Modular laser system

Abstract

A modular laser system comprises a docking system and at least one laser module. The docking system comprises: a frame; a plurality of docks provided on the frame each of the plurality of docks being for releasable engagement with a laser module; and combining optics disposed within the frame. Each laser module is operable to generate a radiation beam and output the laser beam through an output of the laser module, each laser module releasably engaged with a different one of the plurality of docks. The combining optics combines the laser beams from the laser modules to form a single output radiation beam. Each of the plurality of docks is provided with a safety component to block radiation from propagating out of the frame through the window of that dock.

Description

MODULAR LASER SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 729,074 which was filed on 06 December 2024 which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a modular laser system. The present invention also relates to a docking system of a modular laser system. The present invention also relates to a laser module for a modular laser system. The present invention also relates to a laser-produced plasma radiation (LPP) source comprising such a modular laser system. The LPP source may be for generating extreme ultraviolet (EUV) radiation.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective mirrors instead of lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector. The multilayers may be altematingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing thephase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks . For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Fourthly, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] EUV radiation may be produced by a laser produced plasma (LPP) radiation source . Within an LPP radiation source, a laser beam may be used to irradiate fuel droplets so as to produce a plasma which will emit EUV radiation. The laser beam typically has a high power and may be delivered from a laser to the fuel droplets via a beam delivery system.

[0007] It may be desirable to provide a new laser system, for example for use in an LPP radiation source, which at least partially addresses one or more problems associated with known arrangements, whether identified herein or otherwise.SUMMARY

[0008] According to a first aspect of the present disclosure there is provided a modular laser system comprising: a docking system comprising: a housing; a plurality of docks provided on the housing, each of the plurality of docks defining a window in the housing and each of the plurality of docks being for releasable engagement with a laser module; and combining optics disposed within the housing; and at least one laser module, the or each laser module being operable to generate a radiation beam and output the laser beam through an output of the laser module, the or each laser module releasably engaged with a different one of the plurality of docks such that the laser beam propagates through the window of that dock; wherein the combining optics is arranged to receive the laser beam from the or each at least one laser module and to combine them so as to form a single output radiation beam; and wherein each of the plurality of docks is provided with a safety component that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing through the window of that dock.

[0009] The modular laser system according to the first aspect of the present disclosure is advantageous as it allows for a plurality of laser modules to be combined together so as to provide an output radiation beam of increased power. The combining optics may be arranged to combine laser pulses from the plurality of laser modules into a single pulse. Since each laser module is releasably engaged to one of the plurality of docks, each laser module can be removed for servicing, repair or replacement with a new laser module. Furthermore, and advantageously, since each of the plurality of docks is provided with a safety component that is arranged, or is configurable so as to be arranged, toblock radiation from propagating out of the housing through the window of that dock one or more of the laser modules can be safely removed from the dock even while other laser modules connected to other docks are generating radiation without the risk that laser radiation from these operating laser modules exits the housing (where it may pose a safety risk). That is, the provision of the safety features allows for the laser modules to be “hot swapped” from the modular system.

[0010] The housing may be any suitable structure. The housing may generally contain laser radiation within the modular laser system.

[0011] Each of the plurality of docks may alternatively be referred to as a socket or a port.

[0012] It will be appreciated that each of the plurality of docks may be provided with first engagement features, each of the laser modules may be provided with second engagement features and the first and second engagement features may be complementary.

[0013] The window of each of the plurality of docks may comprise an aperture in the housing. It will be appreciated that an interior of the housing may be isolated from the surrounding or ambient environment. For example, in some embodiments, the window of each of the plurality of docks may comprise a transparent material.

[0014] The output radiation beam may have a power of the order of 50 kW or more.

[0015] The safety components may be especially important for such high power laser systems, which pose a significant risk of damage if the laser radiation were able to escape the housing. In some embodiments, the output radiation beam may have a power of the order of 100 kW or more.

[0016] In use, the output radiation beam of such embodiments may form part of a laser-produced plasma radiation source. In particular, the output radiation beam of such embodiments may be directed so as to be incident on a target (for example a tin droplet) so as to generate a tin plasma.

[0017] The or each laser module may comprise a fiber based laser module.

[0018] The or each laser module may be operable to generate radiation with a wavelength between1.5 pm and 2.5 pm.

[0019] Such radiation may be generally referred to as 2 pm radiation. In some embodiments, the or each laser module may be operable to generate radiation with a wavelength of approximately 2 pm.

[0020] The or each laser module may be operable to generate a laser beam with a power of the order of IkW.

[0021] It will be appreciated that each of the plurality of laser modules will, in general, comprise a gain medium, which, in use may be pumped by a suitable pump source.

[0022] The or each laser module may comprise a gain medium.

[0023] For example, the gain medium may comprise an optical fiber, for example doped with rare earth elements.

[0024] Furthermore, in use, each of the plurality of laser modules may be cooled by a cooling system and / or a supplied with purging gas via a gas purging system. In some embodiments, a pluralityof the laser modules may share: a common power source; a common cooling system; and / or a common gas purging system, as now discussed.

[0025] Each of the plurality of docks may be provided with a first power connection and the modular laser system may comprise at least one power supply arranged to provide electrical power the first power connections of each of the plurality of docks.

[0026] The or each laser module may comprise: a second power connection; and a pump generator connected to, and operable to be powered via, the second power connection.

[0027] The second power connections of the laser modules are arranged to engage with the first power connections of the docks. When so engaged, the pump generators of the laser systems can be powered by the at least one power supply of the modular laser system.

[0028] The pump generator may comprise one or more pump diodes.

[0029] The modular laser system may comprise a coolant system. Each of the plurality of docks may be provided with a coolant inlet and a coolant outlet. The coolant system may be operable to provide coolant to the coolant inlet and to receive coolant from the coolant outlet.

[0030] The or each laser module may comprise: a coolant inlet; a coolant outlet; and one or more coolant channels defined between the coolant inlet and the coolant outlet.

[0031] The coolant inlet(s) of the laser modules are arranged to engage with the coolant inlet(s) of the docks. The coolant outlet(s) of the laser modules are arranged to engage with the coolant outlet(s) of the docks. When so engaged, coolant may be pumped through the one or more coolant channels of the laser systems by the coolant system of the modular laser system.

[0032] The coolant inlet and coolant outlet of each of the plurality of docks may each comprise a valve. The valves may be arranged to be closed when there is no laser module engaged with the dock. Similarly, the coolant inlet and coolant outlet of each of the laser modules may each comprise a valve. The valves may be arranged to be closed when that laser module is not engaged with one of the plurality of docks.

[0033] The modular laser system may comprise a purge gas system. Each of the plurality of docks may be provided with a purge gas inlet. The purge gas system may be operable to provide purge gas to the purge gas inlet.

[0034] Optionally, each of the plurality of docks may comprise a purge gas outlet and wherein the purge gas system may be operable to receive purge gas from the purge gas outlet.

[0035] The or each laser module may comprise: a housing defining a purge gas inlet and a purge gas outlet.

[0036] The purge gas inlet(s) of the laser modules are arranged to engage with the purge gas inlet(s) of the docks. When so engaged, purge gas may be pumped through the housing of the laser systems by the purge gas system of the modular laser system. The purge gas outlet(s) of the laser modules may be arranged to engage with the purge gas outlet(s) of the docks (when provided). Alternatively, the purge gas outlet(s) of the laser modules may be open to the ambient environment.

[0037] The purge gas inlet and purge gas outlet (when provided) of each of the plurality of docks may each comprise a valve. The valves may be arranged to be closed when there is no laser module engaged with the dock. Similarly, the purge gas inlet and purge gas outlet of each of the laser modules may each comprise a valve. The valves may be arranged to be closed when that laser module is not engaged with one of the plurality of docks.

[0038] An optical path between the window of each of the plurality of docks and the combining optics may maintain a polarization of a laser beam propagating therethrough.

[0039] For example, in use, laser radiation may be directed from the window of each of the plurality of docks to the combining optics via a polarization maintaining fiber connection.

[0040] The modular laser system may further comprise a phase control mechanism for controlling a phase of a laser beam propagating through the window of each of the plurality of docks.

[0041] For example, in some embodiments each of the plurality of docks may comprise a phase control mechanism for controlling a phase of a laser beam propagating through the window of that dock. Additionally or alternatively, the or each laser module may comprise a phase control mechanism for controlling a phase of the radiation beam output thereby.

[0042] Advantageously, this may allow the combination optics to perform coherent combining.

[0043] Each of the plurality of docks may comprise an alignment mechanism for controlling a position of a laser beam propagating through the window of that dock.

[0044] The modular laser system may further comprise a seed laser module that is operable to produce a seed laser beam and to direct a portion of the seed laser beam to laser modules engaged with at least two of the plurality of docks.

[0045] Advantageously, by providing a portion of the same seed laser beam to each of a plurality of different laser modules (each engaged with a different one of the plurality of docks), there may be a fixed phase relationship between the laser beams from these individual laser modules. In turn, this may allow for the laser beams from the individual laser modules to be combined using coherent combining.

[0046] The seed laser module may, for example, form part of the docking system. In such embodiments, optionally, each of the plurality of docks may be provided with a seed laser window. The seed laser module may be operable to direct a portion of the seed laser beam to the seed laser window of each of the plurality of docks. In such embodiments, the or each laser module may comprise: a seed laser window. Each laser module may be arranged such that when that laser module is engaged with one of the docks the seed laser window of that laser module is aligned with the seed laser window of the dock that it is engaged with.

[0047] The combining optics may be operable to combine the laser beams from the individual laser modules using any type of combination technique.

[0048] The combining optics may comprise at least one component operable to coherently combine at least two laser beams from different laser modules engaged with docks.

[0049] The combining optics may comprise at least one component operable to perform spatial combination and / or polarization combination of at least two laser beams from different laser modules engaged with docks.

[0050] In some embodiments, the combining optics may comprise a plurality of stages. In a first stage groups or sets of laser beams from different laser modules may be combined to form intermediate radiation beams. Subsequently, in a second stage, two or more of the intermediate radiation beams may be combined to form the output radiation beam. It will be appreciated that some embodiments may use additional stages. Note that for embodiments that use both spatial and polarization combining and spatial combining may be used first followed by polarization combining.

[0051] The combining optics may comprise: a plurality of first combining elements arranged to receive a laser beam from laser modules engaged with a different set of the plurality of docks and to combine them so as to form an intermediate radiation beam; and at least one second combining element arranged to receive at least two intermediate radiation beams from the plurality of first combining elements and to combine them so as to at least partially form the output radiation beam.

[0052] In one embodiment, the combining optics may comprise a first stage which uses coherent combining and a second stage which uses polarization and spatial combination. For example, groups of laser beams may be combined at the first stage using coherent combining to create ~10 kW intermediate radiation beams. Subsequently, at a second stage, polarization and spatial combination may be used to generate an output radiation beam having a power of -100 kW.

[0053] The safety component of at least one of the plurality of docks may comprise an optical isolator arranged, or configurable so as to be arranged, to block radiation from propagating out of the housing through the aperture of that dock.

[0054] Such an optical isolator (also known as a Faraday isolator or an optical diode) acts as an optical diode allowing radiation to pass into the housing through the window whilst blocking radiation from exiting the housing via the window.

[0055] It will be appreciated that optical isolators are typically wavelength dependent. It will be further appreciated that the optical isolator of each of the plurality of docks is suitable for blocking radiation from the at least one laser module from propagating out of the housing through the aperture of that dock.

[0056] The safety component of at least one of the plurality of docks may comprise a movable opaque member that is positionable in at least: a first position in which it blocks the window and a second position in which it does not block the window.

[0057] Such a movable opaque member can act as a door. When no laser module is engaged with a dock, or when it is desirable to remove a laser module from a dock, the movable opaque member of that dock may be disposed in the first position. In use, when a laser module is engaged with a dock the movable opaque member of that dock may be disposed in the second position.

[0058] Note that it may be desirable to provide such a movable opaque member in addition to an optical isolator to provide additional safety.

[0059] The modular laser system may further comprise a controller.

[0060] The controller may be configured to: receive one or more signals from each of the plurality of docks; and / or send one or more signals to each of the plurality of docks.

[0061] For example, the controller may be operable to receive one or more signals from each of the plurality of docks that are indicative of a status of those docks, or laser module connected thereto. Additionally or alternatively, the controller may be operable to send one or more signals to each of the plurality of docks so as to control one or more aspects of those docks.

[0062] The controller may be configured to send one or more signals to each of the plurality of docks so as to control operation of a laser module engaged therewith.

[0063] For example, the controller may be operable to control: at least one power supply so as to power a pump generator of a laser module engaged with any of the plurality of docks.

[0064] Additionally or alternatively, the controller may be operable to control: a coolant system so as to provide a flow of coolant to one or more coolant channels of a laser module engaged with any of the plurality of docks.

[0065] Additionally or alternatively, the controller may be operable to control: a purge gas system so as to provide a flow of purge gas to a housing of a laser module engaged with any of the plurality of docks.

[0066] The controller may be configured to: send one or more signals to the at least one power supply.

[0067] For example, the controller may be operable to control: at least one power supply so as to power a pump generator of a laser module engaged with any of the plurality of docks. The controller may, for example, be operable to control power supplied to the first power connection of any one of the plurality of docks (and therefore the second power connection of any laser module engaged with that dock).

[0068] The controller may be configured to: send one or more signals to the coolant system.

[0069] The controller may be operable to control: the coolant system so as to provide a flow of coolant to one or more coolant channels of a laser module engaged with any of the plurality of docks. To achieve this, the controller may, for example, be operable to control operation of one or more valves of any one of the plurality of docks and / or one or more valves of any laser module engaged with that dock.

[0070] The controller may be configured to: send one or more signals to the purge gas system.

[0071] The controller may be operable to control: a purge gas system so as to provide a flow of purge gas to a housing of a laser module engaged with any of the plurality of docks. To achieve this, the controller may, for example, be operable to control operation of one or more valves of any one of the plurality of docks and / or one or more valves of any laser module engaged with that dock.

[0072] The controller may be configured to: send one or more signals to the seed laser module. The controller may be operable to control: the seed laser module so as to produce the seed laser beam and / or to provide a portion thereof to one or more of the plurality of docks.

[0073] The modular laser system may further comprise a radiation sensor that is arranged to monitor whether or not laser radiation is propagating through the window of each of the plurality of docks and to generate a signal indicative thereof.

[0074] For example, in some embodiments each of the plurality of docks may be provided with a radiation sensor that is arranged to monitor whether or not laser radiation is propagating through the window of that dock and to generate a signal indicative thereof. Additionally or alternatively, in some embodiments the or each laser module may be provided with a radiation sensor that is arranged to monitor whether or not a radiation beam is being output by that laser module to generate a signal indicative thereof.

[0075] In some embodiments, signals generated by the radiation sensors (of the plurality of docks or laser modules engaged therewith) may be received by the controller.

[0076] The signals indicative of whether or not laser radiation is propagating through the window of each of the plurality of docks may be received by the controller and the controller may be operable to send a control signal to one or more of the plurality of docks, or a laser module engaged therewith, in dependence thereon.

[0077] The controller may be operable to send a control signal to the movable opaque member of each dock in dependence on the signals indicative of whether or not laser radiation is propagating through the window of each of the plurality of docks.

[0078] For example, upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller may be operable to send a control signal to the movable opaque member of that dock to cause it to be positioned in the first position in which it blocks the window.

[0079] Upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller may be operable to send a control signal to another dock, or a laser module engaged with that other dock to cause a laser module engaged with that other dock to start operating.

[0080] Advantageously, this allows for the modular laser system to have some redundancy (for example more laser modules than are required for the output radiation beam) and to allow the controller to automatically replace a laser module that stops working with another one that has not been operating.

[0081] Upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller may be operable to send a control signal to one or more other docks, or laser module(s) engaged therewith, to cause an output power of a laser module engaged with that one or more other dock to be varied (for example increased).Advantageously, this allows for the controller to automatically increase the output power of one or more a laser modules in the event that one laser module stops working.

[0082] According to a second aspect of the present disclosure there is provided a docking system for the modular laser system according to the first aspect of the present disclosure, the docking system comprising: a housing; a plurality of docks provided on the housing, each of the plurality of docks defining a window in the housing and each of the plurality of docks being for releasable engagement with a laser module; and combining optics disposed within the housing, wherein the combining optics is arranged to receive the laser beam from laser module engaged with any one of the plurality of docks and to combine them so as to form a single output radiation beam; and wherein each of the plurality of docks is provided with a safety component that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing through the window of that dock.

[0083] It will be appreciated that the docking system according to the second aspect of the present disclosure may comprise any of the features of the modular laser system according to the first aspect of the present disclosure.

[0084] Each of the plurality of docks may be provided with a first power connection and wherein the modular laser system may comprise at least one power supply arranged to provide electrical power the first power connections of each of the plurality of docks.

[0085] The docking system may further comprise a coolant system. Each of the plurality of docks may be provided with a coolant inlet and a coolant outlet. The coolant system may be operable to provide coolant to the coolant inlet and to receive coolant from the coolant outlet.

[0086] The docking system may further comprise a purge gas system. Each of the plurality of docks may be provided with a purge gas inlet. The purge gas system may be operable to provide purge gas to the purge gas inlet.

[0087] Optionally, each of the plurality of docks may comprise a purge gas outlet and wherein the purge gas system may be operable to receive purge gas from the purge gas outlet.

[0088] In some embodiments of the docking system, an optical path between the window of each of the plurality of docks and the combining optics may maintain a polarization of a laser beam propagating therethrough.

[0089] For example, in use, laser radiation may be directed from the window of each of the plurality of docks to the combining optics via a polarization maintaining fiber connection.

[0090] Each of the plurality of docks may comprise a phase control mechanism for controlling a phase of a laser beam propagating through the window of that dock.

[0091] Advantageously, this may allow the combination optics to perform coherent combining.

[0092] Each of the plurality of docks may comprise an alignment mechanism for controlling a position of a laser beam propagating through the window of that dock.

[0093] The combining optics may be operable to combine the laser beams from the individual laser modules using any type of combination technique.

[0094] The docking system may further comprise a seed laser module that is operable to produce a seed laser beam and to direct a portion of the seed laser beam to laser modules engaged with at least two of the plurality of docks.

[0095] Advantageously, by providing a portion of the same seed laser beam to each of a plurality of different laser modules (each engaged with a different one of the plurality of docks), there may be a fixed phase relationship between the laser beams from these individual laser modules. In turn, this may allow for the laser beams from the individual laser modules to be combined using coherent combining.

[0096] The combining optics may comprise at least one component operable to coherently combine at least two laser beams from different laser modules engaged with different docks.

[0097] The combining optics may comprise at least one component operable to perform polarization and spatial combination of at least two laser beams from different laser modules engaged with different docks.

[0098] In some embodiments, the combining optics may comprise a plurality of stages. In a first stage groups or sets of laser beams from different laser modules may be combined to form intermediate radiation beams. Subsequently, in a second stage, two or more of the intermediate radiation beams may be combined to form the output radiation beam. It will be appreciated that some embodiments may use additional stages.

[0099] The combining optics may comprise: a plurality of first combining elements arranged to receive a laser beam from laser modules engaged with a different set of the plurality of docks and to combine them so as to form an intermediate radiation beam; and at least one second combining element arranged to receive at least two intermediate radiation beams from the plurality of first combining elements and to combine them so as to at least partially form the output radiation beam.[000100] In one embodiment, the combining optics may comprise a first stage which uses coherent combining and a second stage which uses polarization and spatial combination. For example, groups of laser beams may be combined at the first stage using coherent combining to create ~10 kW intermediate radiation beams. Subsequently, at a second stage, polarization and spatial combination may be used to generate an output radiation beam having a power of -100 kW.[000101] The safety component of at least one of the plurality of docks may comprise an optical isolator arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing through the aperture of that dock.[000102] Such an optical isolator (also known as a Faraday isolator or an optical diode) acts as an optical diode allowing radiation to pass into the housing through the window whilst blocking radiation from exiting the housing via the window.[000103] It will be appreciated that optical isolators are typically wavelength dependent. It will be further appreciated that the optical isolator of each of the plurality of docks is suitable for blocking radiation from the at least one laser module from propagating out of the housing through the aperture of that dock.[000104] The safety component of at least one of the plurality of docks may comprise a movable opaque member that is positionable in at least: a first position in which it blocks the window and a second position in which it does not block the window.[000105] Such a movable opaque member can act as a door. When no laser module is engaged with a dock, or when it is desirable to remove a laser module from a dock, the movable opaque member of that dock may be disposed in the first position. In use, when a laser module is engaged with a dock the movable opaque member of that dock may be disposed in the second position.[000106] Note that it may be desirable to provide such a movable opaque member in addition to an optical isolator to provide additional safety.[000107] The may further comprise a controller.[000108] The controller may be configured to: receive one or more signals from each of the plurality of docks; and / or send one or more signals to each of the plurality of docks.[000109] For example, the controller may be operable to receive one or more signals from each of the plurality of docks that are indicative of a status of those docks, or laser module connected thereto. Additionally or alternatively, the controller may be operable to send one or more signals to each of the plurality of docks so as to control one or more aspects of those docks.[000110] The controller may be configured to send one or more signals to each of the plurality of docks so as to control operation of a laser module engaged therewith.[000111] For example, the controller may be operable to control: at least one power supply so as to power a pump generator of a laser module engaged with any of the plurality of docks.[000112] Additionally or alternatively, the controller may be operable to control: a coolant system so as to provide a flow of coolant to one or more coolant channels of a laser module engaged with any of the plurality of docks.[000113] Additionally or alternatively, the controller may be operable to control: a purge gas system so as to provide a flow of purge gas to a housing of a laser module engaged with any of the plurality of docks.[000114] The controller may be configured to: send one or more signals to the at least one power supply.[000115] For example, the controller may be operable to control: at least one power supply so as to power a pump generator of a laser module engaged with any of the plurality of docks. The controller may, for example, be operable to control power supplied to the first power connection of any one of the plurality of docks (and therefore the second power connection of any laser module engaged with that dock).[000116] The controller may be configured to: send one or more signals to the coolant system.[000117] The controller may be operable to control: the coolant system so as to provide a flow of coolant to one or more coolant channels of a laser module engaged with any of the plurality of docks. To achieve this, the controller may, for example, be operable to control operation of one or more valvesof any one of the plurality of docks and / or one or more valves of any laser module engaged with that dock.[000118] The controller may be configured to: send one or more signals to the purge gas system.[000119] The controller may be may be operable to control: a purge gas system so as to provide a flow of purge gas to a housing of a laser module engaged with any of the plurality of docks. To achieve this, the controller may, for example, be operable to control operation of one or more valves of any one of the plurality of docks and / or one or more valves of any laser module engaged with that dock.[000120] The controller may be configured to: send one or more signals to the seed laser module. The controller may be operable to control: the seed laser module so as to produce the seed laser beam and / or to provide a portion thereof to one or more of the plurality of docks.[000121] Each of the plurality of docks may be provided with a radiation sensor that is arranged to monitor whether or not laser radiation is propagating through the window of that dock and to generate a signal indicative thereof.[000122] In some embodiments, signals generated by the radiation sensors of the plurality of docks may be received by the controller.[000123] The signals indicative of whether or not laser radiation is propagating through the window of each of the plurality of docks may be received by the controller and the controller may be operable to send a control signal to one or more of the plurality of docks, or a laser module engaged therewith, in dependence thereon.[000124] The controller may be operable to send a control signal to the movable opaque member of each dock in dependence on the signals indicative of whether or not laser radiation is propagating through the window of each of the plurality of docks.[000125] For example, upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller may be operable to send a control signal to the movable opaque member of that dock to cause it to be positioned in the first position in which it blocks the window.[000126] Upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller may be operable to send a control signal to another dock, or a laser module engaged with that other dock to cause a laser module engaged with that other dock to start operating.[000127] Advantageously, this allows for the modular laser system which the docking system is part of to have some redundancy (for example more laser modules than are required for the output radiation beam) and to allow the controller to automatically replace a laser module that stops working with another one that has not been operating.[000128] Upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller may be operable to send a control signal to one or more other docks, or laser module(s) engaged therewith, to cause an output power of a lasermodule engaged with that one or more other dock to be varied (for example increased). Advantageously, this allows for the controller to automatically increase the output power of one or more a laser modules in the event that one laser module stops working.[000129] According to a third aspect of the present disclosure there is provided a laser module for use in the modular laser system according to the first aspect of the present disclosure, the laser module being operable to generate a radiation beam and output the laser beam through an output of the laser module, the laser module being releasably engagable with a different one of the plurality of docks of the docking system such that the laser beam propagates through the window of that dock.[000130] The laser module may comprise a fiber based laser module.[000131] The laser module may be operable to generate radiation with a wavelength between 1.5 pm and 2.5 pm.[000132] Such radiation may be generally referred to as 2 pm radiation. In some embodiments, the or each laser module may be operable to generate radiation with a wavelength of approximately 2 pm. [000133] The laser module may be operable to generate a laser beam with a power of the order of IkW.[000134] It will be appreciated that each of the plurality of laser modules will, in general, comprise a gain medium and a pump source. Furthermore, each of the plurality of laser modules may further comprise a cooling system and / or a gas purging system. In some embodiments, a plurality of the laser modules may share: a common power source (for powering the pump source); a common cooling system; and / or a common gas purging system, as now discussed.[000135] The laser module may comprise a gain medium.[000136] For example, the gain medium may comprise an optical fiber, for example doped with rare earth elements.[000137] The laser module may comprise: a second power connection; and a pump generator connected to, and operable to be powered via, the second power connection.[000138] The second power connections of the laser modules are arranged to engage with the first power connections of the docks. When so engaged, the pump generators of the laser systems can be powered by the at least one power supply of the modular laser system.[000139] The pump generator may comprise one or more pump diodes.[000140] The laser module may comprise: a coolant inlet; a coolant outlet; and one or more coolant channels defined between the coolant inlet and the coolant outlet.[000141] The coolant inlet(s) of the laser modules are arranged to engage with the coolant inlet(s) of the docks. The coolant outlet(s) of the laser modules are arranged to engage with the coolant outlet(s) of the docks. When so engaged, coolant may be pumped through the one or more coolant channels of the laser systems by the coolant system of the modular laser system.[000142] The coolant inlet and coolant outlet of each of the plurality of docks may each comprise a valve. The valves may be arranged to be closed when there is no laser module engaged with the dock.Similarly, the coolant inlet and coolant outlet of each of the laser modules may each comprise a valve. The valves may be arranged to be closed when that laser module is not engaged with one of the plurality of docks.[000143] The laser module may comprise : a housing defining a purge gas inlet and a purge gas outlet. [000144] The purge gas inlet(s) of the laser modules are arranged to engage with the purge gas inlet(s) of the docks. When so engaged, purge gas may be pumped through the housing of the laser systems by the purge gas system of the modular laser system. The purge gas outlet(s) of the laser modules may be arranged to engage with the purge gas outlet(s) of the docks (when provided). Alternatively, the purge gas outlet(s) of the laser modules may be open to the ambient environment.[000145] The purge gas inlet and purge gas outlet (when provided) of each of the plurality of docks may each comprise a valve. The valves may be arranged to be closed when there is no laser module engaged with the dock. Similarly, the purge gas inlet and purge gas outlet of each of the laser modules may each comprise a valve. The valves may be arranged to be closed when that laser module is not engaged with one of the plurality of docks.[000146] The laser module may comprise a phase control mechanism for controlling a phase of the radiation beam output thereby.[000147] The laser module may comprise a radiation sensor that is arranged to monitor whether or not a radiation beam is being output by that laser module to generate a signal indicative thereof.[000148] According to a fourth aspect of the present disclosure there is provided a laser-produced plasma radiation source comprising: a fuel generator operable to generate a stream of fuel targets along a fuel target trajectory; the modular laser system according to the first aspect of the present disclosure, wherein the output radiation beam is directed to intersect the fuel target trajectory at a plasma formation region; and collector optics arranged to direct radiation originating from the plasma formation region to an output so as to form a second output radiation beam.[000149] Features of different aspects of the present disclosure may be combined together.BRIEF DESCRIPTION OF THE DRAWINGS[000150] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and further comprising a radiation source according to an embodiment of the present disclosure;Figure 2 schematically depicts a system for (actinic) mask inspection which comprises a radiation source according to an embodiment of the present disclosure;Figure 3 schematically shows a new modular laser system according to the present disclosure, which comprises a docking system and at least one laser module;Figure 4 shows a mutually engaged dock and laser module of the modular laser system shown in Figure 3 showing some additional optional features and an optional power supply;Figure 5 shows a mutually engaged dock and laser module of the modular laser system shown in Figure 3 showing some additional optional features and an optional coolant system;Figure 6 shows a mutually engaged dock and laser module of the modular laser system shown in Figure 3 showing some additional optional features and an optional purge gas system;Figure 7 shows a mutually engaged dock and laser module of the modular laser system shown in Figure 3 showing some additional optional features and an optional controller;Figure 8 shows a mutually engaged dock and laser module of the modular laser system shown in Figure 3 showing some additional optional features and an optional seed laser module;Figure 9 shows a mutually engaged dock and laser module of the modular laser system shown in Figure 3 showing the additional optional features shown in each of Figures 4, 5, 6, 7 and 8;Figure 10 schematically shows an example combining optics that may form part of the modular laser system shown in Figure 3, the combining optics comprising a plurality of stages;Figure 11 shows a docking system according to an embodiment of the present invention that may form part of the modular laser system shown in Figure 3;Figure 12 shows a laser module according to an embodiment of the present invention that may form part of the modular laser system shown in Figure 3; andFigure 13 shows the laser module shown in Figure 12 showing some additional optional features thereof.DETAILED DESCRIPTION[000151] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.[000152] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.[000153] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which areconfigured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).[000154] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.[000155] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.[000156] The lithographic apparatus LA and radiation source SO described herein can be used for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from the radiation source SO to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.[000157] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1 is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.[000158] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.[000159] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or abeam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system (if present) may together be considered to be a radiation system.[000160] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.[000161] Figure 2 schematically depicts a mask inspection system MS for (actinic) mask inspection. The mask inspection system MS can be used to identify or inspect defects in a mask to be used in a lithographic process by means of the lithographic apparatus described in figure 1. The mask inspection system MS comprises a radiation source SO, an illumination system ILM, a detection system DS and a mask stage ME.[000162] The illumination system ILM is configured to condition an EUV radiation beam B before the EUV radiation beam B is incident upon a mask MA supported by the mask stage ME. The illumination system ILM may provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system ILM may comprise a plurality of mirrors 22. The illumination system ILM may comprise one or more faceted mirror devices.[000163] The mask stage ME may be configured to support and move a mask MA relative to the EUV radiation beam B, so that the EUV radiation beam is incident upon different areas of the mask. [000164] The detection system DS comprises a detector 20, and may in addition comprise a plurality of mirrors 24. The plurality of mirrors 24 may be configured to collect EUV radiation BRthat has been reflected from the mask MA, and form an image of the mask MA on the detector 20 (which may be an imaging array). A processor (not depicted) may receive signals output from the detector 20 and use those signals to look for defects in the mask MA.[000165] The radiation source SO of the mask inspection system MS may correspond with the radiation source SO depicted in Figure 1. In the same way as depicted in Figure 1, the radiation source SO of the mask inspection system MS may focus the EUV radiation beam B to form an intermediate focus 6.[000166] Some embodiments of the present disclosure relate to a modular laser system. An example of a new modular laser system 100 according to the present disclosure is shown schematically in Figure 3. The modular laser system 100 comprises: a docking system 200 and at least one laser module 300.[000167] The docking system 200 comprises: a housing 210; combining optics 220; and a plurality of docks 230.[000168] The plurality of docks 230 are provided on the housing 210. Each of the plurality of docks 230 defines a window 232 in the housing 210. Each of the plurality of docks 230 is suitable for releasable engagement with a laser module 300, as discussed further below.[000169] The or each laser module 300 is operable to generate a radiation beam 302 and output the laser beam 302 through an output 304 of the laser module. The or each laser module 300 is releasably engaged with a different one of the plurality of docks 230 such that the laser beam 302 propagates through the window 232 of that dock 230.[000170] Each of the plurality of docks 230 is provided with a safety component 234 that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing 210 through the window 232 of that dock 230.[000171] The combining optics 220 are disposed within the housing 210. The combining optics 220 is arranged to receive the laser beam 302 from the or each at least one laser module 300 and to combine them so as to form a single output radiation beam 222.[000172] The modular laser system 100 shown in Figure 3 is advantageous as it allows for a plurality of laser modules 100 to be combined together so as to provide an output radiation beam 222 of increased power. The combining optics 220 may be arranged to combine laser pulses 302 from the plurality of laser modules 300 into a single pulse 222. Since each laser module 300 is releasably engaged to one of the plurality of docks 230, each laser module 300 can be removed for servicing, repair or replacement with a new laser module 300. Furthermore, and advantageously, since each of the plurality of docks 230 is provided with a safety component 234 that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing 210 through the window 232 of that dock 230 one or more of the laser modules 300 can be safely removed from the dock(s) 230 even while other laser modules 300 connected to other docks 230 are generating radiation 302 without the risk that laser radiation from these operating laser modules 300 exits the housing 310 (where it may pose a safety risk). That is, the provision of the safety features 234 allows for the laser modules 300 to be “hot swapped” from the modular system 100.[000173] The housing 210 may be any suitable structure. The housing 210 may generally contain laser radiation 302 within the modular laser system 100.[000174] Each of the plurality of docks 230 may alternatively be referred to as a socket or a port.[000175] It will be appreciated that each of the plurality of docks 230 may be provided with first engagement features, each of the laser modules 300 may be provided with second engagement features and the first and second engagement features may be complementary.[000176] The window 232 of each of the plurality of docks 230 may comprise an aperture in the housing 210. It will be appreciated that an interior 212 of the housing 210 may be isolated from the surrounding or ambient environment. For example, in some embodiments, the window 232 of each of the plurality of docks 230 may comprise a transparent material. Alternatively, the interior 212 of the housing 210 may be either not isolated from the surrounding or ambient environment or only isolated from the surrounding or ambient environment via the safety component 234. For such embodiments, the window 232 may merely be an aperture in the housing 210.[000177] In some embodiments of the modular laser system 100, an optical path between the window 232 of each of the plurality of docks 230 and the combining optics 220 may maintain a polarization of a laser beam propagating therethrough. For example, in use, laser radiation 302 may be directed from the window 232 of each of the plurality of docks 230 to the combining optics 220 via a polarization maintaining fiber connection 224.[000178] In some embodiments, the output radiation beam 222 may have a power of the order of 50 kW or more. The safety components 234 may be especially important for such high power laser systems 100, which pose a significant risk of damage if the laser radiation were able to escape the housing 210. In some embodiments, the output radiation beam 222 may have a power of the order of 100 kW or more.[000179] In use, the output radiation beam of such embodiments may form part of a laser-produced plasma radiation source. In particular, the output radiation beam of such embodiments may be directed so as to be incident on a target (for example a tin droplet) so as to generate a tin plasma.[000180] In some embodiments, the or each laser module 300 may comprise a fiber based laser module.[000181] In some embodiments, the or each laser module 300 may be operable to generate radiation 302 with a wavelength between 1.5 pm and 2.5 pm. Such radiation 302 may be generally referred to as 2 pm radiation. In some embodiments, the or each laser module 300 may be operable to generate radiation 302 with a wavelength of approximately 2 pm.[000182] In some embodiments, the or each laser module 300 may be operable to generate a laser beam 302 with a power of the order of IkW.[000183] As indicated very schematically in Figure 3, in some embodiments the modular laser system 100 may comprise one or more of the following optional features: at least one power supply 110; a coolant system 120; a purge gas system 130; a controller 140; and / or a seed laser module 150. It will be appreciated that any combination of these optional features (at least one power supply 110; coolant system 120; purge gas system 130; controller 140; and / or the seed laser module 150) may, for example, form part of the docking system 200.[000184] The functionality of these optional features (at least one power supply 110; coolant system 120; purge gas system 130; controller 140; and / or seed laser module 150), and how they may interact with the docking system 200 and the at least one laser module 300 is discussed further below with reference to Figures 4 to 9.[000185] Each of the plurality of laser modules 300 may comprise a gain medium 306, which, in use, may be pumped using a pump source. For example, the gain medium 306 may comprise an optical fiber, for example doped with rare earth elements.[000186] Furthermore, in use each of the plurality of laser modules 300 may be cooled by a cooling system and / or may be supplied with purging gas via a gas purging system. In some embodiments, aplurality of the laser modules 300 may share: a common power source; a common cooling system; and / or a common gas purging system, as now discussed.[000187] In some embodiments, at least two of the laser modules 300 that are engaged with a one of the plurality of docks 230 may be supplied with power via a common power supply 110. The laser modules 300 and the plurality of docks 230, may be arranged to allow this, as now discussed with reference to Figure 4.[000188] As shown in Figure 4, in some embodiments, each of the plurality of docks 230 may be provided with a first power connection 236 and the modular laser system 100 comprises at least one power supply 110 arranged to provide electrical power the first power connections 236 of each of the plurality of docks 230. The at least one power supply 110 may, for example, form part of the docking system 200.[000189] In such embodiments, the or each laser module 300 may comprise: a second power connection 310; and a pump generator 312. The pump generator 312 may be connected to, and operable to be powered via, the second power connection 310. The pump generator 312 may comprise one or more pump diodes.[000190] The second power connections 310 of the laser modules 300 are arranged to engage with the first power connections 236 of the docks 230. When so engaged, the pump generators 312 of the laser systems 300 can be powered by the at least one power supply 110 of the modular laser system 100.[000191] In some embodiments, at least two of the laser modules 300 that are engaged with a one of the plurality of docks 230 may be supplied with coolant via a common coolant supply 120. The laser modules 300 and the plurality of docks 230, may be arranged to allow this, as now discussed with reference to Figure 5.[000192] As shown in Figure 5, in some embodiments, the modular laser system 100 may comprise a coolant system 120. The coolant system 120 may, for example, form part of the docking system 200. Each of the plurality of docks 230 may be provided with a coolant inlet 238 and a coolant outlet 240. The coolant system 120 may be operable to provide coolant to the coolant inlet 238 and to receive coolant from the coolant outlet 240 (as indicated by solid arrows in Figure 5).[000193] In such embodiments, the or each laser module 300 may comprise: a coolant inlet 320; a coolant outlet 322; and one or more coolant channels 324 defined between the coolant inlet 320 and the coolant outlet 322.[000194] The coolant inlet(s) 320 of the laser modules 300 may be arranged to engage with the coolant inlet(s) 238 of the docks 230. The coolant outlet(s) 322 of the laser modules 300 may be arranged to engage with the coolant outlet(s) 240 of the docks 230. When so engaged, coolant may be pumped through the one or more coolant channels 324 of the laser systems 300 by the coolant system[000195] The coolant inlet 238 and coolant outlet 240 of each of the plurality of docks 230 may each comprise a valve 242, 244. The valves 242, 240 may be arranged to be closed when there is no laser module 300 engaged with the dock 230. Similarly, the coolant inlet 320 and coolant outlet 322 of each of the laser modules 300 may each comprise a valve 326, 328. The valves 326, 328 may be arranged to be closed when that laser module 300 is not engaged with one of the plurality of docks 230.[000196] In some embodiments, at least two of the laser modules 300 that are engaged with a one of the plurality of docks 230 may be supplied with a purge gas via a common purge gas supply 130. The laser modules 300 and the plurality of docks 230, may be arranged to allow this, as now discussed with reference to Figure 6.[000197] In some embodiments, the modular laser system 100 may comprise a purge gas system 130. The purge gas system 130 may, for example, form part of the docking system 200. Each of the plurality of docks 230 may be provided with a purge inlet 246. The purge gas system 130 may be operable to provide purge gas to the purge gas inlet 246. Each of the plurality of docks 230 may comprise a purge gas outlet 248. The purge gas system 130 may be operable to receive purge gas from the purge gas outlet 248.[000198] The or each laser module 300 may comprise: a housing 330 defining a purge gas inlet 332 and a purge gas outlet 334. The purge gas inlet(s) 332 of the laser modules 300 may be arranged to engage with the purge gas inlet(s) 246 of the docks 230. When a laser module 300 is engaged with a dock 230, purge gas may be pumped through the housing 330 of the laser system 300 by the purge gas system 130 of the modular laser system 100. The purge gas outlet(s) 334 of the laser modules 300 may be arranged to engage with the purge gas outlet(s) 248 of the docks 230 (when provided). Alternatively, the purge gas outlet(s) of the laser modules 334 may be open to the ambient environment.[000199] The purge gas inlet 246 and purge gas outlet 248 (when provided) of each of the plurality of docks 230 may each comprise a valve 250, 252. The valves 250, 252 may be arranged to be closed when there is no laser module 300 engaged with the dock 230. Similarly, the purge gas inlet 332 and purge gas outlet 334 of each of the laser modules 300 may each comprise a valve 336, 338. The valves 336, 338 may be arranged to be closed when that laser module is not engaged with one of the plurality of docks.[000200] In some embodiments, the plurality of docks 230 may comprise various additional optional components, as now discussed with reference to Figure 7.[000201] In some embodiments, each of the plurality of docks 230 may comprise a phase control mechanism 254 for controlling a phase of a laser beam 302 propagating through the window 232 of that dock. Advantageously, this may allow the combination optics to perform coherent combining.[000202] Additionally or alternatively, in some embodiments, each of the plurality of docks 230 may comprise an alignment mechanism 256 for controlling a position of a laser beam 302 propagating through the window 232 of that dock 230.[000203] Additionally or alternatively, in some embodiments, each of the plurality of docks 230 may comprise a radiation sensor 258 that is arranged to monitor whether or not laser radiation 302 is propagating through the window 232 of that dock 230 and to generate a signal indicative 260 thereof. In some embodiments, signals 260 generated by the radiation sensors 258 of the plurality of docks 230 may be received by the controller 140, as discussed further below.[000204] Note that, although a phase control mechanism 254, an alignment mechanism 256 and a radiation sensor 258 are shown in Figure 7, other embodiments may only comprise one or two of these optional features. Furthermore, as shown in Figure 7 each of the phase control mechanism 254, the alignment mechanism 256 and the radiation sensor 258 are shown in the interior 212 of the housing 210. However, in other embodiments one or more of these may be provided at least partially on the same side of the window 232 as the laser module 300.[000205] Furthermore, in some embodiments, one or more of the following optional features: a phase control mechanism 254, an alignment mechanism 256 and a radiation sensor 258 may be provided at least partially in the laser modules 300. That is, rather than forming part of the docking system 200, any combination of these optional features may be provided as part of the laser modules 300.[000206] In some embodiments, at least two of the laser modules 300 that are engaged with a one of the plurality of docks 230 may be supplied with coolant via a common seed laser module 150. The laser modules 300 and the plurality of docks 230, may be arranged to allow this, as now discussed with reference to Figure 8.[000207] As shown in Figure 8, in some embodiments, the modular laser system 100 may comprise a seed laser module 150. The seed laser module 150 may, for example, form part of the docking system 200. The seed laser module 150 may be operable to produce a seed laser beam 152 to direct a portion of the seed laser beam 152 to a laser module 300 when engaged with any one of a plurality of the plurality of docks 230.[000208] In such embodiments, optionally, each of the plurality of docks 230 may be provided with a seed laser window 262. The seed laser module 150 may be operable to direct a portion of the seed laser beam 152 to the seed laser window 262 of each of the plurality of docks 230 (as indicated by a solid arrow in Figure 8).[000209] In such embodiments, the or each laser module 300 may comprise: a seed laser window 340. Each laser module 300 may be arranged such that when that laser module 300 is engaged with one of the docks 230 the seed laser window 340 of that laser module 300 is aligned with the seed laser window 262 of the dock 230 that it is engaged with. When one of the laser modules 300 is engaged with one of the docks 230, a portion of the seed laser beam 152 may be delivered to the laser module 300 from the seed laser module 150 of the modular laser system 100 via the window 262 of the dock 230 and the window 240 of the laser module 300. In particular, the portion of the seed laser beam 152 may be delivered to the gain medium 306 of the laser module 300 when it is engaged with one of the docks.[000210] Optionally, each of the docks 230 may be provided with a door or shutter to prevent a portion of the seed laser beam 152 from propagating through the seed laser window 262 of that dock 230 when there is no laser module engaged with that dock 230.[000211] Advantageously, by providing a portion of the same seed laser beam 152 to each of a plurality of different laser modules 300 (each engaged with a different dock 230), there may be a fixed phase relationship between the laser beams 302 from these individual laser modules 300. In turn, this may allow for the laser beams 302 from the individual laser modules 300 to be combined using coherent combining.[000212] Figures 4, 5, 6, 7 and 8 each shows only one of the following optional features: at least one power supply 110; a coolant system 120; a purge gas system 130; a controller 140; and / or a seed laser module 150. However, it will be appreciated that any combination of these optional features (at least one power supply 110; coolant system 120; purge gas system 130; controller 140; and / or the seed laser module 150) may be provided. For example, Figure 9 shows a mutually engaged dock and laser module of the modular laser system shown in Figure 3 showing all of these additional optional features (at least one power supply 110; coolant system 120; purge gas system 130; controller 140; and / or the seed laser module 150).[000213] The combining optics 220 may be operable to combine the laser beams 302 from the individual laser modules 300 using any type of combination technique.[000214] In some embodiments, the combining optics 220 may comprise at least one component operable to coherently combine at least two laser beams 302 from different laser modules 300 engaged with docks 230.[000215] In some embodiments, the combining optics 220 comprises at least one component operable to perform polarization and spatial combination of at least two laser beams 302 from different laser modules 300 engaged with docks.[000216] As shown schematically in Figure 10, in some embodiments, the combining optics 220 may comprise a plurality of stages. In a first stage 220a groups or sets of laser beams 302 from different laser modules 300 may be combined to form intermediate radiation beams 227. Subsequently, in a second stage 220b, two or more of the intermediate radiation beams 227 may be combined to form the output radiation beam 222. It will be appreciated that some embodiments may use additional stages.[000217] In some embodiments, the combining optics 220 comprises: a plurality of first combining elements 226 and at least one second combining element 228. The plurality of first combining elements 226 are each arranged to receive a laser beams 302 from laser modules 300 engaged with a different set of the plurality of docks 230 and to combine them so as to form an intermediate radiation beam 227. The or each second combining element 228 is arranged to receive at least two intermediate radiation beams 227 from the plurality of first combining elements 226 and to combine them so as to at least partially form the output radiation beam 222.[000218] In one embodiment, the combining optics 220 may comprise a first stage 220a which uses coherent combining and a second stage 220b which uses spatial combination and / or polarization combination. Note that for embodiments that use both spatial and polarization combining and spatial combining may be used first followed by polarization combining. For example, groups of laser beams 302 may be combined at the first stage 220a using coherent combining to create ~10 kW intermediate radiation beams 227. Subsequently, at a second stage 220b, polarization and spatial combination may be used to generate an output radiation beam 222 having a power of -100 kW.[000219] In some embodiments, the safety component 234 of at least one of the plurality of docks 230 comprises an optical isolator that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing 210 through the aperture 232 of that dock 230.[000220] Such an optical isolator (also known as a Faraday isolator or an optical diode) acts as an optical diode allowing radiation to pass into the housing 210 through the window 232 whilst blocking radiation from exiting the housing 210 via the window 232.[000221] It will be appreciated that optical isolators are typically wavelength dependent. It will be further appreciated that the optical isolator of each of the plurality of docks 230 is suitable for blocking radiation from the at least one laser module 300 from propagating out of the housing 210 through the aperture 232 of that dock 230.[000222] In some embodiments of the modular laser system 100, the safety component 234 of at least one of the plurality of docks 230 may comprise a movable opaque member that is positionable in at least: a first position in which it blocks the window 232 of that dock 230 and a second position in which it does not block the window 232 of that dock 230.[000223] Such a movable opaque member can act as a door. When no laser module 300 is engaged with a dock 230, or when it is desirable to remove a laser module 300 from a dock 230, the movable opaque member of that dock 230 may be disposed in the first position. In use, when a laser module 300 is engaged with a dock 230 the movable opaque member 232 of that dock 230 may be disposed in the second position.[000224] Note that in some embodiments, the safety component 234 of at least one of the plurality of docks 230 may comprise such a movable opaque member in addition to an optical isolator to provide additional safety.[000225] As discussed above, in some embodiments, the modular laser system 100 may comprise a controller 140. The controller 140 may, for example, form part of the docking system 200.[000226] The controller 140 may be configured to: receive one or more signals from each of the plurality of docks 230. For example, the controller 140 may be operable to receive one or more signals from each of the plurality of docks 230 that are indicative of a status of those docks 230, or laser module 300 connected thereto.[000227] The controller 140 may be configured to: send one or more signals to each of the plurality of docks 230. That is, the controller 140 may be operable to send one or more signals 144 to each ofthe plurality of docks 230 so as to control one or more aspects of those docks 230. In some embodiments, the controller is configured to send one or more signals to each of the plurality of docks 230 so as to control operation of a laser module 300 engaged therewith.[000228] Additionally or alternatively, the controller 140 may be configured to: send one or more signals to any one of the at least one power supply 110, the coolant system 120, the purge gas system 130 and the seed laser module 150.[000229] For example, the controller 140 may be operable to control: at least one power supply 110 so as to power a pump generator 312 of a laser module 300 engaged with any of the plurality of docks 230. The controller 140 may, for example, be operable to control power supplied to the first power connection 236 of any one of the plurality of docks 230 (and therefore the second power connection 310 of any laser module 300 engaged with that dock 230).[000230] Additionally or alternatively, the controller 140 may be operable to control: a coolant system 120 so as to provide a flow of coolant to one or more coolant channels 324 of a laser module 300 engaged with any of the plurality of docks 230. The controller 140 may, for example, be operable to control operation of the valves 242, 244 of any one of the plurality of docks 230 and / or the valves 326, 328 of any laser module 300 engaged with that dock 230.[000231] Additionally or alternatively, the controller 140 may be operable to control: a purge gas system 130 so as to provide a flow of purge gas to a housing 330 of a laser module 300 engaged with any of the plurality of docks 230. The controller 140 may, for example, be operable to control operation of the valves 250, 252 of any one of the plurality of docks 230 and / or the valves 336, 338 of any laser module 300 engaged with that dock 230.[000232] In some embodiments, each of the plurality of docks 230 is provided with a radiation sensor 258 that is arranged to monitor whether or not laser radiation 302 is propagating through the window 232 of that dock 230 and to generate a signal indicative 260 thereof. In some embodiments, signals 260 generated by the radiation sensors of the plurality of docks may be received by the controller 140.[000233] In some embodiments, the signals 260 indicative of whether or not laser radiation 302 is propagating through the window 232 of each of the plurality of docks 230 (discussed above with reference to Figure 7) may be received by the controller 140 and the controller 140 may be operable to send a control signal to that dock 230, or a laser module 300 engaged with that dock 230, in dependence thereon.[000234] In some embodiments, the controller 140 may be operable to send a control signal to the movable opaque member (part of the safety component 234) of each dock 230 in dependence on the signals 160 indicative of whether or not laser radiation 302 is propagating through the window 232 of each of the plurality of docks 230. For example, upon receipt of a signal 160 from one of the plurality of docks 230 indicating that laser radiation 302 is not propagating through the window 232 of that dock 230, the controller 140 may be operable to send a control signal to the movable opaque member of that dock 230 to cause it to be positioned in the first position in which it blocks the window 232.[000235] In some embodiments, upon receipt of a signal 260 from one of the plurality of docks 230 indicating that laser radiation 302 is not propagating through the window 232 of that dock 230, the controller 140 may be operable to send a control signal to another dock 230, or a laser module 300 engaged with that other dock 232 to cause a laser module 300 engaged with that other dock 230 to start operating. Advantageously, this allows for the laser to have some redundancy (for example more laser modules 300 than are required for the output radiation beam 222) and to allow the controller 140 to automatically replace a laser module 300 that stops working with another one that has not been operating.[000236] In some embodiments, upon receipt of a signal 260 from one of the plurality of docks 230 indicating that laser radiation 302 is not propagating through the window 232 of that dock 230, the controller 140 may be operable to send a control signal to one or more other docks 230, or laser module(s) 300 engaged with that one or more other dock 232, to cause an output power of a laser module 300 engaged with that one or more other dock 230 to be varied (for example increased). Advantageously, this allows for the controller 140 to automatically increase the output power of one or more a laser modules 300 in the event that one laser module 300 stops working.[000237] Some embodiments of the present disclosure relate to a docking system 200 for use in the modular laser system 100 shown in Figure 3. An example of such a docking system 200 is shown schematically in Figure 11 and may comprise: a housing 210; combining optics 220 disposed within the housing 210; and a plurality of docks 230.[000238] The plurality of docks 230 may be provided on the housing 210 and each of the plurality of docks 230 may each define a window 232 in the housing 210. Each of the plurality of docks 230 may be suitable for releasable engagement with a laser module 300.[000239] The combining optics 220 is arranged to receive the laser beam(s) 302 from laser module(s) 300 engaged with any one of the plurality of docks 230 and to combine them so as to form a single output radiation beam 222.[000240] Each of the plurality of docks 230 is provided with a safety component 234 that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing 210 through the window 232 of that dock 230.[000241] In some embodiments of the docking system 200, an optical path between the window 232 of each of the plurality of docks 230 and the combining optics 220 may maintain a polarization of a laser beam propagating therethrough. For example, as shown in Figure 11, optionally, and in some embodiments, laser radiation may be directed from the window 232 of each of the plurality of docks 230 to the combining optics 220 via a polarization maintaining fiber connection 224.[000242] As shown schematically in Figure 11, in some embodiments the docking system 200 may comprise one or more of the following optional features (as described above): at least one power supply 110; a coolant system 120; a purge gas system 130; and / or a controller 140.[000243] It will be appreciated that the docking system 200 shown in Figure 11 may comprise any of the features of the docking system 200 described above with reference to Figures 4 to 10.[000244] Some embodiments of the present disclosure relate to a laser module 300 for use in the modular laser system 100 shown in Figure 3. An example of such a laser module 300 is shown schematically in Figure 12. In general, the laser module 300 is operable to generate a radiation beam 302 and output the laser beam through an output 304 of the laser module. For example, the laser module may 300 comprise a gain medium 306. For example, the gain medium 306 may comprise an optical fiber, for example doped with rare earth elements.[000245] In general, the module 300 is releasably engageable with a different one of the plurality of docks 230 of the docking system 200 such that the laser beam 302 propagates through the window 232 of that dock 230.[000246] As shown in Figure 13, it will be appreciated that the laser module(s) 300 according to some embodiments of the present disclosure may comprise any of the features of the laser modules 300 described above with reference to Figures 4 to 9.[000247] Some embodiments of the present disclosure relate to laser-produced plasma radiation sources (for example of the type of radiation source SO shown in Figure 1 and as described above). In general, such a laser-produced plasma radiation source SO may comprise: a fuel generator 3 operable to generate a stream of fuel targets along a fuel target trajectory; and a modular laser system 100 as described above with reference to Figures 3 to 13. The output radiation beam 222 may be directed to intersect the fuel target trajectory at a plasma formation region 4. That is, the modular laser system 100 as described above with reference to Figures 3 to 13 may be equivalent to the laser system 1 described above. Furthermore, the output radiation 222 of the modular laser system 100 may, for example, be equivalent to the laser beam 2 shown in Figure 1.[000248] In general, a laser-produced plasma radiation source comprising a modular laser system 100 as described above with reference to Figures 3 to 13 may comprise collector optics 5 arranged to direct radiation originating from the plasma formation region 4 to an output 6, 8 so as to form a second output radiation beam B.[000249] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.[000250] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing ortransmiting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical, and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.[000251] Clauses1. A modular laser system comprising: a docking system comprising: a housing; a plurality of docks provided on the housing, each of the plurality of docks defining a window in the housing and each of the plurality of docks being for releasable engagement with a laser module; and combining optics disposed within the housing; and at least one laser module, the or each laser module being operable to generate a radiation beam and output the laser beam through an output of the laser module, the or each laser module releasably engaged with a different one of the plurality of docks such that the laser beam propagates through the window of that dock; wherein the combining optics is arranged to receive the laser beam from the or each at least one laser module and to combine them so as to form a single output radiation beam; and wherein each of the plurality of docks is provided with a safety component that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing through the window of that dock.2. The modular laser system of clause 1 wherein the output radiation beam has a power of the order of 50 kW or more.3. The modular laser system of any preceding clause wherein the or each laser module comprises a fiber based laser module.4. The modular laser system of any preceding clause wherein the or each laser module is operable to generate radiation with a wavelength between 1.5 pm and 2.5 pm.5. The modular laser system of any preceding clause wherein the or each laser module is operable to generate a laser beam with a power of the order of IkW.6. The modular laser system of any preceding clause wherein the or each laser module comprises a gain medium.7. The modular laser system of any preceding clause wherein each of the plurality of docks is provided with a first power connection and wherein the modular laser system comprises at least onepower supply arranged to provide electrical power the first power connections of each of the plurality of docks.8. The modular laser system of any preceding clause wherein the or each laser module comprises: a second power connection; and a pump generator connected to, and operable to be powered via, the second power connection.9. The modular laser system of any preceding clause comprising a coolant system, wherein each of the plurality of docks is provided with a coolant inlet and a coolant outlet; and wherein the coolant system is operable to provide coolant to the coolant inlet and to receive coolant from the coolant outlet.10. The modular laser system of any preceding clause wherein the or each laser module comprises: a coolant inlet; a coolant outlet; and one or more coolant channels defined between the coolant inlet and the coolant outlet.11. The modular laser system of any preceding clause comprising a purge gas system, wherein each of the plurality of docks is provided with a purge gas inlet; and wherein the purge gas system is operable to provide purge gas to the purge gas inlet.12. The modular laser system of any preceding clause wherein the or each laser module comprises: a housing defining a purge gas inlet and a purge gas outlet.13. The modular laser system of any preceding clause wherein an optical path between the window of each of the plurality of docks and the combining optics maintains a polarization of a laser beam propagating therethrough.14. The modular laser system of any preceding clause further comprising a phase control mechanism for controlling a phase of a laser beam propagating through the window of each of the plurality of docks.15. The modular laser system of any preceding clause wherein each of the plurality of docks comprises an alignment mechanism for controlling a position of a laser beam propagating through the window of that dock.16. The modular laser system of any preceding clause further comprising a seed laser module that is operable to produce a seed laser beam and to direct a portion of the seed laser beam to laser modules engaged with at least two of the plurality of docks.17. The modular laser system of any preceding clause wherein: the combining optics comprises at least one component operable to coherently combine at least two laser beams from different laser modules engaged with docks.18. The modular laser system of any preceding clause wherein: the combining optics comprises at least one component operable to perform spatial combination and / or polarization combination of at least two laser beams from different laser modules engaged with docks.19. The modular laser system of any preceding clause wherein the combining optics comprises: a plurality of first combining elements arranged to receive a laser beam from laser modules engaged with a different set of the plurality of docks and to combine them so as to form an intermediate radiation beam; and at least one second combining element arranged to receive at least two intermediate radiation beams from the plurality of first combining elements and to combine them so as to at least partially form the output radiation beam.20. The modular laser system of any preceding clause wherein the safety component of at least one of the plurality of docks comprises an optical isolator arranged, or configurable so as to be arranged, to block radiation from propagating out of the housing through the aperture of that dock.21. The modular laser system of any preceding clause wherein the safety component of at least one of the plurality of docks comprises a movable opaque member that is positionable in at least: a first position in which it blocks the window and a second position in which it does not block the window.22. The modular laser system of any preceding clause further comprising a controller.23. The modular laser system of clause 21 wherein the controller is configured to: receive one or more signals from each of the plurality of docks; and / or send one or more signals to each of the plurality of docks.24. The modular laser system of clause 21 or clause 22 wherein the controller is configured to send one or more signals to each of the plurality of docks so as to control operation of a laser module engaged therewith.25. The modular laser system of any one of clauses 21 to 23 when dependent either directly or indirectly on clause 7 wherein the controller is configured to: send one or more signals to the at least one power supply.26. The modular laser system of any one of clauses 21 to 24 when dependent either directly or indirectly on clause 9 wherein the controller is configured to: send one or more signals to the coolant system.27. The modular laser system of any one of clauses 21 to 25 when dependent either directly or indirectly on clause 11 wherein the controller is configured to: send one or more signals to the purge gas system.28. The modular laser system of any preceding clause further comprising a radiation sensor that is arranged to monitor whether or not laser radiation is propagating through the window of each of the plurality of docks and to generate a signal indicative thereof.29. The modular laser system of clause 28 when dependent either directly or indirectly on clause 21 wherein the signals indicative of whether or not laser radiation is propagating through thewindow of each of the plurality of docks are received by the controller and wherein the controller is operable to send a control signal to one or more of the plurality of docks, or a laser module engaged therewith, in dependence thereon.30. The modular laser system of clause 29 when dependent either directly or indirectly on clause 21 wherein the controller is operable to send a control signal to the movable opaque member of each dock in dependence on the signals indicative of whether or not laser radiation is propagating through the window of each of the plurality of docks.31. The modular laser system of clause 29 or clause 30 wherein upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller is operable to send a control signal to another dock, or a laser module engaged with that other dock to cause a laser module engaged with that other dock to start operating.32. A docking system for the modular laser system of any preceding clause, the docking system comprising: a housing; a plurality of docks provided on the housing, each of the plurality of docks defining a window in the housing and each of the plurality of docks being for releasable engagement with a laser module; and combining optics disposed within the housing, wherein the combining optics is arranged to receive the laser beam from laser module engaged with any one of the plurality of docks and to combine them so as to form a single output radiation beam; and wherein each of the plurality of docks is provided with a safety component that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the housing through the window of that dock.33. The docking system of clause 32 wherein each of the plurality of docks is provided with a first power connection and wherein the modular laser system comprises at least one power supply arranged to provide electrical power the first power connections of each of the plurality of docks.34. The docking system of clause 32 or clause 33 further comprising a coolant system, wherein each of the plurality of docks is provided with a coolant inlet and a coolant outlet; and wherein the coolant system is operable to provide coolant to the coolant inlet and to receive coolant from the coolant outlet.35. The docking system of any one of clauses 32 to 34 further comprising a purge gas system, wherein each of the plurality of docks is provided with a purge gas inlet; and wherein the purge gas system is operable to provide purge gas to the purge gas inlet.36. The docking system of any one of clauses 32 to 35 wherein an optical path between the window of each of the plurality of docks and the combining optics maintains a polarization of a laser beam propagating therethrough.37. The docking system of any one of clauses 32 to 36 wherein each of the plurality of docks comprises a phase control mechanism for controlling a phase of a laser beam propagating through the window of that dock.38. The docking system of any one of clauses 32 to 37 wherein each of the plurality of docks comprises an alignment mechanism for controlling a position of a laser beam propagating through the window of that dock.39. The docking system of any one of clauses 32 to 38 further comprising a seed laser module that is operable to produce a seed laser beam and to direct a portion of the seed laser beam to laser modules engaged with at least two of the plurality of docks.40. The docking system of any one of clauses 32 to 38 wherein: the combining optics comprises at least one component operable to coherently combine at least two laser beams from different laser modules engaged with different docks.41. The docking system of any one of clauses 32 to 40 wherein: the combining optics comprises at least one component operable to perform spatial combination and / or polarization combination of at least two laser beams from different laser modules engaged with different docks.42. The docking system of any one of clauses 32 to 41 wherein the combining optics comprises: a plurality of first combining elements arranged to receive a laser beam from laser modules engaged with a different set of the plurality of docks and to combine them so as to form an intermediate radiation beam; and at least one second combining element arranged to receive at least two intermediate radiation beams from the plurality of first combining elements and to combine them so as to at least partially form the output radiation beam.43. The docking system of any one of clauses 32 to 42 wherein the safety component of at least one of the plurality of docks comprises an optical isolator arranged, or configurable so as to be arranged, to block radiation from propagating out of the housing through the aperture of that dock.44. The docking system of any one of clauses 32 to 43 wherein the safety component of at least one of the plurality of docks comprises a movable opaque member that is positionable in at least: a first position in which it blocks the window and a second position in which it does not block the window.45. The docking system of any preceding clause further comprising a controller.46. The docking system of clause 45 wherein the controller is configured to: receive one or more signals from each of the plurality of docks; and / or send one or more signals to each of the plurality of docks.47. The docking system of clause 45 or clause 46 wherein the controller is configured to send one or more signals to each of the plurality of docks so as to control operation of a laser module engaged therewith.48. The docking system of any one of clauses 45 to 47 when dependent either directly or indirectly on clause 33 wherein the controller is configured to: send one or more signals to the at least one power supply.49. The docking system of any one of clauses 45 to 48 when dependent either directly or indirectly on clause 34 wherein the controller is configured to: send one or more signals to the coolant system.50. The docking system of any one of clauses 45 to 49 when dependent either directly or indirectly on clause 34 wherein the controller is configured to: send one or more signals to the purge gas system.51. The docking system of any one of clauses 32 to 50 wherein each of the plurality of docks is provided with a radiation sensor that is arranged to monitor whether or not laser radiation is propagating through the window of that dock and to generate a signal indicative thereof.52. The docking system of clause 51 when dependent either directly or indirectly on clause 45 wherein the signals indicative of whether or not laser radiation is propagating through the window of each of the plurality of docks are received by the controller and wherein the controller is operable to send a control signal to one or more of the plurality of docks, or a laser module engaged therewith, in dependence thereon.53. The docking system of clause 52 when dependent either directly or indirectly on clause 44 wherein the controller is operable to send a control signal to the movable opaque member of each dock in dependence on the signals indicative of whether or not laser radiation is propagating through the window of each of the plurality of docks.54. The docking system of clause 52 or clause 53 wherein upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller is operable to send a control signal to another dock, or a laser module engaged with that other dock to cause a laser module engaged with that other dock to start operating.55. A laser module for use in the modular laser system of any one of clauses 1 to 31, the laser module being operable to generate a radiation beam and output the laser beam through an output of the laser module, the laser module being releasably engageable with a different one of the plurality of docks of the docking system such that the laser beam propagates through the window of that dock.56. The laser module of clause 55 wherein the laser module comprises a fiber based laser module.57. The laser module of any one of clauses 55 to 56 wherein the laser module is operable to generate radiation with a wavelength between 1.5 pm and 2.5 pm.58. The laser module of any one of clauses 55 to 57 wherein the laser module is operable to generate a laser beam with a power of the order of IkW.59. The laser module of any one of clauses 55 to 58 comprising a gain medium.60. The laser module of any one of clauses 55 to 59 wherein the laser module comprises:a second power connection; and a pump generator connected to, and operable to be powered via, the second power connection.61. The laser module of any one of clauses 55 to 60 wherein the laser module comprises: a coolant inlet; a coolant outlet; and one or more coolant channels defined between the coolant inlet and the coolant outlet.62. The laser module of any one of clauses 55 to 61 wherein the laser module comprises: a housing defining a purge gas inlet and a purge gas outlet.63. The laser module of any one of clauses 55 to 62 wherein the laser module comprises a phase control mechanism for controlling a phase of the radiation beam output thereby.64. The laser module of any one of clauses 55 to 63 wherein the laser module comprises a radiation sensor that is arranged to monitor whether or not a radiation beam is being output by that laser module to generate a signal indicative thereof.65. A laser-produced plasma radiation source comprising: a fuel generator operable to generate a stream of fuel targets along a fuel target trajectory; the modular laser system of any one of clauses 1 to 31, wherein the output radiation beam is directed to intersect the fuel target trajectory at a plasma formation region; and collector optics arranged to direct radiation originating from the plasma formation region to an output so as to form a second output radiation beam.[000252] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A modular laser system comprising: a docking system comprising: a frame; the frame comprising a plurality of docks, each of the plurality of docks defining an aperture and each of the plurality of docks arranged to be releasably engaged with a laser module; and combining optics disposed within the frame; and a plurality of laser modules, each laser module being operable to generate a radiation beam, each laser module releasably engaged with a different one of the plurality of docks such that the radiation beam propagates through the aperture of that dock; wherein the combining optics is arranged to receive the radiation beam from one of the plurality of laser modules and to combine them so as to form a single output radiation beam; and wherein each of the plurality of docks is provided with a safety component that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the frame through the window of that dock.

2. The modular laser system of claim 1 wherein each laser module comprises a fiber based laser module.

3. The modular laser system of any preceding claim wherein each laser module is operable to generate radiation with a wavelength between 1.5 pm and 2.5 pm.

4. The modular laser system of any preceding claim wherein each laser module is operable to generate a laser beam with a power of the order of IkW.

5. The modular laser system of any preceding claim wherein an optical path between the window of each of the plurality of docks and the combining optics maintains a polarization of a radiation beam propagating therethrough.

6. The modular laser system of any preceding claim further comprising a phase control mechanism for controlling a phase of a radiation beam propagating through the window of each of the plurality of docks.

7. The modular laser system of any preceding claim wherein each of the plurality of docks comprises an alignment mechanism for controlling a position of a radiation beam propagating through the window of that dock.

8. The modular laser system of any preceding claim wherein: the combining optics comprises at least one component operable to coherently combine at least two radiation beams from different laser modules engaged with docks.

9. The modular laser system of any preceding claim wherein: the combining optics comprises at least one component operable to perform spatial combination and / or polarization combination of at least two radiation beams from different laser modules engaged with docks.

10. The modular laser system of any preceding claim wherein the combining optics comprises: a plurality of first combining elements arranged to receive a radiation beam from laser modules engaged with a different set of the plurality of docks and to combine them so as to form an intermediate radiation beam; and at least one second combining element arranged to receive at least two intermediate radiation beams from the plurality of first combining elements and to combine them so as to at least partially form the output radiation beam.

11. The modular laser system of any preceding claim wherein the safety component of at least one of the plurality of docks comprises an optical isolator arranged, or configurable so as to be arranged, to block radiation from propagating out of the frame through the aperture of that dock.

12. The modular laser system of any preceding claim further comprising a controller, wherein the controller is configured to perform at least one of the following actions: receive one or more signals from each of the plurality of docks, and / or send one or more signals to each of the plurality of docks; send one or more signals to each of the plurality of docks so as to control operation of a laser module engaged therewith.

13. The modular laser system of any preceding claim further comprising a radiation sensor that is arranged to monitor whether or not laser radiation is propagating through the window of each of the plurality of docks and to generate a signal indicative thereof.

14. The modular laser system of claim 13 when dependent on claim 12 wherein the signals indicative of whether or not laser radiation is propagating through the window of each of the pluralityof docks are received by the controller and wherein the controller is operable to send a control signal to one or more of the plurality of docks, or a laser module engaged therewith, in dependence thereon.

15. The modular laser system of claim 14 wherein upon receipt of a signal from one of the plurality of docks indicating that laser radiation is not propagating through the window of that dock, the controller is operable to send a control signal to another dock, or a laser module engaged with that other dock to cause a laser module engaged with that other dock to start operating.

16. A docking system for the modular laser system of any preceding claim, the docking system comprising: a frame; the frame comprising a plurality of docks, each of the plurality of docks defining an aperture and each of the plurality of docks arranged to be releasably engaged with a laser module; and combining optics disposed within the frame wherein the combining optics is arranged to receive a radiation beam from one of the plurality of laser modules and to combine them so as to form a single output radiation beam; and wherein each of the plurality of docks is provided with a safety component that is arranged, or is configurable so as to be arranged, to block radiation from propagating out of the frame through the window of that dock.

17. A laser-produced plasma radiation source comprising: a fuel generator operable to generate a stream of fuel targets along a fuel target trajectory; the modular laser system of any one of claims 1 to 15, wherein the output radiation beam is directed to intersect the fuel target trajectory at a plasma formation region; and collector optics arranged to direct radiation originating from the plasma formation region to an output so as to form a second output radiation beam.

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