Optical power meter for safe operation of optical wireless power system

Abstract

A system for measuring power of a laser beam includes a substantially opaque housing from which the laser beam is directed through an exit aperture. The housing includes: a beam splitter configured to transmit a primary portion of the laser beam through the exit aperture and to reflect a secondary portion of the laser beam; a diffuser element positioned such that a reflected secondary portion of the laser beam impinges thereon; at least one detector element in optical communication with the diffuser element, the detector element providing a signal in response to diffused light of the secondary portion of the laser beam impinging thereon; and an absorber element positioned such that a portion of any light that enters the housing through the exit aperture and is reflected by the beam splitter impinges on the absorber element and is substantially absorbed.

Description

[0001] This application is a divisional application of the patent application with the application date of November 1, 2020, the name of “Optical power meter for safe operation of optical wireless power system”, and the application number of 202080076439.6. TECHNICAL FIELD

[0002] The present invention relates to the field of safety systems for remote charging of mobile devices by providing accurate power meters for laser-based charging systems, in particular for preventing overexposure to laser light. BACKGROUND

[0003] In recent years, many battery-powered mobile systems have been developed, whose batteries typically have a capacity of about 1-60 Wh, which allows these devices to operate for hours to weeks before needing to be charged. The charging time of such mobile systems is typically shorter than 12 hours, and therefore the charger needs to provide about 1-5 W of power to charge the battery within this time range. Remote laser charging systems have been developed for such mobile systems, which convert optical energy into electrical energy and do not require the mobile device to be connected to the power source by an electrical cord while charging. Considering the conversion efficiency between optical power and electrical power, the optical power of the light beam carried by the optical charging system is typically several watts.

[0004] Ensuring safety is a requirement for remote, laser-based charging systems. The accessible emission limit (AEL) of Class I lasers is defined in the United States by federal regulation 21 CFR ~ 1040 and other documents. Class 1 lasers are safe under all normal conditions of use. This means that when the laser is observed with the naked eye or with the aid of typical magnifying optics (such as a telescope or microscope), the maximum permissible exposure (MPE) cannot be exceeded. Human exposure to power higher than the acceptable time or power limit does not make the system classified as a Class I laser, thus making such lasers unsuitable for general public use.

[0005] Due to the above-mentioned regulations, the permissible exposure time of a light beam with a power of several watts is very short, about microseconds for a 7 mm Gaussian light beam of 1 W at 1060 nm, so an effective active safety system is needed to promote automatic beam shutdown before the exposure time of the laser exceeds the permissible duration. Any such safety system needs to accurately measure the optical power emitted by the emitter to ensure safe operation.

[0006] Most current laser power transmission systems do not include a power meter, or include a power meter that can not be suitable to provide accurate results in the prolonged operation of the device. Such prior art systems can not take into account the changes in the shape of the light beam, changes in wavelength, or changes in the optical properties of the system itself that naturally occur over time. Therefore, their reliability is not sufficient to ensure safe operation over a long period of time.

[0007] For example, the system described in US 2007 / 0019693 "Wireless power beaming to common electronic devices" by D.S. Graham uses a photodiode (28) to measure the back reflection from the lens. This method is power efficient as it only uses light that would otherwise be wasted as it would not reach the receiver and would not be converted to electrical energy at the receiver. However, the photodiode is sensitive to lens misalignment, dust, wavelength variations (as lenses often have dielectric coatings which are known to be sensitive to wavelength variations), and cannot provide reliable measurements over time without recalibration. It is also very sensitive to illumination of the photodiode from extraneous light sources.

[0008] US 2014 / 0126603 by Della-Pergola, which has common inventors with the present application and is commonly owned by the present applicant, also uses a leaky mirror to direct the beam to a single photodiode. This design can also be susceptible to dust, misalignment, and beam shape variations, especially as lenses are used in some of these configurations. US 9,312,701 "System for optical wireless power supply", which has common inventors with the present application, suggests detecting objects in the beam based on detecting power loss in the beam. US 2014 / 0092929, which also has common inventors and is commonly owned by the present applicant, suggests monitoring the power and shape of the beam to ensure safety.

[0009] Many safety systems rely on measurements of the laser power emitted from the transmitter. Current technology suggests using a power meter to measure the laser power and sometimes specifies coupling some light to a photodiode using the reflection of a "leaky" mirror or lens surface, but fails to provide a system that can maintain an accurately calibrated power meter for long periods of time, thereby requiring frequent recalibration to ensure safe operation.

[0010] Therefore, there is a need for a long-term, accurate, and reliable power meter to ensure safe operation of a remote optical charging system that overcomes at least some of the shortcomings of the prior art systems and methods.

[0011] The disclosures of each of the publications mentioned in this section and in other sections of the specification are hereby incorporated by reference in their entireties. SUMMARY

[0012] A safety system for a laser-based wireless power transmission system is disclosed that ensures safe operation for long periods of time with minimal power loss by using a power measurement module that can ensure reliable and accurate measurement of optical power over long periods of time without the need for recalibration and / or cleaning.

[0013] The power measurement system described in the current disclosure provides long term reliability, safety, and maintenance free operation over more traditional methods that prefer high efficiency and low cost. The system differs from previous systems in that rather than positioning the detector in a direction and position where both reflected laser light and stray light from the environment can easily hit it, the detector is placed near the exit aperture of the laser but facing the laser so that extraneous light entering the enclosure will be absorbed by the walls of the enclosure without hitting the detector. In addition, the power meter of the current disclosure describes many factors that provide stability for the power measurement, low power loss of the main beam and no range loss. These factors include resilience in beam mode, wavelength, changes in temperature, degradation of optical elements and changes in transmissivity and reflectivity, dust accumulation on optical and electronic surfaces, measurement bias from internal reflections ("ghosting") from the main beam, and measurement bias by external illumination.

[0014] Thus the exemplary implementation of the apparatus described in accordance with the present disclosure provides a system for measuring the power of a laser beam, comprising: a substantially opaque enclosure from which the laser beam is directed through an exit aperture, the enclosure comprising: (i) a beam splitter configured to transmit a primary portion of the laser beam through the exit aperture and reflect a secondary portion of the laser beam; (ii) a diffuser element positioned so that the reflected secondary portion of the laser beam impinges thereon; (iii) at least one detector element in optical communication with the diffuser element, the detector element providing a signal in response to diffused light of the secondary portion of the laser beam impinging thereon; and (iv) an absorber element positioned so that any portion of light entering the enclosure through the exit aperture and reflected by the beam splitter impinges on the absorber element and is substantially absorbed.

[0015] In such a system, the laser is disposed in the enclosure or outside the enclosure and directs its beam into the enclosure.

[0016] In addition, the optical coupling efficiency of the detector to the laser can be substantially greater than the optical coupling of the detector to any other location outside the enclosure.

[0017] Further, the wavefront form of the primary portion of the beam transmitted by the beam splitter should be substantially unaffected by the passage through the beam splitter.

[0018] In any of the above systems, the ratio of the power emitted by the laser beam to the power received by the detector element is substantially fixed when the wavelength of the beam is changed from its average value by up to 3 nm. In addition, the ratio of the power emitted by the laser beam to the power received by the detector element is substantially fixed when the polarization of the beam is changed from its average value. Further, the ratio of the power emitted by the laser beam to the power received by the detector element is substantially fixed when the beam profile of the beam is changed from its average value.

[0019] In any of these systems, the beam splitter can include a transparent front surface and an anti-reflective back surface. In addition, the diffuser element is configured to equalize the signal from the beam across the beam profile. The diffuser element can have a concave shape adapted to uniformly illuminate the detector element.

[0020] With respect to the detector element, a pair of adjacent detectors can be included that are positioned such that the detected beam impinges on both of them. In this case, if the laser beam is generated by a laser diode, the pair of detectors should be arranged such that the fast axis of the laser beam is parallel to the line connecting the centers of the detectors.

[0021] Finally, in all of these systems, the major portion of the laser beam transmitted through the exit aperture can be greater than 80% of the source laser beam. BRIEF DESCRIPTION OF DRAWINGS

[0022] The present claimed invention can be more fully understood and appreciated by reading the following detailed description in conjunction with the accompanying drawings, in which:

[0023] Figure 1 It is shown how the maximum allowed exposure time decreases as a function of power for a 1050 nm wavelength and a 7 mm diameter beam;

[0024] Figure 2A and Figure 2B It is shown some differences between the prior art and the beam splitter and detector disclosed in the present description;

[0025] Figure 3 It is shown an exemplary system for accurately measuring the power of an optical charging system over long periods of time without the need for recalibration;

[0026] Figure 4 It is shown a typical beam profile of a diode laser with variable power across the long and short axes, where hot spots are identified;

[0027] Figure 5 It is shown an exemplary laser illustrating the power variation across the long and short axes;

[0028] Figure 6A and Figure 6BAdditional structural components for improving power meter measurement accuracy are shown; and

[0029] Figure 7 Components of a power meter in an example implementation are shown. DETAILED DESCRIPTION

[0030] Reference is first made to Figure 1 This figure shows the maximum allowed exposure time for a 1050 nm wavelength and a 7 mm diameter beam, showing the drop in allowed exposure time as a function of beam power.

[0031] Reference is now made to Figure 2A It shows a prior art beam delivery system illustrating how a small fraction of the emitted laser beam is reflected using a beam splitter in a direction opposite to its propagation direction and towards a detector for providing a signal that is said to be proportional to the output power. Typically, about 98% of the beam is delivered and 2% of the laser light is reflected backwards to hit the detector. However, since the detector faces the aperture through which most of the laser light is delivered, incident light from the environment entering the aperture will also hit the detector and thus can tamper the measurements obtained.

[0032] Many prior art systems suffer from a design problem that makes them susceptible to external lighting interference since they place the power measurement sensor in the so-called backward or backward direction rather than the forward direction.

[0033] The so-called "backward direction of the laser" means: for the wavelength to which the power meter is sensitive, any direction from the laser to the power meter or photodiode for which the transmission efficiency is less than the transmission efficiency from at least one point external to the emitter.

[0034] The so-called "forward direction of the laser" is defined as: for the wavelength to which the power meter is sensitive, the direction from the laser 21B to the power meter or photodiode for which the transmission efficiency is greater than the transmission efficiency from any point external to the emitter.

[0035] Therefore, in the backward direction, the coupling of light from outside the system to the detector 25B is greater than the coupling of the reflected laser light to the detector. For example, a leaky back mirror is often used for power measurements. The back mirror of a laser is designed to have high reflectivity for a specific laser wavelength, and is often transmissive for other wavelengths, which when coming from outside, would falsify the power measurement. In the case where a power meter is placed behind the back mirror of a laser, approximately 0.1% of the laser light can couple to the detector, while up to 5% of the light from outside can couple to the laser. In order to measure the power of a laser, a small portion of the beam is split and directed to a power meter or detector. Both the configuration of the detector relative to the laser, and the angle of the beam splitter affect the accuracy of the detector. US 2007 / 0019693 to Graham uses the back surface of a lens as a beam splitter to measure back reflections from the lens surface. In US 2014 / 0126603, Della Pergola uses the back mirror of a laser as a coupler for power measurements (also in the "backward" direction). In this case, the laser back mirror is the beam splitter.

[0036] Figure 2A A power meter constructed according to the previously used technique is shown, in which the detector 25A is in the backward direction. The beam splitter 23A is positioned such that the laser beam 22A is split by the beam splitter, and a small percentage of the beam 24A is reflected backward and reaches the detector 25A, the main part of the laser beam is emitted from the exit hole. However, in addition, stray light 26A from the outside reaches the detector 25A through the exit hole, affecting the measurement and reducing its accuracy. This is because the detector is facing in the backward direction relative to the laser, i.e., in its direction facing away from the laser, and facing the outside world.

[0037] In Figure 2B an exemplary power meter constructed according to the novel solution to the problem of interference by incident ambient light is shown. In this configuration, the detector 25B is placed near the exit hole but facing in the forward direction relative to the laser, i.e., in its direction facing the laser and with its back facing the outside world. Therefore, stray light entering the housing does not hit the detector and therefore does not affect the reading of the laser power. In this implementation, the beam splitter can again cause 98% of the beam to be transmitted and 2% of the beam to be reflected towards the detector. In this configuration, the detector 25B is positioned such that 2% of the laser beam 24B is deflected after hitting the beam splitter 23B and reaches the detector 25B, but almost no power from the stray beam 26B reaches the detector 25B.

[0038] While the laser is Figure 1-2 is considered to be contained within an opaque housing, but it is understood that the power meter can also be constructed as a separate device for measuring externally generated laser beams. In this case, the housing needs to have an aperture through which the laser is positioned so that the beam can enter. To reduce the entry of extraneous light, it is most advantageous to position the laser as close as possible to the aperture, or even in contact with the housing wall containing the aperture.

[0039] Reference is now made to Figure 3 which schematically illustrates a beam splitter 26 for use in such a power meter module, which is designed to separate a small portion of the laser beam that travels in a forward direction 22 from a laser 21 towards a power detector 27. The beam splitter 26 also diverts light from a backward direction 24 to a loss element 25, which is typically a component that absorbs wavelengths in the range of the light beam, which absorbs most of the reflected light so as to prevent any significant portion of the backward beam from being further reflected to the detector. As Figure 2B shown, these components are sealed in an opaque housing to prevent dust buildup, particularly on the beam splitter 23 and the loss element 25.

[0040] Laser beams, particularly multimode diode laser beams, often undergo beam shape variations. Figure 4 An image of a typical beam shape is now shown, which illustrates a laser beam distribution 32, with the fast axis 33 and the slow axis 34 marked. The so-called "hot spot" shown with the diagonal line 35 often changes shape, position and intensity during normal operation of the laser, as does the shape of the entire beam 32. To avoid such dynamic changes in the beam shape that affect the power measurement, the entire beam must be sampled with uniform efficiency, which is typically achieved by focusing the entire beam onto a detector. However, such a configuration is sensitive to the alignment of the focusing element with the beam and the detector, and tends to drift over time, requiring realignment or recalibration.

[0041] Reference is now made to Figure 5 which schematically illustrates another view of the axes of a diode laser beam. The beam 42 is emitted from a diode laser 41, which can be a single longitudinal mode diode laser or a multi-longitudinal mode diode laser. The beam 42 from such a diode laser typically has two distinct orthogonal axes 43, 44, which behave differently, with the so-called fast axis 43 diverging from the diode emitter at about 25-50°, while the slow axis 44 diverges from the diode emitter at 5-20°. The modes across the slow axis 44 are typically less coherent and less stable than the modes across the fast axis. Thus, the cross-section of the beam in the direction of the fast axis 43 is typically close to a Gaussian shape 45, and does not substantially change over time.

[0042] Because of the lack of power distribution uniformity along the slow axis 44, the sampler needs to sample the entire slow axis 44 of the beam with uniform efficiency. Sampling along the fast axis 43 is less sensitive because the power distribution in this direction is more stable, and sampling at any given point will maintain representativeness of the entire beam distribution in that direction. Therefore, in some cases, non-uniform sampling of a portion of the fast axis 43 in space is sufficient.

[0043] Now for reference Figure 6A and Figure 6B These diagrams schematically illustrate solutions for ensuring accurate measurement of beam power. In some cases, focusing the entire beam onto the detector power meter is impractical or impossible. In such cases, uniform sampling of the split beam is achieved by striking the beam against diffusers 56 and 57, from which diffused light strikes detectors 50 and 59, respectively. The purpose of the diffusers is to ensure that the light collected by the detectors is equally representative of each portion of the beam falling onto the diffusers, thus ensuring a uniform sampling rate for each portion of the beam. If the optical path length of the power meter between such diffusers 56 and 57 and the power detectors 50 and 59 is substantially longer than the beam diameter on the slow axis, where a factor of 30 times the beam diameter is considered sufficient, then a simple diffuser can be used to sample the entire beam almost uniformly through the power meter. In systems with short power meter optical path lengths, distance differences between different points on the diffusers and the power meter can lead to non-uniform sampling, which in turn can result in sensitivity to changes in beam shape. This is evident in… Figure 6A The illustration shows beams from different sides of diffuser 56 striking detector 50, and it is clear that in the example shown, light from the left side of the beam will approach the detector closer than light from the right side of the beam, generating a larger signal in the detector. If the beam distribution exhibits varying intensity at any given location in its distribution, like the slow axis of a diode laser emission, this will generate an over-reliance on variations in the beam distribution in systems with path lengths shorter than those described above.

[0044] In many cases, such a long optical path (30 times the beam diameter) is undesirable for practical reasons, and therefore three alternative solutions can be implemented individually or together. First, tools such as lenses, focusing lenses, or telescopes can be used. Figure 6A and Figure 6B Optical arrangements such as (not shown) are used to compress the optical path so that it can fit in a shorter space while keeping the difference between the nearest point and the farthest point sampled by the diffuser in the beam as small as possible. Second, a diffuser 56 with planned variable reflectivity or transmittance can be used to compensate for different distances and angles between different positions in the beam, thereby making the sampling uniform. Third, asFigure 6B As shown, a shaped diffuser 57 can be used, which is typically recessed along the direction of light incidence along the slow axis, or recessed for both axes, thereby making the optical path length more uniform between different points in the beam. If detectors 50, 59 are placed close to the center of curvature of the diffuser, then the distance from each point in the diffuser to the detector will be the same. Typically, a small deviation from a perfectly circular aperture will allow the beam to uniformly illuminate the diffuser off-axis. These three solutions can be used together or individually; those with ordinary skill in the field of optical design will know how to design and build such systems and their combinations. Thus, the combined implementation of these three features achieves flexibility in response to changes in beam pattern.

[0045] like Figure 6B As shown, it is advantageous to use a pair of detectors 50 and 59 instead of a single detector element. The detectors are positioned such that the line connecting them is optically parallel to the fast axis of the laser diode. Therefore, the two detectors will generate similar signals even if they are slightly off-center from the diffuse beam. Figure 7 This is illustrated more clearly. If the system goes out of calibration range, these similar signals can be used to provide a warning because the two signals will start to differ, thereby enabling corrective measures to be taken to prevent dangerous operations, such as reducing the power level, terminating the beam, and / or calling a technician.

[0046] Detection schemes based on diffusers typically utilize only a small fraction of the total solid angle to collect light via optical detectors, as light propagates in many directions away from the diffuser. Therefore, by implementing multiple such detectors within the diffuse beam, providing multiple simultaneous power indication signals, extensions can be obtained for any of these configurations. All these signals are independent of the characteristics of the original beam (except for power), ensuring that these signals only need to be calibrated relative to each other once, and that this calibration remains effective even if the beam distribution changes. Specifically, in the case of a cylindrical diffuser, multiple detectors can be positioned along the axis of the cylinder.

[0047] Another key aspect of this system is the importance of dustproof sealing, such as... Figure 6B As shown. Dust accumulation on the sampler and other components in the beam path can cause changes in the sampling rate, leading to inaccurate beam power measurements. To avoid such dust problems, the system can be sealed, for example, by a dustproof housing 58, or by extending and complicating the air path between the outside and inside using a labyrinthine air path to prevent dust from reaching the sampling system. The diffuser 57 and detectors 50, 59 can also be enclosed within a light-blocking housing 55, so that any random light entering the module will have a degrading effect on the accuracy of the sampling measurements.

[0048] Now for referenceFigure 7 which schematically illustrates one exemplary scheme for tracking the power of a beam 610 from a laser source 621, which in this exemplary arrangement is located outside the housing 615 of the power meter, with the beam entering through an entrance aperture 602. However, the laser can be located within the housing, as shown in the exemplary system in Figure 2B As in the previous implementation, a beamsplitter 607 is used in order to deflect a portion of the incoming beam towards detectors 601, 609. In this module 600, the beam 610 is split by the beamsplitter 607 into a main beam 611 that exits through an exit aperture 603 and is directed towards a receiver, and a smaller sampling portion 612 that constitutes a known small fraction of the total laser beam. The beam sample 612 enters a light-blocking housing 616, where it impinges on a diffuser element 608. The diffuser element 608 can be spherical, cylindrical, or other shape that helps to perturb the wavefront of the impinging beam so that the direction of each photon is random, or nearly random, relative to the other photons. A portion of the scattered beam impinges on a pair of detectors 601, 609 in such a way that the fast axis of the beam 617 is parallel to the line connecting the centers of the two detectors, with the slow axis in the direction in which the detectors are adjacent to each other. In this configuration, both detectors cover the entire length of the slow axis, so that both detectors cover equally the possible random variations in the beam profile in that direction, while the more stable Gaussian profile along the fast axis can be covered in part by each detector, since each part remains reasonably stable in time.

[0049] If the spatial profile of the original beam is asymmetric in two axes of the incoming beam, for example when using a multimode edge-emitting diode laser, the effect of a cylindrical diffuser is best. The curved optical axis of the diffuser as shown in Figure 6 is aligned with the axis of the beam that is characterized by a stronger spatial variation; in Figure 3 and Figure 4 which are the slow axis 34, 44 of the laser, respectively. Both detectors then convert the optical signal to an electrical signal and provide a measurement of the power of the signal. This information is then used for the safety determination.

[0050] In Figure 7 , it is shown that a portion of the main beam 611 is reflected back into the power meter, and that light 613 enters the system from the outside through the aperture 603. Stray light entering the aperture 603 can also be ambient light from sources other than reflections of the laser beam. This incoming light 613 impinges on the beamsplitter 607, and a portion of the impinging light is reflected as a beam 614 and impinges on a beam absorber 605. The beam absorber 605 prevents the reflected beam 614 from being further reflected within the housing 615 that encloses and surrounds the components of the system 600.

[0051] The use of, for example, a Figure 7The configuration shown, the electrical signal pair produced at detectors 601, 609 is largely insensitive to characteristics of the original light beam other than its power. Such other characteristics include, but are not limited to, polarization, wavelength, and spatial profile. Prior art solutions proposed for measuring power typically suggest using a low coupling coefficient in the beam splitter. Using dielectric coatings to couple light at the beam splitter, coupling coefficients values of less than 0.5% of the power or even less than 0.2% are possible, and additional losses typically occur, especially in the diffuser, but also in other parts. While dielectric beam splitters are available, they do have a number of disadvantages: lasers, especially diode lasers, can experience longitudinal mode hopping and wavelength type changes, which can cause their wavelength and / or bandwidth to drift slightly over time. Dielectric coatings are very sensitive to wavelength changes, and thus, using dielectric coatings can cause the system to become sensitive to these wavelength changes, making it less safe. Wave-length insensitive dielectric coatings can be used, but at a higher cost.

[0052] In another aspect, Fresnel reflections from metallic surfaces are generally less sensitive to wavelength changes. In the presently described system, it is advantageous to use a beam splitter based on Fresnel reflections or based on metallic surface reflections instead of a traditional dielectric coated beam splitter. In some implementations, the surface of the beam splitter opposite the surface on which the output beam is split can be coated with an anti-reflective coating. A round spot metallic coating can also be used in some implementations, especially for higher powers or large beams.

[0053] The first preferred sampler uses the Fresnel reflection from the front face of the normally transparent optical surface of the beam splitter 607, i.e., the face facing the laser beam. The back face is typically coated with an anti-reflective coating to prevent reflections from it, but embodiments that reflect from both surfaces can be advantageous in some cases, especially if the sampler is thin and its faces are slightly non-parallel. The sampler remains spatially uniform over the area covered by the beam, reducing sensitivity to changes in the beam shape; this configuration is well suited to sampling a fixed polarization beam with a fixed wavelength.

[0054] The sampling angle is chosen such that the Fresnel reflection samples a small fraction of the light beam 610. Small angles (typically between 0% and 75% of the Brewster angle) have the advantage of being independent of polarization. Larger angles (between 75% and 120% of the Brewster angle) are more sensitive to polarization, but allow for higher transmission (especially for "P" polarization), thus providing higher efficiency. Angles close to 45° typically allow the most compact structure, which can be advantageous when a compact system is required. In one advantageous embodiment, the main polarization is set to "P" polarization on the sampler axis, so the sampling rate at an incidence angle of about 45° is smaller (about 1-1.3%) relative to sampling at an angle close to 0°, where the sampling rate is typically between 3-5%. In this way, flexibility to wavelength and polarization variations is achieved.

[0055] The detection device in the present invention thus comprises a diffusing element 608 followed by optical detectors 601, 609 that collect light from only a fraction of the solid angle of the diffused light beam. The diffusing element can be transmissive, where the diffusing distribution is generated along the original direction of the light beam, or reflective, where the distribution is generated via reflection off the diffusing surface. Figure 7 The latter case is illustrated. A perfect diffuser generates a universal Lambertian distribution, which is independent of the polarization or wavelength of the sampled light beam. Since the fixed solid angle of this distribution is collected by the optical detectors and converted into a power indication signal, this indication signal is also not sensitive to the polarization and wavelength of the sampled light beam. A practical diffuser is chosen such that the remaining dependence on polarization and wavelength is negligible with respect to the accuracy required by the power tracking system.

[0056] In one embodiment, the optical detectors 601, 609 are positioned at a distance from the diffuser that is large enough such that the entire diffuser area shares essentially the same distance and angle with respect to the detectors. This geometry ensures that all coordinates of the diffuser contribute equally to the power indication signal, thus making this signal insensitive to the spatial distribution of the original light beam. In practice, for a given lateral extent of the sampled light beam, the detection distance is chosen such that the remaining geometric differences produce negligible sensitivity with respect to the accuracy required by the power tracking system.

[0057] The amount of light collected by the optical detector and thus the size of the power indication signal is determined by the collection solid angle, which in turn depends on the detection distance and the detector area. In another embodiment, the detector area is determined by a pinhole aperture placed right above a larger photodetector. This design masks the unintentionally light sensitive areas of the detector, thus providing an effective area of accurate size. The typical diameter of such a pinhole can be, but is not limited to, 300 pm. In another implementation, the detector area is large enough so that the unintentional incidental light sensitivity does not affect the accuracy of the power tracking system. In yet another embodiment, the unintentional light sensitivity is characterized as a fraction of the expected signal.

[0058] In another implementation, the sampling beam is focused onto the diffuser, thus substantially reducing its lateral extent. Such a design enables reducing the detection distance, thus yielding a more compact system, without sacrificing the power indication signal immunity to the spatial profile of the original beam.

[0059] In yet another implementation, the diffuser has a spherical profile; a simple implementation of such an element is a spherical diffuse reflector. When placing the optical detector at the origin of the sphere, all diffuser points share the same distance with respect to the detector, regardless of the detection distance (in this case, the sphere radius). Thus, for any detection distance, it is guaranteed to be independent of the spatial profile of the original beam.

[0060] The detector / sensor is preferably placed away from the focal point of the curved diffuser (cylindrical or spherical), which is typically located at half the radius of curvature of the diffuser surface. Depending on the mechanical design, the wide beam generated by the diffuser can hit certain mechanical elements and be specularly or diffusely reflected towards the optical detector. This contribution increases the power indication signal beyond the desired value by direct collection only. In a preferred embodiment, all potential mechanical reflectors are placed at a distance such that their contribution to the power indication signal is negligible. In another embodiment, baffles are placed to block the indirect collection paths; these are shutters that prevent the self-reflections from reaching the detector. In yet another embodiment, the relevant mechanical elements are coated with an absorbing material in order to minimize their contribution to the power indication signal. Such a coating can be, for example, Metal Velvet® available from Acktar Advanced Coatings Ltd., Kiryat Gat, Israel. TM .

[0061] Those skilled in the art will understand that the application is not limited by what has been specifically shown and described hereinabove. Rather, the scope of the application includes all variations and modifications of the various features that have been presented hereinabove, as well as alternatives and modifications that would be apparent to one skilled in the art upon reading the above description.

Claims

1. A system for measuring the power of a laser beam, comprising: A substantially opaque housing from which the laser beam is guided through an exit aperture, the housing comprising: A beam splitter is configured to transmit the main portion of the laser beam through the exit aperture and reflect the secondary portion of the laser beam. A diffuser element, which is positioned such that a secondary portion of the reflected laser beam strikes it; At least one detector element, which optically communicates with the diffuser element, the detector element providing a signal in response to diffuse light from a secondary portion of the laser beam impacting it; and An absorber element is positioned such that any portion of light that enters the housing through the exit hole and is reflected by the beam splitter strikes the absorber element and is substantially absorbed.

2. The system according to claim 1, wherein, The laser is housed within the housing.

3. The system according to claim 1, wherein, The laser is positioned outside the housing and its beam is directed into the housing.

4. The system according to any one of the preceding claims, wherein, The optical coupling efficiency from the detector to the laser is substantially greater than the optical coupling from the detector to any other location outside the housing.

5. The system according to any one of the preceding claims, wherein, The wavefront form of the main portion of the beam transmitted by the beam splitter is substantially unaffected by the channel passing through the beam splitter.

6. The system according to any one of the preceding claims, wherein, When the wavelength of the beam changes by up to 3 nm from its average value, the ratio of the power emitted by the laser beam to the power received by the detector element remains essentially constant.

7. The system according to any one of the preceding claims, wherein, When the polarization of the beam changes from its average value, the ratio of the power emitted by the laser beam to the power received by the detector element remains essentially constant.

8. The system according to any one of the preceding claims, wherein, When the beam distribution of the laser beam changes from its average value, the ratio of the power emitted by the laser beam to the power received by the detector element remains essentially constant.

9. The system according to any one of the preceding claims, wherein, The beam splitter includes a transparent front surface and an anti-reflective rear surface.

10. The system according to any one of the preceding claims, wherein, The diffuser element is configured to distribute the signal from the beam evenly across the beam.

11. The system according to any one of the preceding claims, wherein, The diffuser element includes a concave shape adapted to uniformly illuminate the detector element.

12. The system according to any one of the preceding claims, wherein, The detector element includes a pair of adjacent detectors, which are positioned such that a detected beam of light strikes both of them.

13. The system according to claim 12, wherein, The laser beam is generated by a laser diode, and the pair of detectors are arranged such that the fast axis of the laser beam is parallel to the line connecting the centers of the detectors.

14. The system according to any one of the preceding claims, wherein, The main portion of the laser beam transmitted through the exit aperture is greater than 80% of the source laser beam.

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