SYSTEM FOR LASER LIGHTING DEVICES

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) of the following co-pending and commonly-assigned applications:

U.S. Provisional Application Serial No. 62/870,900, filed on July 5, 2019, by Caroline E. Reilly, Guillaume Lheureux and Claude C.A Weisbuch, entitled “WAVELENGTH SELECTIVE PHOSPHOR CONVERTING SYTEM FOR LASER LIGHTING DEVICES,” attorneys’ docket number G&C 30794.0733USP1 (UC 2019- 961-1);

which application is incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED

RESEARCH AND DEVELOPMENT

This invention was made with Government support under Grant No. DE- AR0000671 from ARPA-E. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to a wavelength selective phosphor converting system for laser lighting devices.

2. Description of the Related Art.

Laser lighting has become a field of increasing interest in the past few years, especially when combined with phosphors to generate white light. Efficiency is always a concern in these systems, with every small boost in efficiency amounting to increased brightness as well as energy and cost savings during use. In addition, increased efficiency in these systems results in decreased heating and therefore less heat sinking and cooling necessary in the device or in the surroundings. Furthermore, increasing the amount of phosphor light escaping the system allows for less phosphor material to be used for a given device.

Thus, there is a need in the art for improved designs for using phosphors in downconverting light in laser lighting.

SUMMARY OF THE INVENTION

This invention discloses an apparatus and a method of fabricating the apparatus by packaging a light-emitting device, such as a laser diode or a light-emitting diode, with a phosphor component optically coupled to the light-emitting device, wherein a coating, such as a distributed Bragg reflector (DBR) or Fabry-Perot (FP) resonator, is applied to the phosphor component, and the coating is minimally reflective and maximally transmissive for one or more wavelengths of light from the light-emitting device, and the coating is maximally reflective and minimally transmissive for one or more wavelengths of light from the phosphor component. The coating is made of one or more layers of dielectric materials with different optical indices, such as silicon dioxide (SiO ₂ ) and tantalum pentoxide (Ta ₂ O ₅ ). The DBR has a structure with one or more repeating quarter-wavelength (l/4) layers, while the FP resonator has a structure with one or more l/4 layers surrounding a central half-wavelength (l/2) cavity.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 A is a schematic of an integrated laser lighting system, and Fig. IB is a photograph of the integrated laser lighting system. Fig. 2A is a schematic of a single crystal phosphor with a DBR on its backside; Fig. 2B is a schematic of the structure of the DBR with l/4 layers repeating; and Figs. 2C and 2D are graphs of emission or reflectance vs. wavelength l (nm) for simulations of DBR reflectivity with different period numbers and different maximum wavelength reflectivities, indicating DBR reflectivity in a plot with a triangle marker symbol, laser emission line in a plot with a circle marker symbol, and Ce: YAG phosphor emission spectrum in a plot with a square marker symbol.

Figs. 3A, 3B, 3C and 3D are transfer matrix simulations of the reflection coefficient at the phosphor/air interface, wherein Fig. 3 A is a graph of Reflection (R) coefficient vs. angle (degree) at normal incidence; Fig. 3B is a graph of the mean R coefficient as a function of wavelength and angle; Fig. 3C is a graph of the R coefficient vs. wavelength (nm) at the phosphor/DBR interface, at normal incidence of a beam coming from the phosphor, with the emission spectra of the phosphor compound overlaid; and Fig. 3D is a graph of the mean R coefficient of a beam coming from the phosphor with the DBR present as a function of wavelength and angle.

Fig. 4A is a schematic of a single crystal phosphor with an FP resonator on its backside; Fig. 4B is a schematic of the structure of the FP resonator with l/4 layers surrounding a central l/2 cavity; and Fig. 4C is a photograph of the backside of phosphor single crystals, after deposition of the FP resonator, that have been diced into triangles approximately 1 mm per side.

Fig. 5 is a graph of reflectance or emission vs. wavelength (nm), for simulations for wavelength-selective coatings indicating laser wavelength (442 nm), Ce: YAG phosphor emission, simulated normal reflectivity for a DBR (3-period centered at 570 nm), simulated normal reflectivity for the FP, and experimental reflectivity for the FP.

Fig. 6A is a graph of reflectance or emission vs. wavelength l (nm) of spectra collected in an integrating sphere, for emission from the integrated laser lighting system showing phosphor emission for similar thicknesses of single crystal phosphor with and without the FP resonator coatings; and Fig. 6B is a graph of maximum (max) phosphor emission vs. maximum (max) laser light of spectra collected in an integrating sphere, for several phosphors with similar thicknesses with and without FP resonator coatings.

Figs. 7 A, 7B and 7C are schematics of phosphors with DBRs on their back surface and with other changes to improve extraction, wherein Fig. 7A shows shaping, Fig. 7B shows a roughened top surface, and Fig. 7C shows both shaping and a roughened top surface.

Figs. 8A and 8B are schematics illustrating optical losses in a laser-single crystal phosphor device, wherein Fig. 8A is a schematic without any coating and Fig. 8B is a schematic with a wavelength selective reflective coating represented as alternating layers, such that sources of losses enclosed in the dashed area in Fig. 8A are minimized with the addition of the coating in Fig. 8B.

Fig. 9A is a graph of optical efficiency (%) vs. test number and Fig. 9B is a graph of maximum (Max) phosphor intensity vs. maximum (Max) laser intensity, providing comparisons between single crystal Ce: YAG phosphors without and with Fabiy-Perot coatings, wherein Fig. 9A shows the optical efficiency as a function of test number and Fig. 9B shows the maximum phosphor intensity versus the maximum laser intensity.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference is made to the accompanying drawing which forms a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized, and structural changes may be made without departing from the scope of the present invention. Overview

This invention allows for more efficient phosphor-based light-emitting devices. The application of this invention is for a light-emitting device, such as a III-nitride laser diode (LD) or light-emitting diode (LED), in which the emitted light is passed through, and partially converted by, a phosphor component in a transmission geometry. A coating is applied to the phosphor component such that the coating is minimally reflective and maximally transmissive for one or more wavelengths of light from the light-emitting device, and the coating is conversely maximally reflective and minimally transmissive for one or more wavelengths of light from the phosphor component. In this way, the amount of light-emitting device light coupled into the phosphor and the amount of phosphor light being projected upwards in the device are both increased. Both effects contribute to an increase in device efficiency.

The terms“III-nitride”,“group-III nitride”, or "III-N” refer to any alloy composition of (Ga, Al,In,B)N semiconductors having the formula where 0

£ w £ 1 , 0 £x £ 1, 0 £ y £ 1 , 0 £ z £ 1, and w + x + y + z = 1. III-nitride devices may be grown on various planes of the crystal, such as polar, nonpolar and semipolar planes, considering spontaneous and piezoelectric polarization effects of these planes.

The phrase“minimally reflective” for one or more wavelengths of light from the light-emitting device is defined as less than 8% reflective; the phrase“maximally transmissive” for one or more wavelengths of light from the light-emitting device is defined as more than 92% transmissive; the phrase“maximally reflective” for one or more wavelengths of light from the phosphor component is defined as more than 60% reflective; and the phrase“minimally transmissive” for one or more wavelengths of light from the phosphor component is defined as less than 40% transmissive. Technical Description

This invention increases the efficiency of phosphor-based light-emitting devices through the use of a wavelength-selective phosphor coating. In one embodiment, this invention increases the efficiency of a single-crystal phosphor-based laser diode lighting package by a significant margin, with testing showing around a 22% increase in efficacy.

Compared to conventional lighting systems, solid-state lighting has had success partly due to efficiency and form factor benefits provided by semiconductor lighting devices. White solid-state lighting is dominated by III-nitride LEDs, with laser-based lighting becoming a topic of interest more recently. Laser lighting provides benefits in specific applications, such as high-brightness directional emission. Many LEDs need to be used together to provide the same output power as one laser. By increasing the number of LEDs, the area over which the light is emitted also increases. If directionality is desired, the larger area of emission for LEDs will not be as well focused using lossless optical components as will be the smaller emitting area of a laser. To improve the directionality of the LED system, significant efficiency losses will occur, decreasing the brightness of the system. Laser lighting provides an efficient pathway for spotlight-like applications by providing high brightness over a small emitting area, improving the directionality of the light.

Converting from the blue emission from a III-nitride laser to white light requires the use of downconverter materials called phosphors, such as the yellow phosphor

Ce: YAG commonly used in blue light to yellow light conversion, where blue light plus yellow light makes white light. One considers the resulting light as white light although the Ce: YAG phosphor has a broad emission spectrum centered in yellow region of the visible spectrum because the Ce: YAG emission also contains significant components beyond yellow into the orange and red portions of the visible spectrum. One possible way to integrate the phosphor and the laser in a single device is the transmission geometry, in which the laser light passes through the phosphor while being partially converted. In this geometry, the laser light reaching the phosphor and the phosphor light emitted upward should be maximized, in particular by avoiding reflection of laser light at the phosphor surface and backward-emitted phosphor light, both of which could be absorbed by the package.

Fig. 1 A is a schematic of an integrated light-emitting device 100, including a laser diode 101 emitting light 102 located within a TO-9 can package 103, a copper heat sink 104 with a recess 105 on its back side exposing a cathode 106 and an anode 107 of the laser diode 101, and a phosphor holder 108 holding a phosphor component 109 emitting light 110 and a coating 111, wherein the laser 101 light 102 and the phosphor 109 light 110 may be combined, for example, into white light 112. Fig. IB is a photograph of the integrated light-emitting device 100.

This design allows for integration of the phosphor component 109 with the laser 101. The phosphor component 109 may be comprised of a single crystal phosphor, or a phosphor plate, or a phosphor powder in a glass or ceramic matrix, or a phosphor on a substrate, or other forms of phosphor components 109.

When the phosphor component 109 is an optically smooth single crystal Ce:YAG phosphor, the reflectivity at the phosphor-air interface is about 8%, such that 8% of the laser 101 light 102 incident normally on the surface of the phosphor component 109 will be reflected back and will be absorbed by the laser 101 and/or its package 103, contributing to the overall losses of the device 100.

The single crystal phosphor 109 needs to allow the light 102 to be coupled in from the laser 101 and have high extraction efficiency. A wavelength-selective reflective coating 111 has been implemented to address these concerns, which increases the luminous efficacy of the device 100. Engineering the phosphor component 109 using this concept may allow for single crystal phosphors 109 to be viable options for future laser lighting devices 100. Improvements to this reflectivity can be made through anti-reflection coatings 111, to increase the in-coupling of the laser 101 into the phosphor 109. But, this may lower the efficiency of the device 100, since it will, at the same time, favor the extraction of the light 102 in the backwards direction where it will be absorbed by the package 103. There is a need to selectively reflect the phosphor 109 light 110 upwards in these devices

100; therefore, a coating 111 should be wavelength selective to be reflective for the phosphor’s 109 light 110, and transmissive for externally impinging laser’s 101 light 102. One option for this wavelength-selective coating 111 may be a DBR coating 111, which is a design often used for high reflectivity. Another option for the coating 111, which has been experimentally tested by the inventors, is an FP resonator coating 111.

Both DBR and FP resonator coatings 111 are made of layers of dielectric materials with different optical indices. In one embodiment, the materials used for the coating 111 comprise alternating layers of silicon dioxide ( SiO ₂ ) and tantalum pentoxide (Ta ₂ O ₅ ), although other materials may be used. The idea is to design the thickness and the number of periods of the coating 111 to center the reflectivity maximum around the broad emission of the phosphor 109 and, at the same time, have a reflectivity minimum at the wavelength of the laser 101. A variety of materials may be used as the coating 1 11 layers, with optimization necessary for each coating 111 to tune the minimum and maximum wavelengths to match the laser 101 and phosphor 109 emissions, which may be different for various applications.

For a coating 111 comprising a DBR, alternating SiO ₂ and Ta ₂ O ₅ in l/4 thick layers gives rise to a photonic band gap which results in an optical reflectivity close to 1 for a range of wavelengths. At wavelengths below and above this plateau, the reflectivity of the DBR coating 111 oscillates and can be equal to zero for certain wavelengths. The structure of the DBR coating 111 proposed in this invention is shown in Figs. 2A and 2B, along with reflectivity calculations in Figs. 2C and 2D, and more detailed simulations are given in Figs. 3A, 3B, 3C and 3D. Fig. 2A is a schematic of a single crystal phosphor 109 with a DBR coating 111 on its backside; Fig. 2B is a schematic of the structure of the DBR coating 111 with l/4 layers repeating; and Figs. 2C and 2D are graphs of emission or reflectance vs.

wavelength l (nm) for simulations of DBR coating 111 reflectivity labeled with different period numbers (3 period in Fig. 2C and 7 period in Fig. 2D) and different maximum wavelength reflectivities (585 nm in Fig. 2C and 575 nm in Fig. 2D), indicating DBR coating 111 reflectivity in a plot with a triangle marker symbol, laser 101 emission line in a plot with a circle marker symbol, and Ce: YAG phosphor 109 emission spectrum in a plot with a square marker symbol. The thicknesses of the layers are described in terms of optical thicknesses, which correspond to the given material.

Figs. 3 A, 3B, 3C and 3D are transfer matrix simulations of the phosphor/air interface (Figs. 3A and 3B) or of the phosphor/DBR interface (Figs. 3C and 3D), with light impinging from the air or phosphor, respectively, wherein Fig. 3A is a graph of the Reflection coefficient vs. angle (degree) at normal incidence; Fig. 3B is a graph of the mean Reflection coefficient as a function of wavelength and angle; Fig. 3C is a graph of the Reflection coefficient vs. wavelength (nm) at the phosphor/DBR interface, at normal incidence of a beam coming from the phosphor 109, with the emission spectra of the phosphor 109 compound; and Fig. 3D is a graph of the mean Reflection coefficient with the DBR coating 111 of a beam coming from the phosphor 109 as a function of wavelength and angle. The efficient reflection of phosphor 109 light 110 with a wavelength adjusted to the forbidden band of the DBR coating 111 is clearly seen in Fig. 3D for almost all incident light directions.

Fig. 4A is a schematic of a single crystal phosphor 109 with a coating 111 comprising an FP resonator on its backside; Fig. 4B is a schematic of the structure of the FP resonator coating 111 with l/4 thick layers surrounding a central l/2 thick layer comprising a resonator cavity; and Fig. 4C is a photograph of the backside of the single crystal phosphor 109 after deposition of the FP resonator coating 111 that has been diced into triangles having dimensions of approximately 1 mm per side.

The FP resonator coating 111, with the design and an initial deposition as shown in Figs. 4A, 4B and 4C, can also provide a desirable reflectivity spectrum. The wavelength of the reflectance minimum for a FP resonator coating 111 is broad and largely determined by the l/2 cavity in the center of the FP resonator coating 111. This leads to the minimum being experimentally more controllable than in the case of the DBR coating 111, where the minimum is sharper. While the FP resonator coating 111 is more easily experimentally achievable, a DBR coating 111 that is properly calibrated and deposited would provide both a higher maximum reflectivity and a lower minimum reflectivity. This would lead to a greater overall device 100 efficiency improvement in the case of the DBR coating 111.

The FP resonator coating 111 detailed in Figs. 4A, 4B and 4C was deposited onto a single crystal Ce: YAG phosphor 109. The thickness of the phosphor 109 can be varied to optimize the color point of the device 100. Prior to deposition, the phosphor 109 was thinned and polished using chemical mechanical polishing (CMP) to achieve an optically flat surface for deposition. The FP resonator coating 111 was deposited used ion beam deposition (IBD), but could be deposited using other methods.

Simulations indicating the theoretical reflectivity spectra for a representative coating 111 of each type are shown in Fig. 5. The target wavelength maximum for the

DBR coating 111 was 570 nm with three periods, and the target wavelength minimum for the FP resonator coating 111 was 442 nm to match the experimental wavelength of a laser 101 initially tested. The FP resonator coating 111 in Figs. 4A and 4B was deposited on a polished Ce:YAG single crystal phosphor 109 using ion beam deposition. Layer thicknesses and refractive indices were calibrated using ellipsometry and the overall FP resonator coating 111 was calibrated by iterating on depositions and adjusting layer thicknesses. Using a similar method, the deposition of a DBR coating 111 was also attempted. Whereas the FP resonator coating 111 could be calibrated to have a minimum at the laser 101 wavelength, the DBR coating 111 was calibrated with respect to the wavelength of maximum reflectance. The fringes and minima were then heavily dependent on precise control of deposition rates and refractive indices, such that a DBR coating 111 with a minimum at the laser 101 wavelength was difficult to achieve and not reported herein. Normal reflectivity measurements were measured on reference polished sapphire pieces and on the final FP resonator coating 111 deposited on the single crystal phosphor 109 from the backside of the phosphor 109, given in Fig. 5. The theoretical minimum reflectivity was 1.2%, whereas the experimentally achieved minimum value was ~3%. In addition, there was significant overlap of the phosphor 109 emission spectrum and the wavelengths of higher reflectivity.

Figs. 6A and 6B show the difference in phosphor 109 emission from several tests, as well as the comparison of the maximum of the phosphor 109 emission to the maximum of the laser 101 emission. Specifically, Fig. 6A is a graph of reflectance or emission vs. wavelength l (nm), of spectra collected in an integrating sphere, for emission from the integrated laser lighting device 100 showing phosphor 109 emission for similar thicknesses of single crystal phosphor 109 with and without FP resonator coatings 111; and Fig. 6B is a graph of maximum (max) phosphor 109 emission vs. maximum (max) laser 101 light 102, of spectra collected in an integrating sphere, for several phosphors 109 with similar thicknesses with and without FP resonator coatings 111. Overall, the samples with the FP resonator coating 111 have a higher phosphor 109 emission for a given laser 101 emission, supporting the FP resonator coating 111 causing increased phosphor 109 light 110 directed upwards. With this result, this invention gives the additional benefit of lowering the amount of phosphor component 109 needed to create a device 100 of a desired color point This invention can be incorporated for different geometries or along with other light extraction techniques for the phosphor 109 light 110, such as shaping and roughening of the phosphor component 109. For example, Figs. 7A, 7B and 7C are schematics of phosphors 109 with DBR coatings 111 on a back surface of the phosphors 109, wherein Fig. 7A shows shaping of the phosphor 109 to improve extraction, Fig. 7B shows a roughened top surface of the phosphor 109 to improve extraction, and Fig. 7C shows both shaping and a roughened top surface of the phosphor 109 to improve extraction.

Although methods to improve extraction efficiency can be drawn from work done with LEDs, there the phosphor material typically encases the LED where adding the encapsulant improves the out-coupling from the LED. In the situation with the laser 101, the phosphor component 109 does not encase the laser 101, resulting in some losses.

Some sources of loss are depicted in Fig. 8 A. One source of loss is due to the reflection of the laser 101 light 102 at the surface of the phosphor 109, as shown in the dashed area 800. Any amount of laser 101 light 102 that gets reflected back by the phosphor 109 surface is lost due to absorption by the package 103, heat sink 104, or phosphor holder 108. Given the index of refraction of the single crystal phosphor 109 (n = 1.82), the percentage of incoming light 102 reflected at normal incidence for a phosphor 109 with an optically smooth interface was calculated to be 8.5%. This loss can be reduced by the addition of an antireflective (AR) coating such as a quarter-wavelength coating 111 tuned for the laser 101 light 102 wavelength. While an AR coating would serve to help with the laser 101 in-coupling efficiency, it may also serve to increase the amount of yellow light 110 extracted out of the bottom of the phosphor 109. In the transmission geometry, yellow phosphor 109 light 110 extracted in the downwards direction towards the laser 101 would be another source of loss due to absorption by the package 103, heat sink 104, or phosphor holder 108, as shown in the dashed area 800. A coating 111 that is reflective in the yellow range and transmissive in the blue range is therefore desired. As indicated in Fig. 8B, this coating 111 could minimize incoupling losses as well as losses due to downward reflection of yellow light 110, as shown in the dashed area 800. Such a coating 111 could take the form of a DBR or FP resonator coating 111, either of which would be designed to have wavelength dependent reflectivity over the visible wavelength range.

The DBR coating 111 with the structure shown in in Figs. 2A and 2B, and the FP resonator coating 111 with the structure shown in Figs. 4A and 4B, are two examples of potential coatings 111. In both of the DBR coating 111 and the FP resonator coating 111, the optical thicknesses of the thinner layers are l/4 thick, while the resonant Ta ₂ O ₅ cavity was l/2 thick in the FP resonator coating 111. The SiO ₂ side was in contact with the single crystal phosphor 109 for both of the DBR coating 111 and the FP resonator coating 111. The choice of dielectric material and periodicity was not unique, as a variety of designs may achieve varying levels of reflectivity.

A series of phosphors 109 was then tested with and without the presence of a FP resonator coating 111, in order to provide an initial demonstration of the coating 111 technology presented. A single crystal phosphor 109 with an FP resonator coating 1 11 was diced into triangles (~ 1 mm per side, 800 mm thick) and the diced phosphors 109 were compared to a set of uncoated single crystal phosphors 109 (~ 1 mm per side, 850 mm thick), where all phosphors 109 had the same equilateral triangle shape and side length. The marginally thicker phosphor 109 was used as the uncoated sample so that any increase in phosphor 109 extraction provided by the FP resonator coating 111 could be attributed to the coating 111 and not to the thickness difference. Ten tests were conducted in which the laser 101 was tested alone, followed by testing of an uncoated sample, followed by a sample with the FP resonator coating 111. The compact setup was utilized in each case. For each pair of uncoated and coated samples, the wall plug efficiency (WPE) of the laser 101 as tested before the phosphors 109 was considered the WPE of the laser 101 during the phosphor 109 test in order to consider laser 101 fluctuations. The relative optical efficiency (OE) of the device 100 was estimated based on the Equation set forth below, wherein QY is the quantum yield of the phosphor 109, SS is the Stokes Shift of the phosphor 109 conversion, and LER is the luminous efficacy of radiation, with the QY of the phosphor 109 and the SS of the phosphor 109 conversion assumed to be 90% and 78%, respectively. Although the Equation does not consider that the QY and SS only apply to the laser 101 light 102 being absorbed and converted by the phosphor 109, respectively, the relative estimation for OE should be comparable sample to sample.

The average efficacy, LER, and OE were all greater in the samples with the FP resonator coating 1 1 1. The average efficacy increased by 22% with the addition of the FP resonator coating 111. Fig. 9A compares the OE of each sample over the order of testing the samples, showing that effects from any deviations in the laser 101 power were mitigated. The average relative OE was 45% and 52% for the samples without and with the FP resonator coating 111, respectively. The improvement in OE with the addition of the FP resonator coating 111 was attributed to better in-coupling of the laser 101 , as well as better extraction of the phosphor 109 light 110.

Fig. 9B shows a comparison in the maximum of the phosphor 109 peak and the maximum of the laser 101 peak. The phosphors 109 with the coating 111 had more phosphor 109 emission for a given laser 101 peak height. This led to the increase in LER with the introduction of the FP resonator coating 111, as the yellow phosphor 109 light 110 overlaps more with the eye response function than the blue laser 101 light 102. The average LER without the coating 111 was 364 lm/W and with the FP resonator coating 111 was 385 lm/W. The increase in LER also suggests that the use of the FP resonator coating 111 can allow for less phosphor component 109 to be used in order to achieve the desired white light 112 point The increase in LER with the increase in OE together form the basis for the reported increase in luminous efficacy in the device 100.

Alternatives and Modifications

This invention includes a number of alternatives and modifications as set forth below.

Edge-emitting lasers, VCSELs, or other types of light emitters may be used, with different wavelengths than blue, eventually in addition to blue lasers, VCSELs, or other types of light emitters.

Lasers may be incorporated as packaged devices, such as the example of the TO-9 can laser, or may be incorporated into the device as a laser bar with packaging encasing the phosphor.

Various DBR or FP resonator materials may be used, as well as different deposition techniques and various precursors. The invention can be modeled and tuned over any wavelength range of lasers and/or phosphors with proper simulations, calibrations, and deposition techniques.

The invention can be used with types of phosphor other than Ce: YAG.

Multiple phosphors and/or multiple lasers with different wavelengths of emission can be used.

The invention can be used for laser diodes of varied wavelengths, including ultraviolet (UV), blue, green, or red lasers, as well as LEDs.

This invention can be used for types of phosphors other than single crystal, including but not limited to ceramics. The invention may be comprised of multiple phosphors incorporated as powders, layers of single-crystal phosphors, ceramics, deposited in a sol-gel, or other ways.

The invention may be employed to tune the color point of the device by altering the ratios of various wavelengths escaping the device by a proper design of the multilayer coating (beyond the simple DBR or FP resonator designs).

This invention can decrease the amount of phosphor material necessary to achieve a desired color point

This invention may apply in cases where the laser and phosphor are spatially separated and/or separated by other optical elements.

This invention could be applied to other form factors, including designs in which lasers are coupled into fibers and then converted into white light by a phosphor at the output of the fiber, or phosphor shaped as fiber.

The frequency selective coatings described here, both DBR and FP resonator, should be seen as a general coating with wavelength selective functionality. One of skill in the art would know how to produce any desired spectral optical transmission or reflection coefficient by the association of multilayers of different indices and thicknesses, such as the Rugate or the dichroic filters, but not limited to these designs.

The invention reported here describes the general use of a coating highly transmissive of incoming light onto a phosphor and highly reflective of the phosphor emitted light.

Conclusion

This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.