WO2010049875A1 - Laser lighting device - Google Patents

Laser lighting device Download PDF

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Publication number
WO2010049875A1
WO2010049875A1 PCT/IB2009/054724 IB2009054724W WO2010049875A1 WO 2010049875 A1 WO2010049875 A1 WO 2010049875A1 IB 2009054724 W IB2009054724 W IB 2009054724W WO 2010049875 A1 WO2010049875 A1 WO 2010049875A1
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WO
WIPO (PCT)
Prior art keywords
wavelength
material
laser
transparent material
converting material
Prior art date
Application number
PCT/IB2009/054724
Other languages
French (fr)
Inventor
Rifat A.M. HIKMET
Original Assignee
Koninklijke Philips Electronics N.V.
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Filing date
Publication date
Priority to EP08167896 priority Critical
Priority to EP08167896.3 priority
Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2010049875A1 publication Critical patent/WO2010049875A1/en

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Classifications

    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00Semiconductor lasers
    • H01S5/005Optical devices external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping

Abstract

A wavelength converter (10) and a laser lighting device comprising such a wavelength converter are disclosed. The wavelength converter (10) converts laser light of a first wavelength to second light having a different wavelength by means of a wavelength converting material (1), wherein the surface of the wavelength converting material(1) where the laser light enters the wavelength converting material (1) is in good thermal contact with a transparent material (3). The transparent material (3) on the other hand is in good thermal contact with a heat sink (2), which has a window to let the laser light pass before the laser light enters the wavelength converting material (1). The wavelength converter (10) is especially suited for remote laser lighting and particularly the high power densities of lasers and the related local heating of the wavelength converter.

Description

LASER LIGHTING DEVICE

FIELD OF THE INVENTION

The present invention relates to a wavelength converter and a laser lighting device comprising such a wavelength converter.

BACKGROUND OF THE INVENTION

WO 2007/044472 discloses a LED assembly with a light transmissive heat sink. The LED assembly is formed from a high power LED chip having a first surface and a second surface, the first surface being mounted to a substrate, and the second surface being in intimate thermal contact with a light transmissive heat sink having a thermal conductivity greater than 30 watts per meter-Kelvin. Providing the light transmissive heat sink can double the heat conduction from the LED dies, thereby increasing life, or efficiency or luminance or a balance of the three. It is further mentioned that a laser diode may be used as a light producing device instead of an LED, however without recognizing the drawback of the disclosed assembly if a laser diode is used as a light producing device.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the drawbacks of the assembly disclosed in the prior art if a laser is used as a light source. The object is achieved by a wavelength converter according to claiml and a laser lighting device according to claim 12. Preferred embodiments are disclosed in the dependent claims.

According to a first aspect of the present invention, the wavelength converter for converting laser light having at least a first wavelength to second light having at least a second wavelength different from the first wavelength comprises: - at least one wavelength converting material having a first surface where the laser light impinges on the wavelength converting material;

- a transparent material, having a thermal conductivity greater than 0.8 W/(mK), more preferably greater than 10 W/(mK) and even more preferably greater than 30 W/(mK), attached to the first surface of the wavelength converting material;

- a heat sink having a window for letting the laser light pass, and the heat sink is attached to the transparent material.

In light conversion materials, mainly due to the wavelength conversion and partly due to less than 100% quantum efficiency, some of the light power is dissipated as heat. When a laser light beam is focused to a point on a light converting material, due to the low thermal conductivity of the light converting material at the point where the laser light enters the light conversion material, a local high temperature point is produced. With increasing power of the laser source the local temperature can increase such that temperature quenching of luminescent material is induced. It is an idea of the invention that transparent material is attached to the side of a wavelength converting material where laser light emitted by a laser enters the wavelength converting material, and the transparent material may dissipate the heat to a heat sink, the heat sink being arranged in such a way that the laser light, before or during entering the transparent material, first has to pass a window or opening in the heat sink and subsequently enters the wavelength converting material. This is very important especially in applications where a high-power density spot is formed on a small light converting ceramic with a cross section of less than 500*500μm2.

"Attached" means that there is a good thermal coupling between the wavelength converting material and the transparent material, that means, for example, that both materials may be in direct physical contact or may only be separated by means of a thin layer, which thin layer may be an adhesive and/or a layer with an optical functionality such as, for example, being reflective in a certain spectral range. The thermal coupling between the wavelength converting material and the transparent material on the one side and between the transparent material and the heat sink on the other side is needed due to the high power density of laser light that may cause local heating of the wavelength converting material. Heating of the wavelength converting material, for example, a monolithic ceramic luminescence converter as described in WO 2006/087660 Al, phosphor particles embedded in a transparent matrix or any other kind of suitable phosphor material, is highly undesirable as it gives rise to spectral changes in the absorption spectra. Changes in the absorption spectra are undesirable as they lead to changes in the resultant spectra and therefore the colour point. Heating can also give rise to a reduction in the efficiency of the wavelength converting material, which is also highly undesirable.

"Window" means every kind of opening, throughhole or cavity in the heat sink being suited to let the laser light pass the transparent material before entering the wavelength converting material. The cross sectional area or size of the window or opening in the heat sink may be equal to or smaller than the area of the first side of the wavelength converting material and may be limited by the cross-sectional area of the laser light. The smaller the size of the window, the smaller may be the distance between the point where the laser light enters the wavelength converting material and the heat sink and the better the heat dissipation may be. The size of the window can be rather small due to the high energy density of the laser light. This is rather different if LEDs are used as a light source, as the size of the window may strongly restrict the amount of light that can enter the wavelength converting material during a defined period of time.

The heat dissipation may be further improved by choosing a high- transparency material and, in the case of scattering materials, mainly forward scattering properties and low back scattering values (overall transmission higher than 95%) and a suitable thermal conductivity. Ordinary glass has a thermal conductivity of around 0.8 W/(mK) and may be sufficient at low laser light power densities (lower than 108W/m2), depending on the total configuration of the wavelength converter and the power of the laser source emitting the laser light. Another suitable transparent material is single crystal sapphire which has a heat conductivity of around 42 W/(mK) and which may be suited for high power density laser sources (higher than 108W/m2) . Another parameter that may improve the heat dissipation is the choice of a suitable material for the heat sink such as, for example, Indium having a heat conductivity of 82 W/(mK). Again, the choice of the materials may be determined by the total configuration of the wavelength converter and the power of the laser source. The special arrangement of the wavelength converter allows taking into account excessive local heating of wavelength converting material by irradiation with a laser beam or laser light in contrast to the more or less uniform heating of phosphor material if a LED or the like is used as a radiation source. The transparent material may, for example, be arranged between the wavelength converting material and the heat sink. Alternatively, the transparent material may be arranged in the window. In the latter embodiment a mould of transparent material may fill at least a part of a hole (the window) in the heat sink. The hole may be cylindrical, rectangular or of any other shape such as, for example, a combination of a hemispherical hole with a linked cylindrical drilling. It may also be conical, and may also comprise a truncated pyramid with various cross sections (square, hexagonal). The transparent material may fill the hemispherical hole only. The shape of the transparent material in combination with the hole or window may improve the heat dissipation. Depending on the size of the transparent material filled in the window, the heat sink may be attached to the first surface of the wavelength converting material. A good thermal coupling between the first side of the wavelength converting material and the heat sink may further improve heat dissipation. The transparent material may be cylindrical, rectangular or of any other shape such as, for example, a combination of a hemispherical hole with a linked cylindrical drilling. It may also be conical, and may also comprise a truncated pyramid with various cross sections (square, hexagonal). Furthermore, the heat sink may be attached to a second surface of the wavelength converting material. In this case, for example, a disc-shaped phosphor material may be embedded in the heat sink. The wavelength converting material may be cylindrical, rectangular, hemispherical, conical and may also comprise a truncated pyramid with various cross sections (square, hexagonal) or may have any other shape.

Preferred luminescent materials may be selected from garnets and nitrides, especially doped with trivalent cerium or divalent europium, respectively. Embodiments of garnets especially include A3B5O12 garnets, wherein A comprises at least yttrium or lutetium and wherein B comprises at least aluminium. Such garnets may be doped with cerium (Ce), with praseodymium (Pr) or a combination of cerium and praseodymium; especially however with Ce. Especially, B comprises aluminium (Al), however, B may also partly comprise gallium (Ga) and/or scandium (Sc) and/or indium (In), preferably up to about 20% of Al, more preferably up to about 10 % of Al (i.e. the B ions essentially consist of 90 or more mole % of Al and 10 or less mole % of one or more of Ga, Sc and In); B may especially comprise up to about 10% gallium. In another variant, B and O may at least partly be replaced by Si and N. The element A may especially be selected from the group consisting of yttrium (Y), gadolinium (Gd), terbium (Tb) and lutetium (Lu). Further, Gd and/or Tb are especially only present up to an amount of about 20% of A. In a specific embodiment, the garnet luminescent material comprises (Yi_xLux)3B5θi2:Ce, wherein x is equal to or larger than 0 and equal to or smaller than 1. The term ":Ce", indicates that part of the metal ions (i.e. in the garnets: part of the "A" ions) in the luminescent material is replaced by Ce. For instance, assuming (Yi_xLux)3Al5θi2:Ce, part of Y and/or Lu is replaced by Ce. This notation is known to the person skilled in the art. Ce will replace A in general for not more than 10%; in general, the Ce concentration will be in the range of 0.1-4%, especially 0.1-2% (relative to A). Assuming 1% Ce and 10% Y, the full, correct formula could be

(Yo.iLuo.89Ceo.oi)3Al5Oi2. Ce in garnets is substantially or only in the trivalent state, as known to the person skilled in the art.

In an embodiment, the red luminescent material may comprise one or more materials selected from the group consisting of (Ba,Sr,Ca)S:Eu, (Ba,Sr,Ca)AlSiN3:Eu and (Ba5Sr5Ca)2SIsNs :Eu. In these compounds, europium (Eu) is substantially or entirely divalent, and replaces one or more of the indicated divalent cations. In general, Eu will not be present in amounts larger than 10% of the cation it replaces, preferably in the range of about 0.5-10%, more preferably in the range of about 0.5-5%. The term ":Eu", indicates that part of the metal ions is replaced by Eu (in these examples by Eu2+). For instance, assuming 2% Eu in CaAlSiN3 :Eu, the correct formula could be

(Cao.98Euo.o2)AlSiN3. Divalent europium will in general replace divalent cations, such as the above divalent alkaline earth cations, especially Ca, Sr or Ba.

The material (Ba,Sr,Ca)S:Eu can also be indicated as MS:Eu, wherein M is one or more elements selected from the group consisting of barium (Ba), strontium (Sr) and calcium (Ca); especially, M comprises in this compound calcium or strontium, or calcium and strontium, more especially calcium. Here, Eu is introduced and replaces at least part of M (i.e. one or more of Ba, Sr, and Ca). Further, the material (Ba5Sr5Ca)2SIsNSiEu can also be indicated as M2Si5Ns :Eu, wherein M is one or more elements selected from the group consisting of barium (Ba), strontium (Sr) and calcium (Ca). In a further specific embodiment, M consists of Sr and/or Ba (not taking into account the presence of Eu), preferably 50- 100% Ba, more preferably 50-90% Ba and 50-0% Sr, preferably 50-10% Sr, such as Ba1-5Sr0-5Si5N8IEu (i.e. 75 % Ba; 25% Sr). Here, Eu is introduced and replaces at least part of M, i.e. one or more of Ba, Sr, and Ca).

Likewise, the material (Ba,Sr,Ca)AlSiN3:Eu can also be indicated as MAlSiN3 :Eu, wherein M is one or more elements selected from the group consisting of barium (Ba), strontium (Sr) and calcium (Ca); M comprises in this compound preferably calcium or strontium, or calcium and strontium, more preferably calcium. Here, Eu is introduced and replaces at least part of M (i.e. one or more of Ba, Sr, and Ca).

According to a further embodiment of the wavelength converter, a reflective layer may be arranged in a way that most of the second light is reflected in the direction of the wavelength converting material. The efficiency of the wavelength converter may be improved by this measure as losses due to second light leaving the wavelength converter via a hole in the heat sink are at least reduced. The reflective layer may only reflect light in a defined spectral range such that the laser light can pass the reflective layer but the converted second light is reflected by means of the reflective layer. Alternatively, the reflective layer may be highly reflective in the whole visible spectral range and a small opening may be provided in the reflective layer to let the laser light pass.

According to another embodiment of the wavelength converter, the transparent material is adapted to widen the laser light. "Widen the laser light" means that the energy density of the laser light is reduced by means of the transparent material. The reduced energy density of the laser light may reduce local heating of the wavelength converting material as a bigger part of the wavelength converting material may be used to convert the laser light. The laser light may be widened by means of the optical properties of the transparent material. The transparent material may have, for example, a concave surface opposite to the surface of the transparent material attached to the first surface of the wavelength converting material. In this case the transparent material would operate as a diffuser lens. Alternatively or in addition, scattering particles may be embedded in the transparent material.

According to another embodiment the laser light enters the wavelength converter from the same surface as that where the second light leaves the wavelength converter. The wavelength converter operates in this case in a kind of "reflective mode".

According to a second aspect, the invention relates to a laser lighting device comprising a laser and at least one wavelength converter as described above. The laser is adapted to emit laser light of at least the first wavelength, and the laser is further adapted to ensure that the laser light after passing the transparent material and the window hits the first surface of the wavelength converting material. The laser makes it possible to distribute the light power by optical transmission, and the wavelength converter locally converts the light, for example, to diffuse white light that may be used for reading. Furthermore, the laser lighting device may further comprise a light guide, and the light guide is adapted to let the laser light pass the window. The light guide may have the advantage that the laser light can be transmitted from one point to another without using complicated optical arrangements such as combinations of mirrors and lenses. In addition, the safety of the laser lighting device with respect to laser safety may be improved by means of light guides. In addition or alternatively, the laser lighting device may comprise optical means being adapted to focus the laser light.

It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims with a respective independent claim.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings:

Figs. 1 to 12 illustrate schematically sections through different embodiments of the wavelength converter according to the present invention.

Fig. 13 illustrates schematically a section of a laser lighting device according to the present invention. Fig. 14 illustrates experimental results showing the effect of using a wavelength converter according to the present invention in a laser lighting device according to the present invention.

DESCRIPTION OF EMBODIMENTS

Fig. 1 schematically shows a cross section of a first embodiment of a wavelength converter 10 in accordance with the present invention. The wavelength converter comprises a disc-shaped heat sink 2 made of, e.g., Indium with a window (a pinhole) filled with transparent material 3 such as, for example, single crystal Sapphire. A wavelength converting material 1 such as, for example, a monolithic ceramic luminescence converter, is placed on one side of the heat sink 2, covering the pinhole and being attached to the transparent material.

A wavelength converter in accordance with a second embodiment of the present invention shown in Fig. 2 comprises, in comparison to the wavelength converter of Fig. 1, an additional reflective layer 4 between the heat sink 2 and the wavelength converting material 1. The reflective layer 4 is transparent in a first wavelength range, i.e. the wavelength range of a laser emitting laser light, but the reflective layer 4 is reflective in a second wavelength range, i.e. the wavelength range into which the wavelength converting material 1 converts light of the first wavelength. The reflective layer 4 may be a thin sequence of layers having high and low refractive indices. The thermal coupling among the constituents of the transparent material 3 filling a somewhat bigger window in the heat sink 2 in comparison to Fig. 1 is nearly unaffected due to the small thickness of the reflective layer 4.

A third embodiment of a wavelength converter in accordance with the present invention is shown in Fig. 3. A disc-shaped wavelength converting material 1 is embedded in a heat sink 2. Transparent material 3 being in contact with the wavelength converting material 1 is arranged in a part of an opening in the heat sink 3. The opening comprises a spherical cavity with the transparent material 3 and a cylindrical pinhole linked with the spherical cavity such that laser light can irradiate the wavelength converting material 1 after passing the pinhole and the spherical cavity with the transparent material. The transparent material may have various shapes, such as a hemispherical cylinder, parallelepiped, a truncated pyramid with different cross sections (square, hexagonal etc.) etc. In Fig.4, for example, atruncated pyramid-shaped wavelength converting material 1 is embedded in a heat sink 2. Transparent material 3 being in contact with the wavelength converting material 1 is arranged in a part of an opening in the heat sink 3. The opening comprises a disc-shaped cavity filled with the transparent material 3 and a pinhole linked to the cavity such that laser light can irradiate the wavelength converting material 1 after passing the pinhole and the transparent material 3.

In a fourth embodiment of a wavelength converter in accordance with the present invention shown in Fig. 5, the heat sink 2 has a rectangular window filled with a rectangular block of wavelength converting material 1 and a rectangular block of a transparent material 3. The wavelength converting material 1 and the transparent material 3 are in close contact and stacked upon each other. A reflective layer 4, being reflective in the whole visible spectral range, with a small hole is attached to a side of the heat sink 2 such that a laser beam can irradiate the wavelength converting material 1 after passing the small hole and the transparent material 3.

A fifth embodiment of a wavelength converter in accordance with the present invention is shown in Fig. 6. A disc-shaped wavelength converting material 1 is stacked upon a disc-shaped transparent material 3 having a larger diameter than the diameter of the disc-shaped wavelength converting material 1, and the transparent material 3 is stacked upon a heat sink 2 with a cylindrical window having a smaller diameter than the disc-shaped wavelength converting material 1. In a variation of the fifth embodiment, the wavelength converting material 1 may be embedded in the transparent material 3 such that the upper surface of the wavelength converting material (away from the heat sink) is aligned with the upper surface of the transparent material 3, and the transparent material is stacked upon a heat sink 2 with a window as shown in Fig. 7. In a further variation of the wavelength converter shown in Fig. 7, the transparent material 3 with the embedded wavelength converting material may be embedded in a heat sink 2. And a window is aligned with the transparent material 3 as shown in Fig. 8, such that laser light can irradiate the wavelength converting material 1 after passing the pinhole and the transparent material 3.

In a further embodiment of a wavelength converter in accordance with the present invention as shown in Fig. 9, a disc-shaped wavelength converting material 1 is stacked upon a disc-shaped transparent material and both are embedded in a discshaped heat sink 2 such that the upper side of the wavelength converting material 1 is aligned with the upper side of the heat sink 2. On the lower side of the heat sink 2 a window is provided such that a light guide 5 can be adopted in the window, and the light guide is suited for guiding laser light via the transparent material 3 to the wavelength converting material 1.

In accordance with another embodiment of a wavelength converter shown in Fig. 10, a disc-shaped wavelength converting material 1 is stacked upon a disc-shaped transparent material 3 and both are embedded in a block-shaped heat sink 2 such that the upper side of the wavelength converting material 1 is aligned with the upper side of the heat sink 2. A window in the form of a channel being essentially perpendicular to the rotation axis of the disc-shaped wavelength converting material 1, and the disc-shaped transparent material 3 is provided on one side of the heat sink 2 and a light guide 5 is adopted in this channel to contact the transparent material 3. Laser light can be guided via the light guide 5 and irradiate the wavelength converting material 1 after passing the transparent material. The surfaces of the heat sink 2 attached to the disc-shaped wavelength converting material 1 and the disc shaped transparent material 3 are preferably highly reflective.

A further embodiment of a wavelength converter in accordance with the present invention is shown in Fig. 11. A rectangular sheet of wavelength converting material 1 is attached to the upper side of a rectangular sheet of transparent material 3 and the lower side of the transparent material 3 is covered by a reflective layer 4 (a reflective coating) being transparent in a first wavelength range and reflective in a second wavelength range. The stack of these three materials is embedded in a block- shaped heat sink 2 such that the upper side of the wavelength converting material 1 is aligned with the upper side of the heat sink 2. A conical window is provided on the lower side of the heat sink 2, such that the widest opening of the window is located at the surface of the lower side of the heat sink 2 and the window narrows in the direction of the reflective layer 4 such that a small opening to the reflective layer 4 is provided. Furthermore, an optical means 6 like a lens is provided at the widest opening of the window to focus laser light having a wavelength in the first wavelength range to the small opening in order to pass the reflective layer 4 after passing the window in the heat sink 2. The laser light is at least partly converted to second light in the second wavelength range by means of the wavelength conversion material 1. The second light is reflected by the reflective layer 4 and optionally by the walls of the heat sink 2 circumventing the stack of wavelength converting material 1, transparent material 3 and reflective layer 4. The position of the reflective layer 4 which preferentially transmits the laser light and reflects the converted light may also be between the elements 1 and 3. The additional lens may offer the possibility to reduce the energy density of the laser light, for example, for eye safety reasons during the transmission of the laser light, but also enables a spot-like light source by means of the wavelength converter, which may be desirable for certain applications.

Figure 12 shows another embodiment where a single surface is used to pump the wavelength converting material 1 with a laser beam 21 and also extract the converted light 22 through the transparent material 3. The heat sink 2 has a rectangular window filled with a rectangular block of wavelength converting material 1 and a rectangular block of a transparent material 3. The wavelength converting material 1 and the transparent material 3 are in close contact and stacked upon each other such that the upper side or surface of the transparent material 3, which is not in contact with the wavelength converting material, is aligned with the upper side of the heat sink 2, and all surfaces of the wavelength converting material 1 other than the first surface attached to the transparent material are attached to the heat sink 2. The laser beam 21 passes the transparent material 3 in the window and enters the first side of the wavelength converting material 1 The laser light is, for example, converted to red light 23 which re- enters the transparent material 3 and finally leaves the transparent material 3 via the same surface as that where the laser beam 21 enters the transparent material 3, which means that the surface of the transparent material 3 is aligned with the upper side of the heat sink 2.

Fig. 13 schematically shows a laser lighting device in accordance with a second aspect of the present invention. The laser lighting device comprises a laser 20, a wavelength converter 10 and secondary optical means such as a reflector 30. The wavelength converter 10 comprises a heat sink consisting of Indium with an embedded monolithic ceramic luminescence converter used as wavelength converting material and a pinhole filled with single crystal Sapphire used as transparent material. The laser 20 emits a blue laser beam 21 such that the laser beam 21 is focused by an optical means like a lens to a spot size of 30 micron in diameter and passes the Sapphire and subsequently enters the monolithic ceramic luminescence converter. The monolithic ceramic luminescence converter is arranged in a way that the blue laser light is scattered and a first part of the blue light is converted to red light and a second part of the blue light is converted to green light such that white converted light 22 leaves the wavelength converter 10 on the side opposite to that where the laser light 21 enters the wavelength converter 10. As an additional measure the reflector 30 is coupled to the wavelength converter to focus the white light 22 onto a place where the light is needed. Fig. 14 shows the phosphor emission (y-axis, in arbitrary units), that means the power of the radiation emitted by the wavelength converter in relation to the laser power (x-axis, measured in mW) of a laser lighting device similar to the one shown in Fig. 13 (where the laser was focussed to a spot with a diameter of 30 micron), wherein the reflector 30 was omitted. The configuration shown in Fig. 13 was varied with respect to the diameter of the pinhole and the filling material of the pinhole but all other parameters of the wavelength converter were kept constant.

• Curve (a) shows the experimental results if the pinhole has a diameter of 200μm and the pinhole is filled with air. Curve (a) shows an increasing phosphor emission up to 40OmW laser power but a strongly decreasing phosphor emission when the laser power exceeds 40OmW (laser spot size 30 mm diameter A= π* 152 =700 μm2 corresponding to a laser power density of 400/700 mW/μm2=5.7*108W/m2).

• Curve (b) shows the experimental results if the pinhole has a diameter of lOOμm and the pinhole is filled with air. Curve (b) also shows an increasing phosphor emission up to 40OmW laser power, being higher than that shown in curve (a). Above 40OmW the phosphor emission drops again but less than in curve (a).

• Curve (c) shows the experimental results if the pinhole has a diameter of 200μm and the pinhole is filled with single crystal Sapphire. The phosphor emission increases linearly with increasing laser power up to a laser power of

80OmW. Especially in high power density applications it may be advantageous to use a monolithic ceramic luminescence converter as wavelength converting material, as the ceramic can withstand high temperatures. A monolithic ceramic luminescence converter is characterized by its typical microstructure. The microstructure of a monolithic ceramic luminescence converter is polycrystalline, i.e. an irregular conglomerate of cryptocrystalline, microcrystalline or nanocrystalline crystallites. Crystallites are grown so as to come into close contact and share grain boundaries. Macroscopically the monolithic ceramic seems to be isotropic, however, the polycrystalline microstructure may be easily detected by SEM (scanning electron microscopy).

The monolithic ceramic luminescence converter may also contain second phases at the grain boundaries of its crystallites, which change the light scattering properties of the ceramic. Such intergranular material can scatter light and thus increase the light path in the ceramic material due to a refractive index contrast at the grain boundary. The second phase material may be crystalline or vitreous.

Due to their monolithic polycrystalline microstructure, ceramic luminescence converters are transparent or have at least high optical translucency with low light absorption. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The reference signs in the claims should not be construed as limiting the scope of these claims.

Claims

CLAIMS:
1. A wavelength converter (10) for converting laser light (21) having at least a first wavelength to second light having at least a second wavelength different from the first wavelength, the wavelength converter (10) comprising:
- at least one wavelength converting material (1) having a first surface where the laser light impinges on the wavelength converting material;
- a transparent material (3) having a thermal conductivity greater than 0.8 W/(mK), more preferably greater than 10 W/(mK) and even more preferably greater than 30 W/(mK), attached to the first surface of the wavelength converting material (1);
- a heat sink (2) having a window for allowing the laser light (21) to pass, and the heat sink (2) is attached to the transparent material (3).
2. The wavelength converter (10) of claim 1, wherein the window has a cross-sectional area equal to or smaller than the area of the first surface of the wavelength converting material (1).
3. The wavelength converter (10) of claim 1 or 2, wherein the transparent material (3) is arranged between the wavelength converting material (1) and the heat sink (2).
4. The wavelength converter (10) of claim 1 or 2, wherein the transparent material (3) is arranged in the window.
5. The wavelength converter (10) of claim 4, wherein the heat sink (2) is attached to the first surface of the wavelength converting material (1).
6. The wavelength converter (10) of claim 4, wherein the heat sink (2) is attached to a second surface of the wavelength converting material (1).
7. The wavelength converter (10) of claim 5, wherein the heat sink (2) is attached to a second surface of the wavelength converting material (1).
8. The wavelength converter (10) of claim 1 or 2, wherein a reflective layer
(4) is arranged in a way that most of the second light is reflected in the direction of the wavelength converting material (1).
9. The wavelength converter (10) of claim 1 or 2 and the transparent material (2) being adapted to widen the laser light (21).
10. The wavelength converter (10) of claim 9, wherein the transparent material (2) has a concave surface opposite to the surface of the transparent material attached to the first surface of the wavelength converting material (1).
11. The wavelength converter ( 10) of claim 9 and the transparent material (2) comprising scattering particles.
12. A laser lighting device comprising a laser (20) and at least one wavelength converter (10) in accordance with any of the preceding claims, the laser (20) being adapted to emit laser light (21) of at least the first wavelength and the laser being further adapted to ensure that the laser light after passing the transparent material and the window hits the first surface of the wavelength converting material (1).
13. The laser lighting device of claim 12, further comprising a light guide
(5), and the light guide (5) being adapted to let the laser light (21) pass the window.
14. The laser lighting device of claim 12, further comprising optical means
(6) being adapted to focus the laser light.
PCT/IB2009/054724 2008-10-30 2009-10-26 Laser lighting device WO2010049875A1 (en)

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EP08167896 2008-10-30
EP08167896.3 2008-10-30

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Cited By (7)

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JP2015207755A (en) * 2014-04-09 2015-11-19 日亜化学工業株式会社 The light-emitting device
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