IL311259B2 - Laser power meter for high power applications - Google Patents

Description

LASER POWER METER FOR HIGH POWER APPLICATIONS FIELD 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. BACKGROUND 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, at higher powers, generally a flow of coolant, such as water, and at lower powers, even 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 of the publication "2024 Ophir Power Meter Catalog". 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. 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. SUMMARY 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 even 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, and a partially reflective optical element disposed in the beam path before entering the absorbent/coolant, and which reflects a small percentage of the incident laser beam. The majority of the beam enters the coolant vessel, where it is safely absorbed by the fast flowing stream of the absorbent/coolant. The power measurement itself is made on that small reflected percentage of the incident beam, at a significantly lower power level than the incident beam. Such a reflected beam can be more readily handled and measured by conventional high power laser power meters, such as those described in the Background section above. The combination of these two features is achieved by using the partially reflective element as the beam entrance window of the absorber/coolant vessel, through which the beam enters the absorbing coolant volume. The window has to be aligned at an angle to the beam entry direction, conveniently though not necessarily, at 45°, such that the laser beam is not reflected back upon itself. An important inventive feature of the present system is the bi-functional use of a window (i) for inputting the beam into a beam absorbing vessel containing a flowing liquid beam absorber, and (ii) as a beam sampling element to enable the beam measurement to be performed at a power level substantially reduced from that of the very high power incident beam . For very high power beam measurements, such as in the megawatt range, the power reflected 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 1.07µm. Such an uncoated window has the advantages that it is less liable to be damaged by the incident very high power laser beam, and also undergoes ageing more slowly than a window with a multiple layer dielectric coating, which can be designed to provide a lower beam reflection, and hence a more convenient power measurement level, but is more prone to damage and ageing changes. In a case such as that of the uncoated sapphire window, it is possible, according to another implementation of the presently described power meter systems, to provide a second beam splitter entrance window to the coolant vessel, at which the remaining percentage of the laser power reflected from the first window is directed. This second window may too have a reflection at the beam wavelength such that a major percentage of the beam is re-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. 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 environment of the space where the reflected beam is directed. Coolant absorbers 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 window, behind which there is a static interface 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. In addition to the power measurement, the beam profile can be determined by positioning a beam splitter in the path of the low power reflected beam, 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 beam dump. 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: (i) a vessel adapted to contain a liquid which absorbs at least a part of the power of the incident laser beam; and (ii) a first window disposed 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 power of the incident laser beam away from the vessel, and transmits a major part of the power of the incident laser beam into the vessel for absorption by the liquid, the first window having a known reflectivity to the laser beam at the wavelength of the laser beam and at the aligned angle of the window to the laser beam, wherein using the known reflectivity of the incident laser beam from the first window, the power of the incident laser beam can be determined by measurement of the power of the minor part of the incident laser beam reflected by the first window. Such a power meter may further comprise 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 incident laser beam and at the aligned angle of the second window to the reflected minor part of the incident laser beam, the second window being aligned at an angle 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 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. In such a power meter, the measurement of the power of the 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 described in this 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 should be 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. In any of the power meters described in this disclosure, having only a first window, the major part of the power of the incident laser beam should be at least 85% of the incident laser beam power. Additionally, in any of such power meters having a first window and a second window, the majority of the reflected minor part of the power of the incident laser beam should be at least 85% of the reflected minor part of the incident laser beam power. According to yet further implementations of the power meters described in this disclosure, the vessel may be a box-like container having a plurality of outer bounding walls. Alternatively, the vessel may comprise an assembly of tubular elements, and the outer bounding walls include end faces of the assembly of tubular elements. 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. Furthermore, any of the above-described power meters may enable measurement of laser beams having a CW or average power of as high as at least 1 MW.

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. 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: (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. There is further provided in accordance with yet another exemplary implementation of the power meters devices described in this disclosure, a power meter for a incident laser beam having a wavelength, the power meter comprising: (i) a vessel adapted to contain a liquid which absorbs at least a part of the incident laser beam; (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 (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. There is further provided in accordance with an exemplary implementation of the methods described in this disclosure, a method of enabling measurement of a high power laser beam, comprising the steps of: (i) directing the high power laser beam through an optical window into an absorber vessel containing a liquid adapted to absorb any part of the high power laser beam that enters the absorber vessel; and (ii) measuring a minor part of the high power laser beam reflected from the window, at a power level significantly lower than that of the high power laser beam, wherein the window is adapted to operate both as: (a) the entrance window of the absorber vessel in which a major part of the incident laser beam is safely absorbed; and as (b) a beam sampling element, separating out the minor part of the high power laser beam, to enable a beam measurement to be performed at a power level substantially reduced from that of the incident laser beam. The substantially reduced power level could be a reduction of at least 80% or at least 90% or at least 95%, depending on the fraction of the high power laser beam reflected from the window. According to a further method of enabling measurement of a high power laser beam, the minor part of the high power laser beam reflected from the window may be directed onto a second window, where the majority of that minor part of the high power laser beam is input into the absorber vessel, and a smaller part of the laser beam incident on the second window is reflected thereof, at a power level significantly lower than that of the laser beam incident on the second window, thereby enabling measurement of the high power laser beam at an even lower fraction of the power of the high power laser beam. Finally, 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. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig.1 illustrates schematically an exemplary representation of the method by which the power meters described in the present disclosure achieve their objective of measuring the power and other properties of a very high power laser beam, using a partially reflecting window of a vessel containing a liquid capable of absorbing the laser beam; Fig. 2 illustrates schematically a development of the system shown in Fig. 1, using two partially reflecting input windows, enabling use of a lower rated power indicating meter, in order to provide faster response times for the power measurement; Fig. 3 shows a further exemplary implementation of the power measuring generic device shown in Fig 1, but using an absorption/cooling vessel made of tubing, preferably stainless steel tubing, to enable a more compact and lighter configuration than the box-like vessel of Fig. 1; and Fig. 4 illustrates a schematic top view of a practical implementation of a beam measurement system using two partially reflecting input windows, as in Fig. 2, but with the absorption vessel constructed of tubing of the type shown in Fig. 3 to enable good compactness. DETAILED DESCRIPTION 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 completely schematic, and is intended only to show the principle of operation of the system. In Fig. 1, the vessel 10 is shown schematically as a simple rectangular shape, so that the vessel 10 is a box-like container structure having a plurality of outer bounding walls. The incident very high power beam 13 is input into the vessel through an entrance partial reflecting window 14, disposed in an outer bounding wall of the vessel, at which the major percentage part 15 of the beam enters the vessel 10 and is absorbed by the fast flowing coolant. Although the beam 15 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 11 and a coolant outlet 12 through which a high speed flow of coolant is directed, in order to keep the coolant from overheating from the absorption of the 1MW laser beam. The smaller part 16 of the beam is reflected from the input window 14 towards the power meter 17, 18. 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 power meter for measuring the reflected beam comprises an absorbing head 17 and a read-out unit 18. 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. Reference is now made to Fig. 2 which shows, as outlined in the summary section hereinabove, a development of the basic conceptual system shown in Fig. 1, to enable use of a lower power-rated power indicating meter, in order to provide faster response times for the measurement, and a simpler power meter. The implementation of Fig. 2 differs from that in Fig. 1 in that two partially reflective windows 20, 21, are used. The very high power laser beam 24 is incident on window 20, and the major part of the incident beam is transmitted through the window 20 into the absorbing/cooling vessel 22. 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. 2, 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. 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. 2) 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 is directed in the air upwards (relative to the directions in the drawing of Fig. 2) 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 Figs. 3 and 4 hereinbelow. 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 100kW, and the reflected beam 27 from the second window 21, if it too is sapphire, will have a power level in the order of 10kW. Such a power level can be conveniently absorbed by a beam dump 28, which could be the absorber surface of a 10 kW power meter. An additional and alternative power measuring scheme may use a reflecting beam splitter and a Fresnel lens 30 to reflect an even smaller portion of the incident beam into an even lower rated power meter, to provide even faster measurement ability. If the beam splitter reflects 10% of its input beam, this 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. 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. 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 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 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. 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. The use of a large rectangular absorbing vessel 22, such as that shown in Fig. 2, may render the measurement system bulky and unnecessarily heavy. Reference is now made to Fig. 3 which shows a generic rendering of a further exemplary implementation of the power measuring devices of the present disclosure, but having a more compact and lighter configuration, which is also substantially less costly to manufacture. Fig. 3 shows an example of this improved structure, applied to the generic system shown of Fig. 1. In Fig. 3, the required absorption length for the very high power beam is achieved by use of a tube 35 The beam is gradually absorbed in the coolant until a negligible amount of the beam is left at the end of the tubular container. The tubing may advantageously be of stainless steel, which has high strength and low corrosion properties. This tubing configuration can thus withstand the required pressures with much thinner walls than a rectangular box-like structure, thus making the measurement system much lighter in weight. Like the box-shaped embodiment of Fig. 2, the tube embodiment can also operate either connected to a cooling circuit or for a limited time by just recirculating the coolant without removing the heat from the coolant. Reference is now made to Fig. 4, 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 generically in Fig. 3. 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. Figs. 3 and 4 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. To provide an example of the performance achieved by use of the methods of construction described in Fig. 4, 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. 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 (11)

1. 311259/ CLAIMS We 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. 311259/

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 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. 311259/

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.