Laser power meter for high power applications

Abstract

A power meter capable of measuring the power of a laser beam in the megawatt range. A first partially reflective window is disposed in a wall of a vessel containing a stream of beam-absorbing liquid to reflect a minor portion of the beam and transmit a major portion of the beam into the vessel. A second window is disposed in the wall of the vessel at a second location in the path of the minor portion of the beam reflected by the first window and is aligned so that a substantial portion of the reflected minor portion of the incident laser beam is transmitted into the vessel and a small portion of the reflected minor portion of the beam is reflected toward a power meter that measures the power thereof. Knowledge of the reflectivity of the windows enables power measurement of the laser beam.

Description

Technical Field

[0001] This disclosure describes the technology in the field of power measurement of extremely high power laser beams, particularly continuous-wave (CW) beam power or pulsed average beam power up to 1 megawatt or greater. Background Technology

[0002] The most common form of power meter used to measure the power of laser beams at medium to high power levels uses a beam collector that absorbs the laser beam. The power is dissipated by a cooling hotspot, where cooling at higher power levels typically uses a flow of coolant such as water, while at lower power levels, even a flow of cooling air from a fan is used. The temperature drop along the thermal path between the region of the beam absorber and the cooling hotspot should be proportional to the beam power, and measuring this temperature difference determines the beam power. For even higher power levels, the beam may be reflected and spatially diverged, for example, by a reflective cone, and the diverged beam is absorbed on the outer surface of a large-area hotspot through which the coolant passes. Measuring the temperature increase of the coolant (proportional to the power dissipated into the coolant) provides a measure of the beam power. Such power meters have been extensively described, for example, the Ophir Optronics models 10K-W-BB-45 and 120K-W supplied by Ophir Optronics Solutions Ltd. in Jerusalem, Israel, and as described on pages 87 and 89 of the publication "Ophir Power Meter Catalog 2024".

[0003] The ability of such power meters to handle extremely high-power laser beams depends on both the beam absorber's ability to locally withstand the power density of the incident laser beam and the thermal bath's ability to remove heat generated within the beam absorber. Many beam absorbers are constructed of copper with absorbent ceramic surfaces that provide good beam absorption properties and are extremely durable, although other beam-absorbing layers have also been used. However, with the development of laser systems with increasingly higher beam powers, it has become problematic to attempt to scale up the aforementioned existing concepts of beam power measurement to match the increasing power of the laser beams to be measured. This is particularly problematic when laser beam powers reach the hundreds of kilowatts or even megawatts or higher, where it becomes apparent that such beam absorbers cannot withstand the incident laser power required to absorb it for measurement. Furthermore, as the weight of the absorbing elements increases, the absorber's response time to the laser power beam becomes longer, which prolongs the measurement time and reduces the ability to monitor changes in the output power level of the laser beam.

[0004] Measurements of extremely high-power beams can also be performed as follows: a beam splitter is used to capture a small portion of the beam, typically between 1% and 5%, and this captured portion is measured in a power meter with a power level equivalent to that of the split beam. This procedure is commonly used in high-power industrial material handling laser systems, where the main portion of the beam passes through for laser processing of the workpiece (whether for cutting, welding, or heat treatment), while the captured portion of the beam can be easily measured on a power meter with a lower power rating. This application is important in such systems because a rapid response to power measurements is required to provide the optimized responsiveness feedback needed for such material handling machines, and this lower power meter has a faster response than a full-power power monitor used for laser handling machines. One remaining issue with this configuration is that the beam splitter of the laser power meter being sampled is typically installed in the ambient air and must therefore be able to withstand the full-power beam passing through it without damage.

[0005] In beam splitter-type laser power meters used only for measuring beam power without requiring the beam being measured, this problem has been solved by using a cooling liquid that contacts the beam splitter. The liquid acts not only as a coolant for the beam splitter but also as a beam collector. If the cooling liquid is a rapidly flowing cross-flow, many of the problems associated with using solid beam absorbers can be resolved.

[0006] An example of such a laser power meter using this technology is described in Chinese Patent CN114543988B, entitled "Laser Power Meter" and issued to Wuhan Raycus Fiber Laser Technologies Co., Ltd. It shows a beam splitter in the beam path, the opposite side of which is cooled by a flowing water stream, separating a portion of the beam for measurement. However, while this technology may be applicable to CW beams up to 100 kilowatts, measuring even 1% of a megawatt beam, with an acceptablely short measurement response time, presents a daunting task when measuring beams in the megawatt range.

[0007] The disclosure of each publication mentioned in this and other chapters of this manual is hereby referred to in its entirety and is intended to be understood by reference. Summary of the Invention

[0008] This disclosure attempts to provide novel and inventive systems and methods that overcome at least some of the shortcomings of existing systems and methods. This disclosure describes a new exemplary system for measuring the power of laser beams up to 1 megawatt or greater, using a vessel containing a rapidly flowing coolant liquid (typically water or a mixture of water and propylene glycol), which also acts as a beam-absorbing element.

[0009] For extremely 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 lower-power power meter. This may be particularly true if an uncoated window, such as a sapphire window, is used, reflecting approximately 10% of the unpolarized beam in the near-infrared region (e.g., around 1.07 micrometers). Such an uncoated window may even be considered essential for measuring this power level because it is less susceptible to damage from incident high-power laser beams and also undergoes slower aging than windows with multilayer dielectric coatings. Therefore, while multilayer dielectric-coated beam splitters may have the advantage of separating a smaller portion of the beam for measurement compared to the 10% separation by a sapphire beam splitter, they are more susceptible to damage and aging. Consequently, the use of single-reflection beam splitter measurement devices (such as those described in the aforementioned CN114543988B patent) is problematic for megawatt-range power measurement applications.

[0010] To take advantage of uncoated beam splitter windows (e.g., sapphire windows) in the power meter system described herein, a flowing water beam absorber is described, having two beam splitters in the wall of a flowing water vessel, wherein a relatively high percentage of laser power reflected from a first window is directed toward a second beam splitter window. This second beam splitter window also reflects at the beam wavelength, such that the majority percentage of the beam enters the vessel to be absorbed by the flowing coolant therein, while the remaining smaller percentage of the beam is reflected from this second window. Even when the system is limited to using uncoated beam splitter windows (at least for the first window), the reflected portion of the beam should still be at a sufficiently low power level for it to be processed by a lower-rated power meter. The angle at which the second window is aligned should direct the second reflection of the beam toward a convenient direction in which it can be measured without interference from the incident beam reflected from the first inlet window. Using this dual-window architecture, it is possible to absorb 99% of the laser power in the impact in the absorber / coolant vessel, making it convenient to use a 10-kilowatt power meter to measure the power in the space pointed to by the reflected beam for a 1-megawatt beam measurement.

[0011] Coolant absorbers for such extremely high-power beams have been previously used as extremely high-power beam collectors, often redirecting the extremely high-output-power beam from top to bottom onto the surface of coolant flowing in an open vessel. However, this method can result in some potentially dangerous backscattering from the surface of the rapidly flowing coolant. This negative effect is almost entirely absent in the system described herein because the beam enters the coolant absorber through an input window behind which is a surface in contact with the flowing coolant, and the beam enters at a well-defined angle of incidence relative to the beam direction, allowing for reliable safety precautions. The backscattering of the system disclosed herein can be reliably maintained at less than 0.1% of the incident beam and with a substantially unchanged spatial distribution.

[0012] In addition to power measurement, the beam profile can also be determined by positioning an additional beam splitter in the path of the lower-power beam after the second beam splitting action, and positioning an impact plate for the imaging device to view and record the beam profile. Alternatively, the imaging device can directly view the scattered light from the final beam collector.

[0013] Therefore, according to an exemplary embodiment of the device disclosed herein, a power meter is provided for an incident laser beam having a wavelength, the power meter comprising: (i) A vessel, prepared to contain a liquid that absorbs at least a portion of the incident laser beam. (ii) A first window, disposed at a first position on the outer wall of the vessel boundary and aligned at an angle relative to the direction of the incident laser beam, such that the first window reflects a minor portion of the incident laser beam away from the vessel and transmits the major portion of the incident laser beam into the vessel for absorption by the liquid, wherein the first window has a known reflectivity to the incident laser beam at the wavelength of the incident laser beam and at the alignment angle of the first window relative to the incident laser beam, and (iii) A second window, disposed at a second position in the outer wall of the vessel boundary and in the path of the secondary portion of the incident laser beam reflected by the first window, having a known reflectivity at the wavelength of the secondary portion of the incident laser beam and at an alignment angle of the second window relative to the secondary portion of the incident laser beam, the alignment angle of the second window being such that the majority of the reflected secondary portion of the incident laser beam is transmitted into the vessel, and a small portion of the reflected secondary portion of the incident laser beam is reflected away from the vessel. The power of the incident laser beam can be determined by measuring the power of a small portion of the reflected secondary portion of the incident laser beam reflected by the second window, using the known reflectivity of the incident laser beam from the first window and the known reflectivity of the reflected secondary portion of the incident laser beam from the second window.

[0014] In this type of power meter, the power measurement 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 it is reflected only from the first window.

[0015] In any power meter disclosed herein, the known absorptivity of the liquid to the wavelength of the incident laser beam and the path length of the main portion of the incident laser beam in the liquid of the vessel reduce the power of the main portion of the incident laser beam in the vessel to a level determined not to cause damage to the vessel or to cause overheating of the liquid.

[0016] Furthermore, the power of the main portion of the incident laser beam reflected from the first window is likely to be at least 85% of the incident laser beam power. Incidentally, the power of the majority of the reflected minor portion of the incident laser beam reflected from the second window is likely to be at least 85% of the reflected minor portion of the incident laser beam power.

[0017] According to another embodiment of the power meter described in this disclosure, the vessel may be a box-shaped container having several boundary outer walls. Alternatively, the vessel may be an assembly of tubular elements, and the boundary outer walls include the end faces of the tubular element assembly.

[0018] Incidentally, any power meter described herein may further include an imaging camera that is pointed at the plate into which the ultimately reflected beam is incident, such that the beam profile of the incident laser beam can be determined.

[0019] Any power meter disclosed herein may be able to perform measurements of incident laser beams with a CW or average power as high as at least 1 megawatt.

[0020] Furthermore, any power meter described herein should further include liquid inlet and outlet ports for circulating cooled liquid through the vessel. Incidentally, any of the aforementioned power meters may further include external active cooling for the liquid.

[0021] According to other embodiments of the power meter described herein, at least one of the first window or the second window is adapted to operate as follows: (i) a window of a liquid beam absorber in which the main portion of the incident laser beam is safely absorbed; and (ii) a beam sampling element that separates the secondary portion of the incident laser beam, enabling beam measurement at a power level with a substantial reduction in the incident laser beam.

[0022] Finally, in any of the power meters described above, at least the first window may be an uncoated window. This uncoated window may be a sapphire window.

[0023] It should be noted that a particular element of the system may have more than one function or more than one accepted name, and may be referred to in various ways in this disclosure under alternative names. Some such examples are partially reflective elements, also known as beam inlet windows or beam absorbing liquids, also known as coolants or beam cooling liquids for their dual-purpose purpose; or known as beam absorbing plates, which may be freestanding elements, or partial laser power meters, and other minor examples. It should be understood that such alternative naming is not intended to distinguish between different elements, but simply as alternative names that may have been randomly used in this disclosure and intended to be associated with the same element. Attached Figure Description

[0024] The following detailed description of the accompanying diagrams will provide a more complete understanding and appreciation of the invention:

[0025] Figure 1 An exemplary diagram schematically demonstrates a power meter system for measuring the power and other properties of extremely high-power laser beams, using two-part reflective windows containing a vessel with a liquid capable of absorbing the laser beam; and

[0026] Figure 2 A schematic top view of a practical embodiment of the demonstration beam measurement system, as shown. Figure 1 While using two separate reflection input windows, the absorber is constructed from piping, allowing for better compactness. Detailed Implementation

[0027] For reference Figure 1 This schematically demonstrates the method by which the currently described power meter achieves its purpose. The operation of the new power meter is based on the fact that, at the wavelength of interest (typically in the 1070 nm region), ordinary water or a 50:50 mixture of water and propylene glycol (often used for corrosion resistance) has a known absorptivity of up to 11% per centimeter of absorption depth. This means that if a beam travels, for example, through 55 cm of absorber / coolant, only 0.15% of the beam remains unabsorbed. The shape of the vessel can be selected to suit the available volume and manufacturing cost. Figure 1 The shapes of the vessels shown are illustrative and intended only to demonstrate the operating principle of the system. Figure 1The absorption / cooling vessel 22 is schematically shown as a simple rectangle, thus forming a box-like container structure with several boundary walls. A very high-power incident beam 24 enters the vessel through a reflection window 20 located in the boundary walls of the vessel, where a major percentage of the beam 33 enters the vessel 22 and is absorbed by the rapidly flowing coolant. Although the beam 33 in the coolant is shown as a simple extension of the incident beam, it should 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 to guide the high-speed flow of coolant, in order to prevent the coolant from overheating due to the absorption of a 1-megawatt or larger laser beam.

[0028] If the reflector window is made of uncoated sapphire, it reflects approximately 10% of the incident beam and transmits the remainder into the coolant liquid. The device advantageously utilizes reflections from the uncoated window, which has a stable percentage of reflection, thereby enabling accurate monitoring of the laser power, particularly for non-polarized beams, such as those often found in extremely high-power lasers. While using an uncoated sapphire window necessitates measuring the 10% of the incident beam power reflected by the uncoated sapphire window, it is conceivable that other windows with even higher power reflections (e.g., even 15%) could be used, although the greater the power level reflected from the window, the less advantage the currently described system offers. The advantage of using an uncoated window in such high-power laser power monitoring can therefore be described as the window used reflecting more than 5%, or even more than 10%, or even up to 15%, of the full power of the incident laser beam encountered by the first window.

[0029] A secondary portion of the incident beam 26, reflected in the air by the first input window 20, is now introduced into the vessel 22 through the second input window 21 and is largely absorbed in the absorbing cooling liquid. A secondary portion 27 of this second input beam 26 (not transmitted into the absorbing vessel 22) is reflected in a direction away 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 explained further below. The absorbing cooling liquid flows from the input port 23 through the vessel 22 to the output port 25, providing a sufficiently fast flow to remove the heat generated by the absorbed beam, as will be explained further below. Figure 1 When constructed as described, windows 20 and 21 have a dual functional purpose: (i) to input a laser beam into a flowing liquid beam absorber; and (ii) to act as a beam sampling element, separating a small portion of the incident beam power, thereby enabling beam measurement at a power level that substantially reduces the power of the extremely high-power incident beam.

[0030] To provide a convenient geometric configuration and positioning of the reflected beam for the absorption / cooling vessel, the first window 20 may be advantageously vertically aligned (relative to...). Figure 1 The direction of the diagram is 45° to the direction indicated by the incident laser beam, and the second window 21 may be aligned in a direction perpendicular to the direction indicated by the incident laser beam, but tilted at a 45° angle to the vertical direction, so that the second reflected beam 27 is directed upward in the air (relative to the direction indicated by the incident laser beam). Figure 1 The orientation of the windows (as shown in the diagram) is away from the plane of the high-power incident beam 24 and the first reflected beam 26. This geometry provides a compact configuration of the two reflection windows and directs the beam to be measured away from the high-power region. Incidentally, the two windows are at a 45-degree angle to the beam but perpendicular to each other, greatly eliminating any variation caused by polarization in the power of the second reflected beam. However, it is possible to understand any other geometric configurations of the windows that may be used depending on the convenience of the available measurement settings. Particularly convenient alternative configurations are shown below. Figure 2 .

[0031] If a sapphire window is used, as noted above, for a 1 MW incident laser beam 24, the reflected beam 26 from the first entrance window 20 will be in the 100 kW range, and the reflected beam 27 from the second window 21 (if it is also sapphire) will have a power level in the 10 kW range. This power level can be conveniently absorbed by a beam collector 28, which may be the absorber surface 28 of a 10 kW power meter. An additional and alternative power measurement scheme may use a beam splitter 29 and a Fresnel lens 30 to reflect a smaller portion of the incident beam into a lower-rated power meter 31 for faster measurement capabilities. If the beam splitter 29 reflects 10% of the input beam, the remaining beam power level is in the 1000 watt range, which can be measured even more easily than a 10 kW beam. This system, based on reflecting the beam from an uncoated window with a stable percentage of reflection, can accurately measure laser power, especially for unpolarized beams, which are often extremely high-power lasers.

[0032] In addition to measuring beam power, the system can also measure other parameters of the laser beam, particularly the beam profile. An exemplary method to achieve this is to use an imaging camera 32 to view the beam scattered from the absorber plate 28 incident on the final beam 27, either by placing the imaging camera horizontally to the side of the system and viewing the reflection from the absorber plate 28 of the beam collector at the beam splitter 29, or by placing the imaging camera under the system and pointing it upwards to view the light scattered from the absorber plate 28 directly.

[0033] Regarding the flowing liquid beam absorber used, a flow of ordinary water or a water / diol mixture absorbs approximately 11% of the 1070 nm laser radiation per centimeter, a wavelength typically used for extremely high-power laser emission. This means that after passing through 55 cm of water, only about 0.15% of the beam remains unabsorbed. Therefore, for a 1 MW incident beam, from a beam safety perspective, a water absorption length of less than 100 cm is sufficient to reduce the beam power to a negligible level. Thermal simulations show that for a 1 MW beam, a flow rate of approximately 350 liters per minute is sufficient to prevent the coolant from heating to a level that could bring it close to boiling. With this coolant flow rate, a relatively high pressure differential (up to 3 bar) is required between the inlet port 23 and the outlet port 25 of the cooling vessel 22, depending on the geometry of the vessel 22 and the inner diameter of the coolant port. This pressure differential can be supplied by a coolant pump with a suitable output. This high internal pressure necessitates the construction of the outer boundary wall of vessel 22 with reinforced properties, such as increased wall thickness or the application of reinforcing ribs along the boundary wall. The exemplary design of using a sapphire window is partly due to its high strength and good thermal conductivity, which reduces the degree of thermal distortion this element may experience when handling high-power beams. Calculations show that for a window absorbing approximately 80 parts per million per centimeter, it will heat to less than 10°C above the coolant temperature. Maintaining the thermal stability of the sapphire window can be improved by positioning the inlet port 23 so that it directs the coolant inlet flow directly to the back of the inlet window 20. If the flow system has a cooling system capable of removing the heat added to the coolant, the system can operate continuously for extended periods. If the system only circulates the coolant without cooling, the system can operate for approximately 1 minute before overheating. If an additional 500-liter pressurized tank is added to the coolant circuit, it may operate for approximately 5 minutes before heating to the permissible limit. Alternatively, a cooling coil or external cooler can be used to slowly cool the coolant until there may be another 5-minute interval.

[0034] By adding appropriate absorbing materials to the coolant, the absorption of the coolant at that wavelength can be adjusted to the desired value to accommodate other wavelengths of the incident laser beam.

[0035] Use a large rectangular absorbent 22 (e.g.) Figure 1 (As shown) this could make the measurement system large and unnecessarily heavy. See now for reference. Figure 2 A schematic top view showing an actual embodiment of the beam measurement system, using a tube elbow 36 to form Figure 1The schematically shown beam absorber is of a type that creates a convenient and compact architecture. The piping may be assembled such that the first input window 20 and the second input window 21 are configured at an angle to each other, such that the beam reflected from the first window is directly incident on the second window, which is itself aligned such that the reflected beam from the second window is directed away from the plane of the diagram toward the power measurement unit and any other beam characteristic measurement unit. Figure 2 The diagram clearly shows how by precisely aligning the coolant input 11 with the inner surface of the partially reflective input window, the window can be cooled efficiently and uniformly, ensuring a long service life and reducing thermal stress caused by a lack of heating uniformity.

[0036] In order to provide usage Figure 2 The performance example achieved by the described construction method, for a 1-megawatt beam with a diameter of 80 mm, can be conveniently achieved using standard 14-inch stainless steel piping. This measurement system has dimensions of 135 cm × 120 cm and weighs only approximately 140 kg, with an additional approximately 250 kg for the absorbent / coolant. The system can be used to measure an incident beam of 1 megawatt power for up to approximately one minute without active cooling, before the heating effect is considered excessive. If a coolant circuit with cooling capacity of at least the entire laser power is used, the system can operate continuously. For safety, a flow meter may be useful, and incidentally, a temperature sensor on the input window mount is important to maintain the proper operation of this critical component and detect overheating.

[0037] Exemplary embodiments are provided so that this disclosure will be thorough and fully convey its scope to those skilled in the art. Numerous specific details, such as examples of particular components, apparatus, and methods, are listed to provide a thorough understanding of the embodiments of this disclosure. Those skilled in the art will understand that specific details are not necessary, and that exemplary embodiments may be embodied in many different forms, neither of which should be construed as limiting the scope of this disclosure. Furthermore, those skilled in the art recognize that the invention is not limited to what has been specifically shown and described herein. Rather, the scope of the invention includes both combinations and sub-combinations of the various features described above, as well as variations and modifications that would occur to those skilled in the art upon reading the foregoing description and not found in the prior art.

Claims

1. A power meter for an incident laser beam having a wavelength, the power meter comprising: A vessel, adapted to contain a liquid that absorbs at least a portion of the incident laser beam; A first window, disposed at a first position on the outer wall of the vessel boundary and aligned at an angle relative to the direction of the incident laser beam, such that the first window reflects a minor portion of the incident laser beam away from the vessel and transmits the major portion of the incident laser beam into the vessel for absorption by the liquid, and that the first window has a known reflectivity to the incident laser beam at the wavelength of the incident laser beam and at the aligned angle relative to the incident laser beam; and A second window, disposed at a second position in the outer wall of the vessel's boundary and in the path of a secondary portion of the incident laser beam reflected by the first window, at that wavelength of the secondary portion of the incident laser beam and at an aligned angle relative to the secondary portion of the incident laser beam, has a known reflectivity. The aligned angle of the second window is such that the majority of the reflected secondary portion of the incident laser beam is transmitted into the vessel, and a small portion of the reflected secondary portion of the incident laser beam is reflected away from the vessel. The power of the incident laser beam is determined by measuring the power of a small portion of the incident laser beam reflected from the second window, using the known reflectivity of the incident laser beam from the first window and the known reflectivity of the secondary portion of the incident laser beam reflected from the second window.

2. The power meter of claim 1, wherein the measurement of the power of the incident laser beam reflected from the first window and the second window is performed at a power level lower than that of the incident laser beam after it has been reflected only from the first window.

3. The power meter according to any one of the preceding claims, wherein the known absorptivity of the liquid to the wavelength of the incident laser beam and the path length of the main portion of the incident laser beam in the liquid of the vessel reduce the power of the main portion of the incident laser beam in the vessel to a level determined not to cause damage to the vessel or overheating of the liquid.

4. The power meter according to any one of the preceding claims, wherein the power of the main portion of the incident laser beam reflected from the first window is at least 85% of the power of the incident laser beam.

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

6. The power meter according to any one of the preceding claims, wherein the vessel is a box-shaped container having a plurality of boundary outer walls.

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

8. The power meter according to any one of the preceding claims, further comprising an imaging camera pointed at the plate into which the reflected final beam is incident, such that the beam profile of the incident laser beam is measured.

9. The power meter according to any one of the preceding claims, which is capable of performing the measurement of the incident laser beam having a continuous wave (or average power) of at least 1 megawatt.

10. The power meter according to any one of the preceding claims, further comprising a liquid inlet and an outlet port for circulating the cooled liquid through the vessel, and optionally further comprising external active cooling for the liquid.

11. The power meter according to any one of the preceding claims, wherein at least one of the first window or the second window is adapted to operate as follows: (i) A window of a liquid beam absorber, wherein the main portion of the incident laser beam is safely absorbed; and (ii) A beam sampling element that separates the secondary portion of the incident laser beam, enabling beam measurement at a power level that is reduced from the power level of the incident laser beam.

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

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

14. The power meter of claim 12, wherein at least the first window is capable of reflecting more than 5% of the incident laser beam incident on the first window.

15. The power meter of claim 12, wherein at least the first window is capable of reflecting more than 10% of the incident laser beam incident on the first window.

16. The power meter of claim 12, wherein at least the first window is capable of reflecting up to 15% of the incident laser beam incident on the first window.

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