region, the adaptive optical element is configured to apply a first correction for correcting for aberrations at the first target location, andduring formation of the second modified region, the adaptive optical element is configured to apply a second correction, different from the first correction, for correcting for aberrations at the second target location.

10. A system according to claim 9, whereinthe first target location is located within a first region of a first field of view of the objective lens and the first correction is an average correction for correcting aberrations at locations within the first region; andthe second target location is located within a second region of the first field of view of the objective lens and the second correction is an average correction for correcting aberrations at locations within the second region.

11. A system according to claim 10, wherein the system is further configured to:form a plurality of first modified regions in the sample within the first region of the first field of view, using the first correction; andform a plurality of second modified regions in the sample within the second region of the first field of view, using the second correction.

12. A system according to any one of claims 9 to 11, wherein the system further comprises a motion device for moving the sample and / or the objective lens relative to one another, wherein the system is further configured to:move the sample and / or the objective lens relative to one another; andform a third modified region at a third target location in the sample, wherein during formation of the third modified region, the adaptive optical element is configured to apply a third correction for correcting for aberrations at the third target location.

13. A system according to claim 12, wherein the third target location is located outside the first field of view of the objective lens.

14. A system according to any one of claims 9 to 13, wherein the control system is configured to:obtain a pattern of modifications to be formed in the sample;divide the pattern of modifications into a plurality of regions; anddetermine, for each region of the plurality of regions, a respective correction to be applied to the adaptive optical element for correcting aberrations when forming parts of the pattern comprised within said region.

15. A system according to claim 14, wherein the control system is further configured to, for each region of the plurality of regions:configure the adaptive optical element to apply the respective correction to the laser beam; andwhile the respective correction is applied to the laser beam, modify the sample to form the modifications corresponding to parts of the pattern comprised within said region.

16. A system according to any one of the preceding claims, wherein the control system is further configured to:adjust an axial position of the focus of the laser beam until the focus is at a first location on the surface of the sample to thereby determine an axial distance of the sample relative to the objective lens.

17. A system according to claim 16, wherein the control system is further configured to: adjust an orientation of the or each scan mirror to adjust a transverse position of the laser beam to a second location at the surface of the sample; andadjust an axial position of the focus of the laser beam until the focus is at the second location on the surface of the sample to thereby determine a tilt of the surface of the sample relative to a focal plane of the objective lens.

18. A system according to any one of the preceding claims, configured to:adjust a respective orientation of the or each scan mirror to form a plurality of modified regions at a plurality of different respective target locations in the sample; andat each target location, adjust a configuration of the adaptive optical element, prior to producing the respective modified region, to vary the axial position of the focus of the laser beam dependent on an orientation of the or each scan mirror and a determined tilt of the sample.

19. A system according to any one of the preceding claims, further comprising an imaging system for imaging the laser beam at the surface of the sample.

20. A system according to any one of the preceding claims, further comprising a delivery system configured to deliver a sample to the sample receiving area.

21. A method for modifying a sample to form a modified region at a target location in the sample using a laser system according to any one of claims 1 to 20, the method comprising:forming a first modified region at a first target location in the sample;adjusting an orientation of the or each scan mirror to adjust a location of the focus of the laser beam in the sample from the first target location to a second target location in the sample; andforming a second modified region at the second target location.

22. A system or method according to any one of the preceding claims, wherein the sample is at least one of: a diamond, a gemstone, a melee diamond, a melee gemstone, a coloured gemstone, a diamond plate, a rough diamond, SiC, or hBN.

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