WO2013153371A1

WO2013153371A1 - Laser focusing method and apparatus with control system for correction of the optical aberration

Info

Publication number: WO2013153371A1
Application number: PCT/GB2013/050908
Authority: WO
Inventors: Patrick SALTER; Martin Booth
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2012-04-13
Filing date: 2013-04-09
Publication date: 2013-10-17
Anticipated expiration: 2014-10-13
Also published as: GB201206542D0; GB2501117A

Description

LASER FOCUSING METHOD AND APPARATUS WITH CONTROL SYSTEM FOR CORRECTION OF THE OPTICAL ABERRATION

This invention relates to laser focusing techniques. Laser focusing is for example used in laser fabrication systems and also in analytical apparatus such as laser microscopes.

Ultrafast laser material processing permits three dimensional fabrication inside transparent substrates. The non-linearity of any absorption coupled with the ultrashort nature of the pulse allows the generation of embedded features confined to the focal volume, without any damage to surrounding regions or the surface. The technique is of increasing interest in the fabrication of a range of devices, such as artificial bandgap materials, microfluidic devices, metallic nanostructures and photonic waveguide circuits.

The fidelity of fabrication depends strongly on the quality of the focal spot. In many cases, the quality of the focus is impaired by aberrations. A common problem is a mismatch between the refractive indices of the processed material and the objective immersion medium, generating a spherical aberration to the focal spot. It has been shown that it is possible to compensate such aberrations and restore diffraction limited performance using adaptive optical elements, both in microscopy and more recently in laser microfabrication.

There are further unique challenges presented for aberration compensation in laser microfabrication. Taking the example of embedded waveguides in bulk glass, the substrate is often translated perpendicular to the optical axis to create the guiding structure. Near the edge of the substrate, the fabrication efficiency decreases and the effect eventually disappears since a portion of the focussed light passes through the side facet of the substrate.

Figure 1 shows Ray trace diagrams showing the refraction of rays focussed through the top and side surfaces of a substrate with differing refractive index to that of the lens immersion medium. The sequence of ray trace diagrams in Figure 1 illustrates the effect for a nominal point of focus moving nearer to the edge of the substrate. The diagrams provide a view of a plane containing the optical axis of the lens and perpendicular to the focal plane. It can be seen that the focal splitting between the different rays increases as the edge is approached. Refraction of rays from just a single surface causes focal distortion, but refraction from both the side and top surfaces leads to an additional focal splitting. Thus any fabrication close to the side surface is severely impaired. However, for the manufacture of a waveguide, the structure needs to be constructed all the way to the side surface in order to achieve efficient coupling of light in and out of the waveguide chip by external optical fibre.

Thus, currently the ends are polished back to a point where the fabricated waveguide structure is well formed. This step is not only wasteful, but also highly time consuming.

There is therefore a desire for a method that enables structures to be fabricated right to the edge of a transparent substrate.

Similar issues apply in the field of imaging, where edge effects also influence the imaging quality. There is therefore also a desire for a method that enables high quality imaging of a feature of a structure which is at an edge.

Laser fabrication and laser imaging systems each comprise "laser focusing" systems, and this invention applies generally to such systems.

The invention provides a system and method as defined in the independent claims.

According one aspect of the invention, there is provided a laser focusing system comprising:

a laser;

a spatial light modulator for controlling a phase pattern of the laser output; an optical system for providing the phase pattern controlled laser output to a sample in which a structure is to be fabricated;

a translation stage for controlling the position of the sample relative to the phase pattern controlled laser output; and

a control system for controlling the phase pattern applied by the spatial light modulator in dependence on the position of the sample relative to the phase pattern controlled laser output, thereby to provide correction for optical aberration resulting from a refractive index boundary at an edge of the sample, which edge has a lateral distance to the laser output which varies as the relative position of the sample is varied. This system enables focusing right up to a lateral edge of a sample - i.e. an edge which extends in a plane aligned with the optical axis of the incident laser fabrication beam. The edge need not be perfectly parallel to the optical axis, and any shape can be modelled. Thus, the sample does not have to have perpendicular sides and top surface as in the examples below.

By controlling the phase pattern, a desired focal intensity distribution is determined for light focused into the sample (which is a transparent medium) near the edge of the sample. The phase pattern can split the pupil function with a phase discontinuity, and this enables the aberrations introduced for rays focused both through the top and side facets of the sample to be compensated.

The laser focusing system can be a laser fabrication system. The system can for example be used within a femtosecond microfabrication system, for example for fused silica. This system allows controlled sub-surface fabrication of a transparent sample right to the edge of the substrate, which, for example, is of particular interest for the manufacture of waveguides and photonic crystals.

The spatial light modulator functions to change the propagation direction of light, and functions in a similar manner to a blazed grating, in that there is constructive interference of all light in a particular direction from the surface normal, which has a transverse component in the same direction as the periodicity.

The control system can further provide correction for optical aberration resulting from a refractive index boundary at a top surface of the sample perpendicular to the laser illumination direction. This type of aberration correction is already known.

The translation stage preferably provides a position feedback signal to the control system to provide the information concerning the position of the sample relative to the phase pattern controlled laser output. The degree of overlap of the laser fabrication beam with the sample edge varies as the relative position is changed, and a feedback control ensures correct adaptation of the phase pattern.

The spatial light modulator can comprise a phase only reflective liquid crystal spatial light modulator. A high resolution deformable mirror device is a possible alternative, and an amplitude only LC spatial light modulator may also be used. A detection system can also be used for detecting plasma emission intensity, and derive a quality feedback control signal indicative of a fabrication quality. This quality feedback signal can also be provided to the control system. This provides a second feedback control approach, based on the quality of fabrication at the intended fabrication site within the sample.

As mentioned above, the system can be a waveguide writing system, and the structure is to be fabricated comprises a waveguide to be fabricated fully to the lateral edge of the sample. It can be used for other structures such as gratings and photonic crystals.

The invention also provides a laser focusing method comprising:

controlling a phase pattern of a laser output;

providing the phase pattern controlled laser output to a sample in which a structure is to be fabricated;

controlling the position of the sample relative to the phase pattern controlled laser output during fabrication; and

controlling the phase pattern applied by the spatial light modulator during fabrication in dependence on the position of the sample relative to the phase pattern controlled laser output, thereby to provide correction for optical aberration resulting from a refractive index boundary at an edge of the sample, which edge has a lateral distance to the laser output which varies as the relative position of the sample is varied.

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows ray diagrams showing how a laser fabrication signal overlaps a side edge of a sample when fabrication is near the sample edge, and shows how defocusing arises;

Figure 2 shows parameters used in a mathematical analysis;

Figure 3 shows example phase patterns;

Figure 4 shows an example of a fabrication system of the invention;

Figure 5 shows measurements of plasma emission from the focal volume for use as a feedback parameter; Figure 6 is a first diagram to show how the aberration correction of the invention improves the quality of fabricated structures;

Figure 7 is a second diagram to show how the aberration correction of the invention improves the quality of fabricated tracks up to an edge of a sample; and Figure 8 shows three common examples of sample edge to which the invention can be applied.

The invention provides a laser fabrication system in which a phase pattern of a laser fabrication signal is controlled to provide correction for optical aberration resulting from a refractive index boundary at a lateral edge of the sample. The lateral edge has distance to the laser signal which varies as the structure is fabricated, giving different degrees of overlap of the laser fabrication signal over the edge of the sample. This aberration correction enables focusing of a laser output right up to the edge of a sample.

The optical problem which arises at edges of samples will first be discussed, in particular the point spread caused by refractive optical aberration.

Figure 2 shows how light is focused to a point F at a depth d beneath the upper surface T of a substrate, and a distance g from the side facet∑. The line

CC divides the focusing cone into rays which pass through and are refracted by the top surface T and the side facet∑. The line OPF is the optical axis for the focusing lens, while the perpendicular line O'QF represents the optical axis for a virtual pupil to describe rays passing through the side facet.

Light is shown as focussed into a substrate of refractive index (Rl) n₂ from an immersion medium of Rl n-i . The substrate is bounded by two planes: T (the top surface) with normal parallel to the z axis and∑ (the side surface) normal to the x axis. The line of intersection of the two planes (the edge of the substrate) is parallel to the y axis.

The optic axis OPF is parallel to the z axis. F is the geometric, aberration free point of focus within the substrate, while P is the intersection of the optic axis with the top surface T. The line O'QF is along the x axis and the point Q designates the intersection of this line with the side surface ∑. The focussing depth PF is d, and the distance from the edge of the substrate QF is g. The intersection of the focussing cone with the top surface T is a circular segment bounded by the chord CC, while the intersection with the side surface∑ is a hyperbolic segment likewise terminated by the chord CC.

If the focussing objective lens has a numerical aperture NA = n-isina-i, then the radius of the circular segment on T is given by R = dtana-i. A general point A within this circular segment on T is described by coordinates (r_t, Q ) as shown. In the pupil of the focussing optic this corresponds to coordinates (p, Θ), where the pupil radius is normalised to one. Given that the ray passing through point A makes an angle φ with respect to the surface normal of T, and assuming the objective obeys the sine condition, the following transformation is valid: p= sincp/sina-i. Meanwhile the azimuthal angle must match: θ=θ₍. Simple geometry gives:

Hence, using the above relationships, it is straightforward to transform the chord CC into the pupil as:

it c — g (2)

These relationships provide a boundary within the pupil discriminating between rays passing through either the top or side surface of the substrate. In Figure 2(a), the rays passing through the top surface T are situated in the shaded part of the pupil inset, while those hitting the side surface∑ pass through the unshaded area of the pupil.

Having identified whether rays pass through the top or side surface, it is necessary to consider the respective phase aberrations encountered. Such aberrations will arise due to refraction at the substrate surfaces, caused by the mismatch in Rl between the sample and immersion media. For rays passing through the top surface, the spherical aberration is the same as for the corresponding ray focussing through an Rl mismatch without the edge. The appropriate phase function Ψ (ρ,θ) which should be applied to the pupil in order to cancel such an aberration is given by:

Ψ( , Θ) = ···-···· dNA i cosec¼2 ···· p² ···· cosec¼j ····· p² ) (4) where NA = nisinai = n₂sina₂ and λ is the wavelength of light in vacuum.

Now it is necessary to consider the aberrations induced by refraction for rays passing through the side of the substrate∑. Consider a ray which intercepts ∑ at point B and makes an angle ψ with the normal to∑. The same ray will pass the plane of the top surface T (although not through T itself) at some point A and make angle φ with the normal to T. It is convenient to introduce a virtual pupil, with optical axis O'QF and a numerical aperture

If rays passing through the surface∑ can be considered as originating from the virtual pupil, then the aberration is equivalent to the form presented in Equation 5 with the appropriate substitutions:

^■

(ρ Θ^' ) — ^ gNA casec² i— pⁿ - eosec½; - p^a) (5) Here NA = n-isina'i = n₂sin a'₂. In order to determine the appropriate phase required in the real pupil, it is necessary to relate α- ,ρ', θ' to the variables a-i, p, Θ, g, and d.

Consulting Figure 2(d), it can be seen that the length Cr, where Γ is the midpoint of CC, can be represented as:

while from Figure 2(c) Cf is also given by

such that:

From above it is known that

and similarly R' = gtanaV Inserting these two relations into Equation (6) allows us to determine the angle αΊ as:

The point B on ∑ has coordinates (r_s, 6_S), as indicated by Figure 2(d). Clearly θ= θ', while r_s and p' are connected by a similar expression to Equation (1 ). Noting that φ and Ψ are angles for a particular ray to the z and x axes respectively, we introduce the quantity X to represent the angle that ray forms with the y axis. A unit vector along FB then clearly has a y component cosX. Alternatively, by projecting onto the top surface T, the y component can equally be written as -sin(psin6_t. Thus, using direction cosines and remembering that θ= 6_t the following holds:

cos² - co " ψ ~ s n^ Φ hr 9 =^■ 1 (8)

Again assuming that the objective obeys the sine rule p= sincp/sina-i, and by analogy in the virtual pupil p'= sinM-Vsina-i, equation (8) gives a definition for p' as:

Hence the phase distribution required in the pupil of the objective to counter any aberrations introduced by the refraction of rays at the side facet of the substrate∑, can be ascertained by combining Equations (5), (7) and (10) to give:

It is interesting to note that Equation (1 1 ) is solely a function of pcos0= x and, hence, in this region of the pupil there are only variations in phase along the x axis. As a demonstration, Figure 3 shows typical pupil phase patterns for a 0.85 NA objective air lens ni = 1 focussing light of wavelength λ =790nm at a depth of 50pm in a fused silica substrate n₂ = 1 .45, for various distances from the edge (values of g).

Equation (3) gives the boundary CC in the pupil, while Equations (4) and (1 1 ) give the respective corrective phase distributions. There is necessarily a discontinuity in the pupil phase along the chord CC when separating the pupil into ray passing through T and∑. However, these can be easily accommodated by a liquid crystal phase-only spatial light modulator (SLM), which operates in the range

[0,2TT].

The phase maps of Figure 3 are for a focus at a distances (a) g=0 pm, (b) g=12 pm, (c) g=24 pm and (d) g=40 pm from the side facet∑. A background phase is added to the calculated phase distributions, which takes into account any system aberrations including the initial flatness correction for the SLM. It should be noted that often when an adaptive optics correction for spherical aberration is used, either in microscopy or microfabrication, it is common to remove the defocus element of Equation (4). In doing so, the focal distortion due to the Rl mismatch is removed, but the focal depth is not restored to the geometric focus. However, in this situation, it is important to retain the full form of Equation (4), such that rays intercepting T and∑ overlap at the geometric focus.

Figure 4 shows a schematic of an experimental layout. This is one example of a possible setup to demonstrate the practical benefits of the invention. Other experimental setups are of course possible, and the invention can be implemented in many different ways in a fabrication environment.

The experimental layout shown could be used for fabrication using all or a subset of the components (for example omitting optical analysis), so Figure 4 shows not only an experimental layout but also an example of the components of a laser fabrication system in accordance with the invention.

The pulses are emitted from a regeneratively amplified titanium sapphire laser 10 (Solstice, Newport/Spectra Physics, pulse length 150 fs, repetition rate 1 kHz, central wavelength 790 nm) and they are attenuated using a rotatable half- wave plate 12 and a Glan-Laser polariser 14. The expanded beam is directed onto a reflective liquid crystal phase-only SLM 16 (X10468-02, Hamamatsu Photonics). The output from the SLM is reflected by mirror 17 to a path directed to the sample. The SLM and the pupil plane of the objective 22 are imaged onto one another by a 4f system, composed of two achromatic doublet lenses 18. A dichroic reflector 19 reflects the fabrication laser light to the sample but allows other wavelengths to pass, such as LED light for sample analysis as well as emitted plasma as discussed below.

A 500pm diameter pinhole 20 on an adjustable mount is inserted into the Fourier plane of the SLM and initially positioned to transmit all light incident on the SLM. The beam fully illuminates the back aperture of a Leitz NPI 50x objective 22 with a 0.85 numerical aperture. The substrate is mounted on a three axis air- bearing translation stage 24.

For analysis purposes, an LED illuminated transmission brightfield microscope can be used to illuminate the specimen during fabrication. The LED is shown as 25. Plasma emission measurements can be used as part of a feedback mechanism. When an ultrafast laser is focused into a transparent material, a plasma is generated within the focal volume where there is structural modification. The plasma is generated by multiphoton absorption and avalanche effects and does not necessarily indicate the destruction of the surrounding material matrix.

Under strong focusing conditions, the plasma emission is mostly isotropic and unpolarized. Previously it has been shown that the plasma emission intensity is an appropriate feedback metric for performing aberration correction using adaptive optical elements during fabrication.

The plasma emission intensity is greatest when any aberrations are minimised producing the smallest fabricated features for a given input pulse energy.

Measurements of the focal plasma emission intensity can be used to modify the phase pattern displayed on the SLM and minimise aberrations generated when focusing near the edge of a transparent substrate.

A CCD 26 is used for this purpose.

The same CCD is used to analyse the plasma emissions and the LED light. The plasma is a supercontinuum (containing all wavelengths over a broad spectrum).

The samples employed in the test procedure were high purity fused silica

(Schott Lithosil Q0) with both the top and side facets polished to an optical quality. With a flat phase pattern assigned to the SLM, the top surface was located by noting the z position of the specimen which had the lowest threshold pulse energy for surface fabrication.

The sample was then translated to a nominal depth d_n0m and distance from the edge facet g_n0m- Using the values of g_n0m and d_n0m as a starting point, various phase patterns were applied to the SLM and the sample was irradiated with a continuous train of pulses with energy below that for void formation. To measure the plasma emission intensity, the LED illumination was switched off and the focal volume imaged onto the CCD. A typical image is shown in the inset of Figure 5(b) described below.

Figure 5 shows measurements of the net intensity of plasma emission from the focal volume when focussing at a nominal depth of 50pm in fused silica, at nominal distances of 5 m, 10 m and 15 m from the edge of the substrate. In the process giving the results shown in Figure 5 the sample was kept stationary and the SLM phase pattern altered while the plasma intensity was monitored.

In Figure 5(a), the degree of edge correction in region A of the SLM phase pattern is altered and the depth correction is constant. This corresponds to changing the value of g in Equations (3) and (1 1 ) which alters both the position of the chord CC and the form of the phase modulation in region A of the SLM as denoted in the inset of Figure 5(a). The form of the phase pattern in region B of the SLM depends solely on d, which remained constant as d_n0m- Six sets of measurements were taken at different points along the substrate edge by both increasing and decreasing applied values of g either side of g_n0m-

Figure 5(a) shows that there is a sharp peak in the plasma emission intensity, for values of g close to g_n0m- This point corresponds to rays passing through both the top surface and side facet focussing to a common point, and hence generating the maximum plasma at the focus. This should also lead to fabrication of the tightest features at the lowest pulse energies. High repeatability in the peak position and relatively low error across the measurements sets (~2%) indicate the reliability of the technique.

Decreasing g from the optimum value, it is obvious that the plasma emission should decrease, because the change in the chord CC renders a greater portion of the pupil subject to Equation (1 1 ). Thus, rays actually passing through the top surface of the substrate are corrected as though they were passing through the side surface. This will lead to a decrease in focal intensity. However, increasing g from the optimum value should have no effect on rays passing through the top surface. Hence, the sharp drop in plasma intensity for g>g_n0m demonstrates that Equation (1 1 ) is appropriate for the phase correction of rays passing through the side surface, and removes any focal splitting.

In Figure 5(b), the plasma emission intensity is monitored as a function of differing depth corrections with the sample stationary at a nominal depth of 50pm and 10pm from the side surface. This corresponds to changing the value of d in Equations (3) and (1 1 ), again altering the position of the chord CC, but now the form of the phase modulation in region B of the SLM. The measurements were made in a similar manner to those presented in Figure 5(a), only by altering d and keeping g fixed at g_n0m-

There is again a peak in the plasma emission intensity as the value of d varied through d_n0m- The peak is not as sharp as that found when varying g, possibly related to the natural focal elongation along the axial direction. However, this still demonstrates the necessity to have the correct values of both d and g in order to provide appropriate aberration correction, relating to maximum overlap of rays in the focal volume.

The plasma emission measurements exhibit the success of the edge correction technique. The same results are obtained by observing results of the improvement in fabrication.

Figure 6 shows images of voids fabricated at a depth of 50pm in fused silica at a distance of 10pm from the edge of the substrate.

Sets of 3 voids, separated laterally by 5pm were generated by a burst of 100 pulses with energies of 90, 1 10 and 130n J as indicated in Figure 6.

Voids were generated firstly with aberration correction for rays passing through both the top and side surfaces ("Edge corrected"), and subsequently with aberration correction purely for the Rl mismatch at the top surface neglecting the edge of the substrate ("No edge correction").

In the x-y plane (with the optic axis along z) the void fabrication is seems stronger for a given power with the edge correction applied. By viewing in the x-z plane, Figure 6(b), it is clear that when the edge correction is applied, there is greater axial confinement in the void features. With no edge correction applied, there is evidence of focal splitting due to rays passing through the side facet of the substrate.

One of the main areas of application for this technique promises to be the means for generating high quality direct laser written waveguides right up to the edge of a substrate, eliminating the current time-consuming requirement of a final polish. To investigate this possibility, tracks up to the edge of the fused silica sample used in the previous section for void formation have been written.

The translation speed was 0.5pm/s perpendicular to the edge at a depth of 50pm. Feedback from the stages was used to automatically update the amount of edge correction applied to phase patterns displayed on the SLM. The emission from the laser was a continuous pulse train (repetition rate 1 kHz) and the power kept constant. The results are shown in Figure 7(a). Tracks (i) and (iii) were written with automatically updated edge correction while the SLM phase pattern was kept constant during fabrication of track (ii). It can be seen that the edge correction has a beneficial effect in maintaining fabrication up to the side surface for (i) and (iii) in contrast to track (ii). However, it is clear that the degree of fabrication is reduced as the focus nears the side surface. Such an effect is indeed to be expected since reflection of incident light from the side facet will be stronger than that from the top surface as the rays are incident at greater angles to the surface normal. Henceforth, to fabricate a spatially invariant structure some variation in the writing power is also required.

The laser power is generally controlled in the current experimental layout using a combination of a half waveplate and polariser immediately following the laser output. However, for the purposes of automation it is more convenient to attenuate the writing beam using the SLM, since this is continually updating to provide the optimum edge correction. Therefore a blazed grating was overlaid onto the SLM phase pattern directing the proportion of light needed for fabrication into the first order. The zero order was blocked by adjusting the pinhole position in the Fourier plane of the SLM (see Figure 4). A modulation depth of the blazed grating equal to 2π provides maximum power in the fabrication beam, and subsequent variation in the modulation depth enabled simple power adjustment.

The power variation required was firstly ascertained by monitoring the plasma emission intensity as a function of distance from the edge of the substrate. The blazed grating modulation depth was then adjusted to maintain a constant plasma emission intensity at all points. This was done both when the SLM phase pattern was updated to give edge correction and for a static phase pattern on the SLM providing only depth correction.

The tracks written using the SLM to provide edge correction and power variation (Figure 7(b)) create uniform features right up to the edge of the substrate. This is not possible without the edge correction routine, as seen in Figure 7(c), as there is evidence of focal splitting near the substrate edge.

Thus, there are two possible feedback mechanisms which can be combined. One is feedback based on the position of the sample relative to the laser, so that the edge correction is adapted to take account of the boundary between the light cone part reaching the top surface and the light cone part reaching the side face. The second is based on plasma emission intensity and can be used to control the system to control the focal point quality. This can be implemented by controlling a blazed grating modulation depth

Waveguides made by the fabrication method of the invention can have a typical length of 50 to 200mm, so they generally have to be fabricated by translating along a direction oblique to the optic axis of the focussing lens. As mentioned above, the sample may not have perpendicular ends. For example, it may have chamfered or bevel edges.

Three examples of substrate edge are shown in Figure 8. Figure 8(a) shows a square edge as modelled above. Figure 8(b) shows a 45 degree chamfered edge and Figure 8(c) shows a curved fillet radius edge. Different phase patterns will be appropriate for the different edge types but the principles remain the same.

The invention avoids thermal effects previously encountered resulting from the use of increased power at the edge of a sample, as well as avoiding the need for edge polishing to remove significant parts of the sample edge (removing 200 pm at each end of a waveguide is typical in the prior art).

A manufacturing setup may not need the CCD or LED illumination explained above for the test environment.

As mentioned above, the invention is of interest for the fabrication of waveguides, but it can be applied to the fabrication of any structure right to the edge of a sample, including gratings and photonic crystal structures.

It will be clear from the above description that the invention is of particular interest for laser fabrication systems and methods. However, the invention has possible uses in laser microscopes, which are optically almost identical to the fabrication system described above. The structuring of the laser focus for a multiphoton microscope that needs to image near the edge of a block/interface can be controlled in the same way as described above.

Other laser focusing systems may also benefit from the approach of the invention, such as optical tweezers, optical data storage etc.

Various modifications will be apparent to those skilled in the art.

Claims

1 . A laser focusing system comprising:

a laser;

a control system for controlling the phase pattern applied by the spatial light modulator in dependence on the position of the sample relative to the phase pattern controlled laser output, thereby to provide correction for optical aberration resulting from a refractive index boundary at an edge of the sample, which edge has a lateral distance to the laser output which varies as the relative position of the sample is varied.

2. A system as claimed in claim 1 , wherein the control system further provides correction for optical aberration resulting from a refractive index boundary at a top surface of the sample perpendicular to the laser illumination direction.

3. A system as claimed in claim 1 or 2, wherein the translation stage provides a position feedback signal to the control system to provide the information concerning the position of the sample relative to the phase pattern controlled laser output.

4. A system as claimed in any preceding claim, wherein the spatial light modulator comprises a phase only reflective liquid crystal spatial light modulator.

5. A system as claimed in any preceding claim comprising a laser fabrication system, and further comprising a detection system for detecting plasma emission intensity, and deriving a quality feedback control signal indicative of a fabrication quality, and providing the quality feedback signal to the control system.

6. A system as claimed in claim 5, further comprising a blazed grating associated with the spatial light modulator, wherein the blazed grating modulation depth is adjusted in dependence on the quality feedback control signal.

7. A system as claimed in any preceding claim, comprising a laser writing system, wherein a structure to be fabricated comprises a feature to be fabricated fully to the lateral edge of the sample.

8. A laser focusing method comprising:

controlling a phase pattern of a laser output;

providing the phase pattern controlled laser output to a sample;

controlling the position of the sample relative to the phase pattern controlled laser; and

controlling the phase pattern applied by the spatial light modulator in dependence on the position of the sample relative to the phase pattern controlled laser output, thereby to provide correction for optical aberration resulting from a refractive index boundary at an edge of the sample, which edge has a lateral distance to the laser output which varies as the relative position of the sample is varied.

9. A method as claimed in claim 8, comprising a laser fabrication method, and a structure is to be fabricated in the sample, wherein the the position of the sample relative to the phase pattern controlled laser is controlled during fabrication and the phase pattern applied by the spatial light modulator is controlled during fabrication.

10. A method as claimed in claim 8 or 9, wherein the controlling further comprises providing correction for optical aberration resulting from a refractive index boundary at a top surface of the sample perpendicular to the laser illumination direction.

1 1 . A method as claimed in claim 8, 9 or 10, wherein the method includes providing a position feedback signal to provide the information concerning the position of the sample relative to the phase pattern controlled laser output.

12. A method as claimed in any one of claims 8 to 1 1 , wherein the phase pattern is controlled by a spatial light modulator.

13. A method as claimed in claim 9, further comprising detecting plasma emission intensity from the sample, and deriving a quality feedback control signal indicative of a fabrication quality, and providing the quality feedback control signal to the control system.

14. A method as claimed in claim 13, further comprising a blazed grating associated with the spatial light modulator, wherein the method comprises adjusting the blazed granting modulation depth in dependence on the quality feedback control signal.

15. A method as claimed claim 13 or 14, for writing a feature fully to the lateral edge of the sample.