Laser power meter for high power applications

Abstract

A power meter capable of measurement of a laser beam having an average power in the MW range. A first partially reflective window is disposed in a wall of a vessel containing a flow of beam absorbing liquid, to reflect a minor part of the beam, and to transmit a major part of the beam into the vessel. A second window is disposed at a second location in the wall of the vessel, in the path of the minor part of the beam reflected by the first window, and aligned such that a majority of the reflected minor part of the incident laser beam is transmitted into the vessel, and a minority of the reflected minor part of the beam is reflected towards a power meter where its power is measured. Knowledge of the reflectivity of the windows enables the measurement of the power of the laser beam.

Description

[0001] LASER POWER METER FOR HIGH POWER APPLICATIONS

[0002] FIELD

[0003] The present disclosure describes technology related to the field of the measurement of the power of very high power laser beams, especially those having continuous-wave (CW) beam powers or pulsed average beam powers of up to 1MW or more.

[0004] BACKGROUND

[0005] The most common form of power meter for measuring the power of laser beams of medium and high power levels, uses a beam dump which absorbs the laser beam, whose power is dissipated by a cooled heat sink feature, cooled at higher powers generally by a flow of coolant, such as water, and at lower powers even by a flow of cooling air from a fan. The temperature drop on the heat path between the region of the beam absorbers which absorbs the laser beam, and the cooled heat sink should be proportional to the power of the beam, and measurement of this temperature difference enables the beam power to be determined. For higher power levels, the beam may be reflected and spatially dispersed, such as on a reflective cone, and the dispersed beam absorbed on the outer surface of a large area heatsink, through which the coolant is passed. The increase in temperature of the coolant, being proportional to the power dissipated into the coolant, is measured and this then provides a measure of the beam power. Such power meters have been widely described, such as the Ophir Optronics models 10K-W-BB-45 and 120K-W as supplied by Ophir Optronics Solutions Ltd., of Jerusalem, Israel, and as described on pages 87 and 89 of the publication “2024 Ophir Power Meter Catalog”.

[0006] The ability of such power meters to handle very high power laser beams is dependent both on the ability of the beam absorber to locally withstand the power density of the incident laser beam, and on the ability of the heat sink to remove the heat generated in the beam absorbers. Many beam absorbers are constructed of copper with an absorbing ceramic surface that provides good beam absorbing properties, and is highly durable, though other beam absorbing layers have also been used. However, as laser systems having higher and higher beam powers are developed, it has been found problematic to attempt to scale the above described prior art concepts of beam power measurement to match the increase in laser beam powers which need to be measured. This is particularly problematic when the laser beam power reaches levels of hundreds of kilowatts or even the megawatt region or more, where it becomes clear that such beam absorbers cannot withstand the incident laser power which they are required to absorb, in order to perform the measurement. Furthermore, as the weight of the absorbing element rises, the response time of the absorber to the laser power beam becomes longer, lengthening the measurement time, and reducing the ability to monitor changes in the output power level of the laser beam.

[0007] The measurement of a very high power beam can also be performed by using a beam splitter for extracting a small part of the beam, typically of the order of between 1% and 5% of the beam, and of measuring the small extracted part of the beam in a power meter having a capacity commensurate with the power level of the split-off beam part. This procedure is commonly used in high power industrial material processing laser systems, where the main part of the beam passes on for use in the laser processing of the workpiece, whether for cutting, welding or heat treating, while the small extracted part of the beam can be readily measured on a power meter with a lower power rating. This use is important in such systems since a fast response to the power measurement is required in order to provide the optimally responsive feedback required in such material processing machines, and such a lower powered meter has a speedier response than a power monitor for the full power of the laser processing machine. There is still one problem with such arrangements, in that the beam splitter of a pass-through, sampling laser power meter is generally mounted in the ambient air environment, and therefore must be capable of withstanding the full power beam passing through it, without damage.

[0008] In a beam splitter laser power meter for use only in measuring the beam power, without the need to use the beam being measured, this problem has been solved by using a cooling liquid in contact with the beam splitter, the liquid acting as a beam dump besides its coolant function for the beam splitter. If the cooling liquid is a fast flowing stream, many of the problems stated above in the use of solid beam absorbers, can be solved.

[0009] One example of such a laser power meter using this technique, is described in Chinese Patent No. CN114543988B entitled Laser Power Meter, and granted to Wuhan Raycus Fiber Laser Technologies Co. Ltd., which shows a beam splitter in the beam path, cooled on its reverse side by a stream of flowing water, and splitting off a part of the beam for measurement. However, while this technique may be useable for CW beams of up to lOOkW, once beams in the MW range need to be measured, measurement of even only 1% of the MW beam presents a formidable task to achieve with an acceptably short measurement response time.

[0010] The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.

[0011] SUMMARY

[0012] The present disclosure attempts to provide novel and inventive systems and methods that overcome at least some of the disadvantages of prior art systems and methods. The present disclosure describes new exemplary systems for measuring the power of laser beams of up to 1 MW or more, using a vessel containing a fast flowing coolant liquid, typically water or a mixture of water and propylene glycol, the coolant also acting as the beam absorbing element.

[0013] For very high power beam measurements, such as in the megawatt range, the power reflected from a single beam splitter may still be too high for convenient measurement by a power meter for lower powers. This may be particularly so if an uncoated window is used, such as a sapphire window, which for an unpolarized beam, reflects approximately 10% in the near infrared region, such as around 1.07pm. Such an uncoated window may even be considered to be essential for measurements of such power levels since it less liable to be damaged by the incident high power laser beam, and also undergoes ageing more slowly than a window with a multiple layer dielectric coating. Thus, although the multiple layer dielectric coated beam splitter may have the advantage of enabling a much smaller part of the beam to be split off for measurement, than the 10% of a sapphire beam splitter, it is more prone to damage and ageing changes. Therefore, the use of a single reflection beam splitter measurement device, such as that described in the above- mentioned CN114543988B patent, is problematic for use in the MW range of power measurements. In order to make use of the advantages of an uncoated beam splitter window, such as a sapphire window, in the presently described power meter systems, there is described a flowing water beam absorber having two beam splitters in the walls of the flowing water vessel, and in which the comparatively high percentage of the laser power reflected from the first window is directed at a second beam splitting window. This second beamsplitting window too reflects at the beam wavelength, such that a major percentage of the beam is entered into the vessel, to be absorbed by the flowing coolant therein, while the remaining, smaller percentage of the beam, is reflected from that second window. That reflected part of the beam should then be at a sufficiently low power level that it can be readily handled by a lower rated power meter, even when the system is limited to the use of uncoated beam- splitting windows, at least for the first window. The second window should be aligned at an angle which diverts the second reflection of the beam in a convenient direction where it can be measured without interference from the incident beam reflected from the first entrance window. Using such a double window configuration, it is possible to absorb in the absorbent / coolant vessel, 99% of the impinging laser power, such that for a 1 MW beam measurement, a 10 kW power meter head may be conveniently used to perform the power measurement itself, in the space towards where the reflected beam is directed.

[0014] Coolant absorbers for such very high power beams have previously been used as a very high power beam dump, usually by diverting the output very high power beam from the top, down onto the surface of coolant flowing in an open vessel. However, this method may result in some potentially dangerous backscatter from the surface of the fast-flowing coolant. Such a negative effect is almost entirely absent in the presently described systems, since the entry of the beam into the coolant absorber takes place through the input windows, behind which there is a surface in contact with the flowing coolant, and the beam entry is at a well defined angle of incidence relative to the beam direction, such that safety precautions can be reliably taken. The backscatter of the systems of the present disclosure can be reliably kept below 0.1% of the incident beam, and at an essentially unchanging spatial distribution.

[0015] In addition to the power measurement, the beam profile can be determined by positioning an additional beam splitter in the path of the lower power beam after the second beamsplitting action, and an impingement plate viewed by an imaging device for recording the beam profile. Alternatively, the imaging device can look directly at the scattered light from the final beam dump.

[0016] There is thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a power meter for an incident laser beam having a wavelength, the power meter comprising:

[0017] (i) a vessel adapted to contain a liquid which absorbs at least a part of the incident laser beam,

[0018] (ii) a first window disposed at a first location in an outer bounding wall of the vessel and aligned at an angle relative to the direction of the incident laser beam, such that the first window reflects a minor part of the incident laser beam away from the vessel, and transmits a major part of the incident laser beam into the vessel for absorption by the liquid, the first window having a known reflectivity to the incident laser beam at the wavelength of the incident laser beam and at the aligned angle of the first window relative to the incident laser beam, and

[0019] (iii) a second window disposed at a second location in the outer bounding wall of the vessel and in the path of the minor part of the incident laser beam reflected by the first window, the second window having a known reflectivity at the wavelength of the minor part of the incident laser beam and at the aligned angle of the second window relative to the minor part of the incident laser beam, the aligned angle of the second window being such that a majority of the reflected minor part of the incident laser beam is transmitted into the vessel, and a minority of the reflected minor part of the incident laser beam is reflected away from the vessel, wherein using the known reflectivity of the incident laser beam from the first window, and the known reflectivity of the reflected minor part of the incident laser beam from the second window, the power of the incident laser beam can be determined by measurement of the power of the minority of the reflected minor part of the incident laser beam, reflected by the second window.

[0020] In such a power meter, the measurement of the power of the incident laser beam reflected from the first window and the second window can be performed at a power level lower than that of the incident laser beam after reflection from the first window only. In any of the power meters of the present disclosure the known absorptivity of the liquid to the incident laser beam wavelength, and the path length of the major part of the incident laser beam in the liquid in the vessel are such that the power of the major part of the incident laser beam in the vessel is reduced to a level which is determined not to cause damage to the vessel, or to cause overheating of the liquid.

[0021] Furthermore, the major part of the power of the incident laser beam reflected from the first window may be at least 85% of the incident laser beam power. Additionally, the majority of the reflected minor part of the power of the incident laser beam reflected from the second window may be at least 85% of the reflected minor part of the incident laser beam power.

[0022] According to yet another implementation of the power meters described in this disclosure, the vessel maybe a box-like container having a plurality of outer bounding walls. Alternatively, the vessel may be an assembly of tubular elements, and the outer bounding walls include end faces of the assembly of tubular elements.

[0023] Additionally, any of the power meters described in this disclosure may further comprise an imaging camera directed at a plate on which the final reflected beam is incident, such that the beam profile of the incident laser beam can be determined.

[0024] Any of the power meters of the present disclosure may enable the measurement of incident laser beams having a CW or average power of as high as at least 1 MW.

[0025] Furthermore, any of the power meters described in this disclosure should further comprise liquid input and output ports for circulating the cooling liquid through the vessel. Additionally, any of the above-described power meters may further comprise external active cooling for the liquid.

[0026] According to yet other implementations of the power meters described in this disclosure, at least one of the first window or the second window is adapted to operate as:

[0027] (i) the window of a liquid beam absorber in which the major part of the incident laser beam is safely absorbed, and (ii) a beam sampling element, separating out a minor part of the incident laser beam, to enable a beam measurement to be performed at a power level substantially reduced from that of the incident laser beam.

[0028] Finally, in any of the above described power meters, at least the first window may be an uncoated window. That uncoated window may be a sapphire window.

[0029] It is to be noted that a specific element of the described systems, which may have more than one function or more than one accepted name, may have been variously cited in this disclosure by alternative names. Some such examples are the partially reflective elements, also known as beam entrance windows, or the beam absorbing liquid, also known as the coolant or beam cooling liquid because of its double functional purpose, or the beam absorption plate, which can be a self standing element, or part of a laser power meter, and other minor examples. It is to be understood that such alternative designations are not intended to delineate between different elements, but are simply alternate names which may have been used randomly in the disclosure, but which are intended to relate to the same element.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

[0032] Fig.l illustrates schematically an exemplary representation of a power meter system achieve its objective of measuring the power and other properties of a very high power laser beam, using two partially reflecting windows of a vessel containing a liquid capable of absorbing the laser beam; and

[0033] Fig. 2 illustrates a schematic top view of a practical implementation of a beam measurement system using two partially reflecting input windows, as in Fig. 1, but with the absorption vessel constructed of tubing to enable better compactness.

[0034] DETAILED DESCRIPTION

[0035] Reference is now made to Fig. 1, which illustrates schematically the method by which the presently described power meters achieve their objective. The operation of the new power meter is based on the fact that at the wavelength of interest, typically in the region of 1070nm, ordinary water, or a 50:50 mixture of water and propylene glycol that is often used for corrosion resistance, has a known absorptivity of 11% per cm of absorption depth. This means that if the beam travels, for instance, through 55cm of the absorber / coolant, only 0.15% of the beam remains unabsorbed. The shape of the vessel can be chosen to suit the available volume and cost of manufacture of the vessel. The shape of the vessel shown in Fig. 1 is schematic, and is intended only to show the principle of operation of the system. In Fig. 1, the absorbing / cooling vessel 22 is shown schematically as a simple rectangular shape, so that it is a box-like container structure having a plurality of outer bounding walls. The incident very high power beam 24 is input into the vessel through an entrance partial reflecting window 20, disposed in an outer bounding wall of the vessel, at which the major percentage part 33 of the beam enters the vessel 22 and is absorbed by the fast flowing coolant. Although the beam 33 within the coolant is shown as a simple extension of the incident beam, it us to be understood that in practice, it will be gradually absorbed into the cooling liquid as a function of the path length within the liquid. The vessel has a coolant inlet 23 and a coolant outlet 25 through which a high speed flow of coolant is directed, in order to keep the coolant from overheating from the absorption of the 1MW or more laser beam.

[0036] If the reflector window is made of uncoated sapphire, it reflects some 10% of the incident beam and transmits the rest of the beam into the coolant liquid. The device advantageously uses reflection from uncoated windows with a stable reflection percentage, thereby enabling accurate monitoring of the laser power, especially for unpolarized beams, as is usually the case for very high-power lasers. Although the use of an uncoated sapphire window results in the need to measure 10% of the incident beam power that is reflected from the uncoated sapphire window, it is to be understood that use of other windows having an even higher power reflected, such as even 15%, may also be envisaged, though the greater the power level reflected from the window, the less the advantage provided by the presently described systems. The advantage of using an uncoated window in such high power laser power monitors can therefore be described as using windows enabling reflections from at least the first window, which encounters the full power of the incident laser beam, of more than 5%, or even more than 10% or even as much as 15% The minor part 26 of the incident beam, reflected in the air by the first input window 20, is now input into the vessel 22 through the second input window 21 , to be largely absorbed within the absorbing and cooling liquid. The minor part 27 of that second input beam 26, not transmitted into the absorbing vessel 22, is reflected in a direction remote from the plane of the incident beam 24 and the second input beam 26, to a region where its power can be conveniently measured, and other properties of the beam, such as the beam profile, can also be conveniently determined, as will be further explained hereinbelow. The absorbing and cooling liquid is passed through the vessel 22 from an input port 23 to an output port 25, providing a flow sufficiently fast to remove the heat generated by the absorbed beams, as will be further explained hereinbelow. When configured as described in Fig. 1, the windows 20, 21 have bi-functional use (i) for inputting the laser beam into a flowing liquid beam absorber, and (ii) as beam sampling elements separating out a small part of the incident beam power, to enable the beam measurement to be performed at a power level substantially reduced from that of the very high power incident beam.

[0037] To provide a convenient geometrical arrangement for the absorbing / cooling vessel, and the reflected beam positioning, the first window 20 may advantageously be aligned vertically (relative to the directions in the drawing of Fig. 1) and at an angle of 45° to the direction at which the incident laser beam is directed, and the second window 21 may be aligned in a direction perpendicular to the direction at which the incident laser beam is directed, but tilted at an angle of 45° to the vertical, so that the second reflected beam 27 is directed in the air upwards (relative to the directions in the drawing of Fig. 1) away from the plane of the high power incident beam 24 and the first reflected beam 26. This geometry provides a compact arrangement of the two reflecting windows, and directs the beam ultimately measured away from the high power region. In addition, the two windows being at 45 degrees to the beam but perpendicular to each other, largely cancels out any variation due to polarization in the power of the second reflected beam. However, it is to be understood that any other geometric arrangement of the windows may be used, according to the convenience of the measurement setup available. An alternative particularly convenient arrangement is shown in Fig. 2 hereinbelow.

[0038] If sapphire windows are used, then, as indicated hereinabove, for a 1 MW incident laser beam 24, the reflected beam 26 from the first entrance window 20, will be of the order of lOOkW, and the reflected beam 27 from the second window 21, if it too is sapphire, will have a power level in the order of lOkW. Such a power level can be conveniently absorbed by a beam dump 28, which could be the absorber surface 28 of a 10 kW power meter. An additional and alternative power measuring scheme may use a reflecting beam splitter 29 and a Fresnel lens 30 to reflect an even smaller portion of the incident beam into an even lower rated power meter 31, to provide even faster measurement ability. If the beam splitter 29 reflects 10% of its input beam, the remaining beam power level is of the order of 1000W, which can be even more easily measurable than a 10 kW beam. Such a system based on reflection of the beam from uncoated windows with a stable reflection percentage, enables accurate measuring of the laser power, especially for unpolarized beams, which is often the case for very high -power lasers.

[0039] In addition to the measurement of the beam power, the system is able to measure other parameters of the laser beam, especially the beam profile. One exemplary method of achieving this is by using an imaging camera 32, which can view the beam scattered from an absorber plate 28 on which the final beam 27 is incident, either by reflection in the beam splitter 29 from the beam dump absorber plate 28 if the imaging camera is placed horizontally at a side of the system, or, directly if the imaging camera is located beneath the system and is directed upwards to view the light scattered from the absorbing plate 28.

[0040] With regard to the use of a flowing liquid beam absorber, a flow of ordinary water or a water / glycol mixture, absorbs approximately 11% per cm, of 1070 nm laser radiation, which is a wavelength commonly used for very high-power laser emission. This means that after transmission through 55 cm of water, only about 0.15% of the beam remains unabsorbed. Therefore, for a 1 MW incident beam, an absorption length of water of less than 100cm. is amply sufficient to reduce the beam power to negligible levels, from the point of view of beam safety. Thermal simulation shows that for a beam of 1 MW, a flow of approximately 350 liters / minute is sufficient to prevent the coolant from heating up to a level which may bring the coolant close to boiling level. With such a coolant flow rate, a comparatively high pressure differential is required in the cooling vessel 22 between the input port 23 and the output port 25, as high as 3 bar, depending on the geometry of the vessel 22 and the internal diameter of the coolant ports. This pressure differential can be supplied by a coolant pump having a suitable output. Such a high internal pressure would require that the outer bounding walls of the vessel 22 be constructed with strengthening features, such as an increased wall thickness, or the application of strengthening ribs along the outer bounding walls. The exemplary design for the system to use sapphire windows, is in part because of the high strength and good thermal conductivity of a sapphire window, the high thermal conductivity reducing the level of thermal distortion which such an element may undergo when handling a high power beam. Calculations show that for a window absorbing some 80 parts per million / cm, the window will heat up to less than 10°C above the coolant temperature. Maintaining the thermal stability of the sapphire windows can be enhanced by locating the input port 23 such that it directs the input flow of cooling liquid directly onto the back of the input window 20. If the flow system has a cooling system capable of removing the heat added to the coolant, the system can operate for an indefinite time. If the system only circulates the coolant without cooling, the system can operate for about 1 minute before heating up excessively. If an additional ballast tank of 500 liter is added to the coolant circuit, about 5 minutes of operation is possible before heating up to the limit allowed. Cooling coils or an external chiller can also be used to slowly cool down the coolant until another 5 minute session is possible.

[0041] Other wavelengths of the incident laser beam can be accommodated by adding an appropriate absorbing material to the coolant, to adjust the absorption of the coolant at that wavelength to the desired value.

[0042] The use of a large rectangular absorbing vessel 22, such as that shown in Fig. 1, may render the measurement system bulky and unnecessarily heavy. Reference is now made to Fig. 2, which shows a schematic top view of a practical implementation of a beam measurement system using tubing elbows 36 to form a beam absorption vessel of the type shown schematically in Fig. 1. Such a construction forms a convenient and compact configuration. The tubing parts may be assembled such that the first input window 20 and the second input window 21 are mutually disposed and angled such that the beam reflected from the first window is incident directly onto the second window, which itself is aligned such that the reflected beam from the second window is directed out of the plane of the drawing towards the power measurement unit and any other beam characteristic measurement units. Fig. 2 clearly show how, by aligning the coolant input 11 immediately opposite the inside surface of the input partially reflecting widow, the window may be cooled efficiently and uniformly, to ensure long usage life and to reduce thermal stress due to lack of heating uniformity.

[0043] To provide an example of the performance achieved by use of the methods of construction described in Fig. 2, for a 1 MW beam of diameter 80mm, standard 14” stainless steel tubing parts may be conveniently used. Such a measurement system having dimensions of 135cm. x 120 cm. has a weight of only approximately 140 kg, with the absorber / coolant adding approximately another 250kg. The system can be used for measuring a 1MW power incident beam for a duration of up to approximately 1 minute without active cooling, before the heating effect is considered to be excessive. If a coolant circuit having a cooling capacity of at least the power of the entire laser, is used, the system can be operated indefinitely. As a safety measure, a flow meter may be useful, and in addition, a temperature sensor on the input window mount would be important to maintain correct operation of that critical element and to detect overheating.

[0044] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. Furthermore, it is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

CLAIMSWe claim:

1. A power meter for an incident laser beam having a wavelength, the power meter comprising: a vessel adapted to contain a liquid which absorbs at least a part of the incident laser beam; a first window disposed at a first location in an outer bounding wall of the vessel and aligned at an angle relative to the direction of the incident laser beam, such that the first window reflects a minor part of the incident laser beam away from the vessel, and transmits a major part of the incident laser beam into the vessel for absorption by the liquid, the first window having a known reflectivity to the incident laser beam at the wavelength of the incident laser beam and at the aligned angle of the first window relative to the incident laser beam; and a second window disposed at a second location in the outer bounding wall of the vessel and in the path of the minor part of the incident laser beam reflected by the first window, the second window having a known reflectivity at the wavelength of the minor part of the incident laser beam and at the aligned angle of the second window relative to the minor part of the incident laser beam, the aligned angle of the second window being such that a majority of the reflected minor part of the incident laser beam is transmitted into the vessel, and a minority of the reflected minor part of the incident laser beam is reflected away from the vessel, wherein using the known reflectivity of the incident laser beam from the first window, and the known reflectivity of the reflected minor part of the incident laser beam from the second window, the power of the incident laser beam can be determined by measurement of the power of the minority of the reflected minor part of the incident laser beam, reflected by the second window.

2. The power meter according to claim 1, wherein the measurement of the power of the incident laser beam reflected from the first window and the second window can be performed at a power level lower than that of the incident laser beam after reflection from the first window only.

3. The power meter according to any of the previous claims, wherein a known absorptivity of the liquid to the incident laser beam wavelength, and a path length of the major part of the incident laser beam in the liquid in the vessel are such that the power of the major part of the incident laser beam in the vessel is reduced to a level which is determined not to cause damage to the vessel, or to cause overheating of the liquid.

4. The power meter according to any of the previous claims, wherein the major part of the power of the incident laser beam reflected from the first window is at least 85% of the incident laser beam power.

5. The power meter according to any of the previous claims, wherein the majority of the reflected minor part of the power of the incident laser beam reflected from the second window is at least 85% of the reflected minor part of the incident laser beam power.

6. The power meter according to any of the previous claims, wherein the vessel is a box-like container having a plurality of outer bounding walls.

7. The power meter according to any of the previous claims, wherein the vessel is an assembly of tubular elements, and the outer bounding walls include end faces of the assembly of tubular elements.

8. The power meter according to any of the previous claims, further comprising an imaging camera directed at a plate on which the final reflected beam is incident, such that the beam profile of the incident laser beam can be measured.

9. The power meter according to any of the previous claims, enabling the measurement of incident laser beams having a CW or average power of as high as at least 1 MW.

10. The power meter according to any of the previous claims, further comprising liquid input and output ports for circulating the cooling liquid through the vessel, and optionally further comprising external active cooling for the liquid.

11. The power meter according to any of the previous claims, wherein at least one of the first window or the second window is adapted to operate as:(i) a window of a liquid beam absorber in which the main part of the incident laser beam is safely absorbed; and(ii) a beam sampling element, separating out a minor part of the incident laser beam, to enable a beam measurement to be performed at a power level reduced from that of the incident laser beam.

12. The power meter according to any of the previous claims, wherein at least the first window is an uncoated window.

13. The power meter according to any of the previous claims, wherein the at least first window is an uncoated sapphire window.

14. The power meter according to claim 12, wherein at least the first window may enable reflections of the laser beam incident on it of more than 5%.

15. The power meter according to claim 12, wherein at least the first window may enable reflections of the laser beam incident on it of more than 10%.

16. The power meter according to claim 12, wherein at least the first window may enable reflections of the laser beam incident on it of up to 15%.

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