WO2008107823A1

WO2008107823A1 - System for providing a laser light output

Info

Publication number: WO2008107823A1
Application number: PCT/IB2008/050718
Authority: WO
Inventors: Holger Moench
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-03-05
Filing date: 2008-02-28
Publication date: 2008-09-12

Abstract

The invention describes a system (1) for providing a laser light output (Ls), which system comprises a laser light source (2, 20) for generating a high-intensity laser light beam (La), an output coupler (4) for passing a fraction of the high-intensity laser light, and a scattering element (40, 41, 42) for irreversibly scattering the passed fraction, whereby the output coupler (4) and the scattering element (40, 41, 42) are combined such that the laser light source (2, 20) is rendered inoperative when the scattering element (40, 41, 42) is removed. The invention further describes an output coupler safety element (3) comprising an output coupler (4) for reflecting back into a laser light source (2, 20) a laser beam (La) generated by the laser light source (2) and for passing a fraction of the laser light beam (La), and a scattering element (40, 41, 42) for irreversibly scattering the passed fraction. The invention also describes a method of fabricating a system (1) for providing a laser light output (Ls), which method comprises obtaining a laser light source (2, 20) for generating a high-intensity laser light beam (La), obtaining an output coupler (4) for reflecting the high-intensity laser beam (La) back into the laser light source (2, 20) and passing a fraction of the high-intensity laser light beam (La), and obtaining a scattering element (40, 41, 42) for irreversibly scattering the passed fraction, wherein the scattering element (40, 41, 42) is combined with the output coupler (4) in such a way that the laser light source (2, 20) is rendered inoperative when the scattering element (40, 41, 42) is removed.

Description

SYSTEM FOR PROVIDING A LASER LIGHT OUTPUT

FIELD OF THE INVENTION

The invention describes a system for providing a safe laser light output. The invention also relates to an output coupler safety element and to a method for providing a safe laser light output. BACKGROUND OF THE INVENTION

Laser light generated by a laser has properties that are useful in many applications such as eye surgery, dermatology, welding, cooling, communications, distance measurement, etc. Laser light is characterised by a high degree of spatial coherence, so that a laser beam can propagate over long distances with little divergence, and can be accurately focussed onto very small areas. Furthermore, a high degree of temporal coherence of the laser light allows a very narrow spectral bandwidth to be obtained, giving laser light of a certain colour depending on the type of laser used to produce the beam.

Different types of laser include gas lasers, fibre lasers, solid state lasers and semiconductor lasers, in decreasing order of size. The power of the emitted laser beam can range, depending on laser type, from thousands of watts down to a few milliwatts. The power and wavelength of the emitted beam of a laser governs the applications for which the laser is suited.

The basic principle of a laser is that light is first generated and then made to circulate, or resonate, in a gain medium contained in a cavity between two mirrors. The 'mirror' can be any reflective surface, such as a dielectric mirror. The gain medium is pumped, for example optically or electrically, depending on the type of laser, to amplify the circulating light. If one of the mirrors is partially transmissive, a fraction of the light will eventually pass through this partially transmissive mirror as a useful high-intensity output laser beam.

Safety is a key consideration in the use of all laser devices. Very serious damage can be caused by a laser beam of only a few milliwatts, particularly to the eye, because even a low-intensity laser beam can be focussed by the lens of the eye to a point on the retina, where serious permanent damage can instantly occur. Other serious types of injury can involve burns to the skin, photochemical changes in the skin, and even skin cancer. Because of this potential to cause damage, operation of high-power laser devices is generally only allowed by skilled and trained personnel, and only under strict usage conditions, for example while wearing protective eye-gear. Laser products are subject to safety restrictions, for example safety standard IEC 60825-1 published by the International Electrotechnical Commission, which specifies that devices for use by untrained users may only incorporate laser sources of very low power. For this reason, devices such as laser pointers may only comprise a laser diode emitting visible red laser light at an output power of only a few milliwatts. Devices for home use are therefore very limited in the laser sources that can be implemented, even though there are many types of product, for example image projectors, that could in theory make use of a high- power semiconductor laser to deliver high performance levels at low production cost.

With the developments being made in the field of semiconductor lasers, it is possible to economically manufacture large numbers of lasers, since many lasers can be fabricated on a single wafer. Furthermore, their extremely small size makes them very attractive from the point of view of incorporating them in portable consumer electronics devices. For many applications, it would be very desirable to be able to use these high- power laser light sources without the need for safety training or protective eye-wear, and without the devices presenting a safety hazard. Particularly for display applications, use of protective eye-wear is not realistic, since the laser light is intended to be seen by the observer.

Several approaches have been suggested for making laser light 'safe' for use in various environments. Generally, these systems operate by spreading the power of a narrow high-intensity laser beam to give a broad low-intensity beam of light. For example, US 2005/0213180 describes a laser light source such as a laser diode for producing a high-intensity beam of laser light, and a means of diffusing the high-intensity beam of light. However, none of the prior art systems offer an entirely safe solution, since a potentially dangerous high-intensity laser beam can be restored in these systems by accident or by intent, by simply removing the diffuser in each case.

It is therefore an object of the invention to provide a laser light source that can be implemented without risk to the users. SUMMARY OF THE INVENTION

To this end, the present invention describes a system for providing a laser light output, which system comprises a laser light source for generating a high-intensity laser light beam, an output coupler for passing a fraction of the high-intensity laser light, and a scattering element for irreversibly scattering the passed fraction, whereby the output coupler and the scattering element are combined such that the laser light source is rendered inoperative when the scattering element is removed.

An obvious advantage of the system according to the invention is that the laser light that is passed by the output coupler, also referred to as the 'emitted' laser light, is scattered before being output, and the scattering is performed in such a way that the scattered light cannot be recombined - either intentionally or inadvertently, using lenses or other optical elements - to recover a coherent, collimated laser light beam. Also, the power density of the scattered laser light is greatly reduced, so that the light output of the system according to the invention can be considered to be essentially harmless.

Furthermore, a laser light source cannot function without an output coupler, since this is required to allow the amplified light to resonate in the cavity. The output coupler and scattering element of the system according to the invention are combined so that the laser light source of the invention cannot generate a laser light beam once the scattering element is removed. Therefore, a particularly advantageous feature of the system according to the invention is that it provides safe laser light output, and can be used in consumer electronics products, for example projectors, home cinema systems, etc., without requiring the users of these products to undergo specialist training or to wear protective devices. An appropriate output coupler safety element according to the invention comprises an output coupler for reflecting back into a laser light source a laser beam generated by the laser light source and for passing a fraction of the laser light beam, and a scattering element for irreversibly scattering the passed fraction.

An appropriate method of fabricating a laser light source for providing a laser light output comprises obtaining a laser light source for generating a high-intensity laser light beam, an output coupler for reflecting the high-intensity laser beam back into the laser light source and for passing a fraction of the high-intensity laser light beam. The method also comprises obtaining a scattering element for irreversibly scattering the passed fraction, wherein the scattering element is combined with the output coupler in such a way, that the laser light source is rendered inoperative when the scattering element is removed. Here, the term 'obtaining' an element can mean manufacturing the relevant element or simply acquiring or purchasing it, since devices such as semiconductor lasers or output couplers can be available from specialist suppliers.

The dependent claims and the subsequent description disclose particularly advantageous embodiments and features of the invention. The system and method according to the invention can be used with various different types of laser light source such as those mentioned in the introduction. However, the described system and method are particularly suitable for use in conjunction with semiconductor lasers, so that, in the following, without restricting the invention in any way, the laser light source is assumed to be a semiconductor laser. As mentioned in the introduction, an amplified beam of light is stimulated in a laser source by reflecting the beam back and forth between two reflectors. In a horizontally emitting laser diode, the stimulated light is reflected back and forth in a cavity formed by the cleaved edges of the diode structure. At least one of the edges has a dichroic coating to make it highly reflective and to direct the light output to the other edge, where the laser light exits the device. In a surface emitting semiconductor laser, such as a Vertical Cavity Surface-Emitting Laser (VCSEL), the amplified light beam can exit the diode structure at the top or bottom surface. For such top-emitting and bottom- emitting lasers, the reflecting surfaces are 'grown' epitaxially in the diode structure during the semiconductor manufacturing process, and generally comprise multiple layers of alternating materials with different refractive index, so that each layer causes a partial reflection of the amplified light wave. An example of such a reflector is the distributed Bragg reflector (DBR). One of the reflectors can comprise layers of p-doped AlGaAs and GaAs, while the other mirror can comprise layers of n-doped AlAs and GaAs, so that the combination of these reflectors acts as a diode junction. One or more quantum wells are sandwiched between the two DBRs. The manner in which a semiconductor diode structure is built up will be known to a person skilled in the art, and need not be explained in further detail at this point.

A semiconductor diode for which the cavity is external to the diode structure is referred to as a Vertical External Cavity Surface-Emitting Laser (VECSEL), since the cavity is now between the diode structure and an external reflector. The diode structure and the external mirror can be separated by a short distance of, for example, a few millimetres. Such lasers can be pumped optically, while recent developments have also led to electrically pumped VECSELs which can be used to produce green or blue laser light by frequency doubling. These devices can provide an output power in the region of 100 milliwatts, and an output power of several watts can be achieved by using arrays of multiple devices, thus making them very attractive for applications such as high-quality image projection. In systems using DLP^® (digital light processing), light is projected onto a micro-mirror display unit to generate an image which is then reflected onto the projection area. The micro-mirror display unit is also fabricated in a very compact manner using semiconductor technology, so that a projector system using DLP^® and VECSEL laser light sources could be very light and portable compared to state-of-the-art devices.

As already described, the amplified laser light of a prior art VECSEL exits at the surface and circulates in an external cavity, being almost totally reflected by a semi-transparent dielectric mirror or output coupler which also emits a small fraction of the circulating intra-cavity amplified light beam, thus providing a useful laser output. If the external cavity contains a frequency doubling element, the output coupler can be realised to be transparent only for that fraction of the laser light resonating in the cavity that has been frequency-doubled by the frequency doubling element. For example, such an output coupler may effectively cause the infra-red fundamental laser beam to be almost totally reflected within the cavity, so that the fraction that is passed comprises green or blue laser light.

For many VECSEL laser light sources, as already mentioned, the output coupler can be placed at a short distance away from the diode structure, so that the laser cavity is in a so-called 'free space' region between the diode structure and the output coupler. This is especially useful if other optical elements, such as e.g. a frequency doubling crystal, are to be incorporated inside the cavity. Alternatively, the output coupler can be superimposed directly onto the diode structure. An example of an output coupler is the Bragg grating, which takes the form of a transparent element with a certain periodic structure with which nearly total reflection of the amplified laser beam can be achieved. A Bragg grating made of a solid such as photo -thermo -refractive glass (PTR glass) or a polymer is known as a Volume Bragg Grating (VBG), as will be known to a person skilled in the art. Examples of VBGs are described in US 6,673,497 B2.

The principles of operation of the reflectors and gratings described above will be known to a person skilled in the art, and need not be explained in excessive detail here. Some state-of-the-art lasers that are equipped with an external output coupler are also equipped with an additional diffuser, as mentioned already, for the purpose of spreading the light output so as to significantly reduce the radiation density of the light. However, such systems are not without risk, since it is still possible to remove the diffuser and so expose a potentially dangerous laser beam. Therefore, in the system according to the invention, the output coupler and the scattering element are combined such that the scattering element may not be removed without also removing or destroying the output coupler. Without the output coupler, it is not possible to generate a laser light beam.

The scattering element can be realised as a layer that is attached in some way to the output coupler to give an output coupler safety element, so that the emitted light from the output coupler passes directly from the output coupler into the scattering element, where it is then irreversibly scattered and spread. Such an output coupler safety element could be constructed, for example, by gluing a scattering element to the exit face of an output coupler. In a particularly preferred embodiment of the invention, however, the output coupler and the scattering element are realised as a single entity, or integrated output coupler safety element, so that it is not necessary to physically attach a separate scattering element to the output coupler. Evidently, the scattering element can be of the same material as the output coupler, for example the output coupler safety element can be realised as a type of structure known as a 'volume hologram', which can be made of a polymer or glass with a lower layer or level acting as a Volume Bragg Grating with the periodic structure required to provide almost total reflection and to pass a small fraction of the amplified laser light, and an upper layer or level with a different structure to randomly scatter the passed fraction, which then exits the output coupler safety element at an exit face. Here, the term 'lower' refers to the side of the output coupler nearest to the diode structure, and the term 'upper' refers to the opposite side of the output coupler. The term 'exit face' then evidently refers to the surface of the output coupler safety element facing away from the diode structure.

In another preferred embodiment, a frequency multiplier element, usually a frequency doubling crystal, can be used inside the cavity. In this case, the output coupler safety element almost totally reflects the infrared fundamental laser light, while transmitting and scattering the frequency-multiplied, preferably frequency-doubled, visible light. A VBG element with small spectral reflectivity at the IR wavelength will be transparent at the frequency doubled wavelength. As will be known to a person skilled in the art, such output coupling of the laser light can also be achieved by using "stacked functional layers", or the output coupler safety element can be realised as a single volume holographic element having different functions for different wavelengths.

In another preferred embodiment, the scattering element can be obtained by treating the exit face of a Volume Bragg Grating output coupler in some appropriate manner. For example, the exit face of the VBG could be etched mechanically or chemically, or the exit face could simply be sandblasted, to obtain a randomised alteration of the structural properties of the VBG, resulting in a random scattering of the emitted light. In another alternative embodiment, the scattering element can comprise a dichroic filter with a roughened outer edge to act as the exit face.

As previously mentioned, lasers light sources are used in applications where an intense collimated light source is desired. The effect of the scattering element according to the invention is to throw the rays of light into random directions, thus effectively cancelling any collimation and coherence that the emitted laser light beam would otherwise have had. However, the light scattered in this way may not be sufficiently bright or intense for certain applications. Therefore, in a further preferred embodiment of the invention, the system comprises a collimator for re-collimating the scattered light. There are various ways of obtaining a collimated light beam. In a simple approach, a type of grating might be used, having narrow slits in only one direction, for example in a direction perpendicular to the exit face of the output coupler safety element, so that only light rays travelling in that direction are passed by the collimator. A disadvantage of this approach is that much of the light is lost, and the net light output may not be sufficiently intense.

In a preferred embodiment of the invention, therefore, a collector is used to collect the scattered light and reflect it so that most of the rays of light exit the reflector in an essentially parallel direction. An example of such a reflector is a compound parabolic collector (CPC), or 'Winston collector'. This type of collector (also referred to as a 'concentrator') has an essentially parabolic shape so that light rays are reflected off the inside 'walls' of the collector and back out in a direction parallel to the longitudinal axis of the collector. Such collectors can be hollow or solid. In the case of a solid CPC, less light rays can escape the collector in a non-parallel manner owing to total internal reflection, so that this type of concentrator is preferred. The scattered light has sufficient brightness or intensity to make it suitable for various applications requiring intense light, while still being well below a hazardous level.

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 presents a cross-section of a state-of-the-art VECSEL; Fig. 2 shows a cross-section of a state of the art laser light source for obtaining a laser light output of reduced radiation density;

Fig. 3 a shows a cross-section of a first embodiment of a system for providing a safe laser light output according to the invention;

Fig. 3b shows a variation of the embodiment of Fig. 3a with an additional frequency doubler crystal;

Fig. 4a shows a first embodiment of an output coupler safety element according to the invention;

Fig. 4b shows a second embodiment of an output coupler safety element according to the invention;

Fig. 4c shows a third embodiment of an output coupler safety element according to the invention;

Fig. 5 shows a cross-section of a second embodiment of a system according to the invention for providing a collimated safe laser light output; Fig. 6a shows a cross-section of a third embodiment of a system according to the invention for providing a collimated safe laser light output; Fig. 6b shows a cross-section of a fourth embodiment of a system according to the invention for providing a collimated safe laser light output.

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 presents a very simplified cross-section of a state-of-the-art VECSEL 10 showing the relevant levels in its construction. Electrical contacts 15 are mounted on a diode structure comprising a substrate 14 and quantum wells 12 sandwiched between distributed Bragg reflectors 11, 13. This semiconductor laser 10 is pumped, in this case electrically, to generate a beam L_a of amplified light which exits the diode structure in a direction perpendicular to the surface. A fraction of this amplified light beam L_a is emitted on the exit face of an output coupler 16 as a laser beam L₆. The output coupler 16 in this example is a volume Bragg grating (VBG) which reflects most of the light back into the diode structure for amplification, while emitting only the small fraction L₆. The output coupler 16 is constructed such that the divergence of the emitted laser beam L₆ is very low, for example in the range of 1°. In the diagram, for the sake of simplicity, the output coupler is shown at a distance from the diode structure. Alternatively, a VECSEL realisation can comprise an output coupler which is incorporated in a transparent block positioned directly on top of the diode structure. In the following diagrams, this realisation is implicitly assumed even when the output coupler is shown at a distance removed from the diode structure.

Fig. 2 shows a cross-section of a state of the art laser light source 10 with additional components 17, 18 for obtaining a light output of reduced intensity. Again, the laser light source 10 comprises an output coupler 16 as described above, for reflecting most of the amplified light beam L_a and only allowing a small fraction through to give the useful laser light beam L₆. The laser light beam L₆ is spread by an optical element 17 to give a diverging beam La of reduced intensity, which is then re-collimated in a collimator 18 to give a light output L_c with essentially constant radius, but whose radiation density is much lower than that of the original laser beam L₆. This collimated light beam L_c still features desirable properties of the laser beam L from which it was derived, for example a high degree of brightness coherence, but whose potential to cause damage is reduced owing to the spread in power density. The disadvantage of such a prior art laser light source, as already explained above, is that the potentially hazardous laser light beam L₆ can easily be recovered or exposed by simply removing the collimator 17 and diffuser 18. In similar state of the art realisations in which a laser beam is only optically spread to reduce the power density, the output light could conceivably be converted back into a hazardous laser beam, intentionally or by accident, by placing suitable lenses or reflective surfaces in its path. It is because of these dangers that the use of high-power laser diodes in consumer products is subject to strict limitations. Fig. 3a shows, in cross-section, a first embodiment of a system 1 according to the invention for providing a safe laser light output L_s with a high-power semiconductor laser light source 2. The diagram shows a cross-section of a VECSEL 2, essentially as described in Figs. 1 and 2. In this embodiment according to the invention, a combined output coupler safety element 3 acts as both transmissive mirror and diffuser, i.e. a fraction of the amplified light L_a is allowed to pass through a lower layer 4 or output coupler 4 of the output coupler safety element 3, but does not emit as an intense laser beam since an irreversible scattering and spreading is immediately performed on this passed fraction in an upper layer 40 or scattering element 40 of the output coupler safety element 3. An actual laser beam such as that produced by the state-of-the-art system described in Fig. 2 above cannot be emitted by the system 1 according to the invention. Instead, the light output comprises irreversibly scattered light L_s, which, unlike the input amplified laser light L_a, is neither collimated nor spatially coherent. Removal of the combined output coupler safety element 3 would make the laser light source 2 inoperative. If the scattering element 40 were to be forcibly separated from the output coupler 4, for example by cutting or sawing, the critical properties of the output coupler 4 would be damaged or destroyed, so that the exposed output coupler 4 would no longer be able to adequately reflect any light emanating from the diode structure, and an amplified light beam could as a result no longer be generated.

Fig. 3b shows a further development of the embodiment described in Fig. 3 a. Here, a frequency doubling crystal 45 is incorporated in the cavity and serves to double the fundamental frequency of the laser light generated in the diode structure. In this embodiment, the output coupler 3 is realised so that it passes essentially only the frequency-doubled light, while light of the fundamental frequency continues to resonate within the cavity.

The irreversible scattering of the passed fraction of the amplified laser light L_a can be achieved in a number of ways. Figs. 4a - 4c show a number of realizations of an output coupler safety element 3 according to the invention. In each embodiment, a Volume Bragg Grating 4 acts as output coupler 4. The amplified laser light L_a shown in each of Figs. 4a - 4c can have been generated with or without a frequency doubler crystal as described above in Fig. 3b. In Fig. 4a, a separately manufactured scattering element 40 can be fused or otherwise attached to the exit face of the Volume Bragg Grating 4. The scattering element 40 can comprise a material such as a glass or polymer with appropriate refractive properties that result in a scattering of the passed fraction of the amplified light L_a to give scattered light output L_s. In the diagram, the exit face of the output coupler 4 is indicated by the horizontal thick black line between the Volume Bragg Grating 4 and the scattering element 40.

Fig. 4b shows another simple realization of the output coupler safety element 3, in which a diffusing layer 41 is obtained by etching or sandblasting the upper surface of a top layer of the Volume Bragg Grating 4. The diffusing layer 41 obtained in this way performs a random scattering of the passed fraction of the amplified laser light L_a, so that the collimation and spatial coherence of the passed fraction are largely destroyed.

In Fig. 4c, the output coupler safety element 3 comprises a holographic scattering element 42 made of the same material as the Volume Bragg Grating 4, and is manufactured in one piece. As mentioned above, this type of realisation is known as a volume hologram. In the embodiment shown in this diagram, the output coupler safety element 3 is a solid block of photo -thermo -refractive glass in which a lower layer 4 is treated to exhibit a periodic structure with modulation in the direction of propagation of the amplified beam of laser light L_a, thereby giving the output coupler 4, and an upper layer 42 is treated to give the scattering element 42 whose periodicity is essentially perpendicular to the direction of the amplified light beam L_a. The result of these layers is that the passed fraction 44 of the amplified light L_a does not exit the output coupler safety element 3 as a narrow laser beam, but rather as a scattered light output L_s. The upper layer 42 does not necessarily have to have a regular periodic structure, as indicated by the cross-hatching in the diagram, but can exhibit a randomised structure. This type of output coupler safety element 3, made of one piece of glass or polymer, can be placed directly on top of the diode structure of a VECSEL, as already mentioned above.

For each of the embodiments shown in Figs. 4a - 4c, the resulting light output L_s is still bright and intense, but can be regarded as non-hazardous. This scattered light L_s, lacking any spatial coherence, cannot be converted back into a hazardous laser beam using optics. Neither can a laser beam be obtained by removing the scattering element 40, 41, 42 with the intention of revealing the exit face of the output coupler 4, since the scattering element 40, 41, 42 and output coupler 4 are combined as a single element. Removal of the scattering element 40, 41, 42 would necessitate destroying or removing the output coupler 4 as well. Without the highly reflective output coupler 4, the semiconductor laser light source is inoperative and cannot output amplified light L_a.

In most applications, the scattered light shown in Figs. 4a - 4c is of limited use. It is usually more desirable to have collimated light, i.e. a beam of light, for use in optical elements, with a maximum angle of divergence, (e.g. +/- 15°). One way of achieving this is shown in Fig. 5. In this embodiment, a compound parabolic collector 5, or CPC for short, is mounted onto the output coupler safety element 3 of a semiconductor laser light source 2 so that the light L_s scattered by the diffuser 41 is collected in the CPC 5 (any of the output coupler safety elements 3 shown in Figs. 4a - 4c could be used here). In the interior of the CPC 5, the rays of scattered light L_s undergo multiple reflections before eventually leaving the CPC as essentially parallel rays of collimated scattered light L_sc. In the diagram, only a few reflections are shown for the sake of clarity. The ray traces shown are only intended to give an impression of the multiple reflections within the CPC 5 and should not be taken to give an exact representation. The light rays are brought into an essentially parallel formation by the physical characteristics of the CPC. In the diagram, the collimated light is shown to exit the CPC as parallel rays, but, in practice, the collimated output beam will exhibit a small amount of divergence, typically in the region of a few degrees. The collimator 5 shown here is drawn, for the sake of simplicity, as a hollow structure. Naturally, a solid CPC could be used to better effect, since light rays that would escape from a hollow concentrator undergo total internal reflection in a solid CPC, allowing a more efficient light output L_sc to be achieved.

Figs. 6a and 6b show two further embodiments of the system according to the invention. In Fig. 6a, a diode-pumped solid-state laser (DPSS) 20 is shown, in which a blue laser diode 21 is used to optically pump a solid-state waveguide laser 22. The beam of laser light is reflected back and forth in the waveguide by means of an output coupler placed at the waveguide output. Using a prior art output coupler with a height of 0.7μm, such a solid-state waveguide laser 51 could deliver a laser beam with divergence of 10x20° and a power output of 5OmW, which is far too high for use in commercial appliances, since the radiation density of such a device is in the region of 100kW/mm²/sr. In the embodiment shown in the diagram, an output coupler safety element 3 and collimator 5 according to the invention are positioned at the waveguide output, so that the fraction of the amplified laser beam L_a passed by the output coupler 4 is irreversibly scattered by the scattering element 42 and then re-collimated to give a useful light output L_sc. Experimental values show that the radiation density of the scattered and collimated light L_sc is reduced by a factor of 17 to give an output power of approximately 6kW/mm²/sr. To reduce this radiation density even further, a second diffuser 43 and collimator 5' can be attached to the first collimator 5, as shown in Fig. 6b. Here, the same diode-pumped solid-state laser 20 is shown, with an output coupler safety element 3 positioned at the waveguide output and followed a first collimator 5. A second diffuser 43 scatters and spreads the light that has been collimated by the first collimator 5, and this scattered light is once more re-collimated to give a broader light output of even further reduced intensity L_sc. Experimental values have shown a decrease in overall power density by a factor of 55,000.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

Claims

CLAIMS:

L A system (1) for providing a laser light output (L_s), which system comprises a laser light source (2, 20) for generating a high-intensity laser light beam

an output coupler (4) for passing a fraction of the high-intensity laser light; and a scattering element (40, 41, 42) for irreversibly scattering the passed fraction, whereby the output coupler (4) and the scattering element (40, 41, 42) are combined such that the laser light source (2, 20) is rendered inoperative when the scattering element (40, 41, 42) is removed.

2. A system according to claim 1, wherein the scattering element (40, 41, 42) and the output coupler (4) are combined such that the scattering element (40, 41, 42) may not be separated from the output coupler (4) without destroying the output coupler (4).

3. A system according to claim 1 or claim 2, wherein the scattering element (40, 41, 42) and the output coupler (4) are physically attached to each other to give an output coupler safety element (3).

4. A system according to claim 1 or claim 2, wherein the scattering element (40, 41, 42) and the output coupler (4) are realised as a single output coupler safety element (3).

5. A system according to claim 4, wherein the scattering element (40, 41, 42) comprises a treated exit face of the output coupler (4).

6. A system according to any of claims 3 to 5, wherein the output coupler safety element (3) comprises a Volume Bragg Grating.

7. A system according to any of the preceding claims, wherein the system comprises a frequency multiplier element (45), which frequency multiplier element (45) multiplies a fundamental frequency of the laser light generated by the system to give laser light of a multiple of the fundamental frequency, and the output coupler (4) of the system is realized to essentially completely reflect laser light of the fundamental frequency and to pass the laser light of the multiple of the fundamental frequency.

8. A system according to any of the preceding claims, comprising a collimator (5) for re- collimating the scattered light (L_s) to give a re-collimated laser light output (L_sc).

9. A system according to any of the preceding claims, wherein the laser light source (2, 20) comprises a semiconductor laser.

10. An output coupler safety element (3) comprising an output coupler (4) for reflecting back into a laser light source (2, 20) a laser beam (La) generated by the laser light source (2, 20) and for passing a fraction of the laser light beam (La), and a scattering element (40, 41, 42) for irreversibly scattering the passed fraction, whereby the output coupler (4) and the scattering element (40, 41, 42) are combined such that the laser light source (2, 20) is rendered inoperative when the scattering element (40, 41, 42) is removed.

11. A method of fabricating a system (1) for providing a laser light output (L_s), which method comprises obtaining a laser light source (2, 20) for generating a high-intensity laser light beam

an output coupler (4) for reflecting the high-intensity laser beam (L_a) back into the laser light source (2, 20) and for passing a fraction of the high- intensity laser light beam (L_a); and a scattering element (40, 41, 42) for irreversibly scattering the passed fraction; wherein the scattering element (40, 41, 42) is combined with the output coupler (4) in such a way, that the laser light source (2, 20) is rendered inoperative when the scattering element (40, 41, 42) is removed.