## Description 

[0001] The invention is related to laser beam delivery. A laser beam may for example be delivered in order to treat, e.g. clean, a surface, e.g. by laser ablation. As a side effect of such a treatment, by-products, e.g. includ- ing debris and/or evaporated material, are often formed at the surface, wherein the by-products can travel away fromthe surface and towardsunwanted areas,forexam- ple towards one or more optical components. When by- products reach an optical component, e.g. a lens or a protective window, they can contaminate or even dam- age the component and/or occlude a laser beam path at the component,such thatefficiency ofthe lasertreatment is reduced and/or one or more components require re- placement. For example, by-products can damage an optical component, altering its optical characteristic s such astransmission,absorption and/orreflectance.Due to anincreased absorption,the laserlightwillincreasingly heat the optical component, possibly leading to fracture of the component. In particular, in practice, frequent re- placement of a protective window (usually arranged in front of a beam exit opening) is often required, which is cumbersome and costly and can slow down the overall beam delivery process and/or treatment. It is therefore desired to prevent that such by-products can affect opti- cal components associated with the beam delivery. 

30 35 40 45 [0002] Various efforts have been made to address the above-mentioned problems. For US2003/0197909 discloses a method forpreventing con- tamination of an optical element in an imaging head for imaging a media that produces imaging debris. The method comprises the steps of: collimating a source of pressurized fluid to producea substantiallynon-turbulent fluid flow;and directingthenon-turbulentfluid flow across an opticalelementsuch thatthe fluid flow formsa barrier between the opticalelementand the imaging debris.Be- cause the fluid flow is merely directed across (i.e. along a surface of) the optical element, its effectiveness in keeping by-products (debris) away from the optical ele- ment is limited. Moreover, a large flow rate and flo w speed are required to realize an appropriate buffer layer of flowing fluid. This can be prohibitively impractical de- pending on the specific application, e.g. depending on the speed of the by-products and the acceptable rate of replacing the optical element. 

[0003] An object of the present invention is to solve at leastone ofthe above mentioned problems.In particular, an objectistoprovideanimproved beamdeliverysystem and a method for treating a surface with a laser beam. 

[0004] To this end, anaspect ofthe invention provides a beamdeliverysystemaccordingtothe features ofclaim 1. 

[0005] Advantageously,the systemcomprisesa shield which includes at least one beam passage for transmis- sion of at least one laser beam; an optics assembly con- figured to atleastpartlyfocus the atleastone laserbeam on the at least one beam passage; and means for pro- 

vidinga fluid flow through the atleastonebeampassage. 

[0006] It will be appreciated that in the context of the present disclosure, ’at least partly focus’ means that a laserbeam is substantially concentrated,e.g.inthe form of a point focus or a line focus or the-like, such that it transitions from substantially convergentto substantially divergent along at least one spatial dimension (but not necessarily along multiple spatial dimensions). By at least partly focusing the laser beam on the at least one beam passage, a large portion, e.g. substantially all, of the laser beam (i.e. of the beam’s energy, e.g. energy intended for laser ablation) can reach a surface to be treated (the surface being arranged ata certain treatment distance behind the shield, facing the shield during op- eration).The shield cansignificantlylimitexposure ofthe optics assembly to harmful by-products, e.g. ablated de- bris that emanates from a surface to be treated. By pro- vidinga fluid flow through the atleastonebeampassage, afore-mentioned by-productscanbe significantlyorsub- stantiallyprevented fromtravelling towards the opticsas- sembly. The exposure of optics to such by-products can thus be complimentarily further reduced, thus limiting la- ser beam efficiency loss (e.g. due to occlusion) and in- creasing the functionallifetime ofan opticalelement,e.g. a replaceable protective window of the optics assembly. 

exam ple, [0007] Focusing the laser beam onto the beam pas- sage of the shield can in particular mean, that a respec- tive focus point or focus line of the optics assembly is situated in the beam passage, preferably centrally (and on an optical axis of the beam e.g. extending centrally, preferably normally, through the beam passage). In this way, the beam passage can be relatively small, which advantageouslyimprovesprotection oftheoptics assem- bly against by-products, while at the same time the laser beamcan substantially travelthroughthe beampassage. [0008] Further, the shield as such is preferably made ofmaterialable toblock laserlight,e.g.secondary and/or tertiary light reflections resulting from a laser beam de- livery to a target area. 

[0009] The system is preferably (but not necessarily) configuredforperforminglaserablation usingthe atleast one laser beam, because the invention finds particular benefitin laserablation applications due tothe prominent problem of by-product formation from laser ablation. It will however be appreciated that the invention can alter- natively and/or additionally find benefit in other laser beam delivery applications including but not limited to laser heating, laser cleaning, laser cutting, laser welding and laser etching. 

[0010] In an embodiment, the optics assembly is con- figured to partly focus the at least one laser beam on a firstfocalplane,the firstfocalplane having an intersection with the at least one beam passage. In particular, the beam passage can coincide with the first focal plane of the optics assembly. 

[0011] The optics assembly may be further configured to partly focus the at least one laser beam also on a sec- ond focalplane whichextends ata distance from the first 

