WO2006045897A1 - Light emitting device and method for directing light - Google Patents

Light emitting device and method for directing light Download PDF

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Publication number
WO2006045897A1
WO2006045897A1 PCT/FI2005/050373 FI2005050373W WO2006045897A1 WO 2006045897 A1 WO2006045897 A1 WO 2006045897A1 FI 2005050373 W FI2005050373 W FI 2005050373W WO 2006045897 A1 WO2006045897 A1 WO 2006045897A1
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WO
WIPO (PCT)
Prior art keywords
light emitting
reflecting surface
emitting diode
resonant cavity
light
Prior art date
Application number
PCT/FI2005/050373
Other languages
French (fr)
Inventor
Pekka Iso-Ketola
Jukka Vanhala
Original Assignee
Pekka Iso-Ketola
Jukka Vanhala
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Pekka Iso-Ketola, Jukka Vanhala filed Critical Pekka Iso-Ketola
Publication of WO2006045897A1 publication Critical patent/WO2006045897A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/48—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor body packages
    • H01L33/58—Optical field-shaping elements
    • H01L33/60—Reflective elements
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
    • G02B6/4214—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms the intermediate optical element having redirecting reflective means, e.g. mirrors, prisms for deflecting the radiation from horizontal to down- or upward direction toward a device
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/10—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector
    • H01L33/105—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector with a resonant cavity structure

Definitions

  • the present invention relates to a light emitting device comprising a light emitting diode and a reflecting surface, said light emitting diode being adapted to emit a diverging light beam into a direction, and said reflecting surface being adapted to direct light emitted from said light emitting diode substantially into said direction.
  • the present invention relates also to a method to direct light using said reflecting surface.
  • the present invention relates to a combination of an optical waveguide and said light emitting device. Yet, the present invention relates to a display unit comprising said light emitting device.
  • Light emitting diodes emit a beam with high divergence. This is a drawback in applications in which high optical power should be concentrated into a predetermined direction. On the other hand, light emitted to unwanted directions may cause dazzling (glare) of human observers or cross coupling between separate signal channels, for example.
  • the divergence of the beam may be reduced using optical elements such as lenses, reflectors or combinations thereof.
  • Japanese patent publication JP10221574 discloses a method to couple light from a light source to an optical fiber using a conical reflecting surface. According to the teaching, the half-cone angle of the reflecting surface is advantageously selected according to the acceptance angle of the optical fiber, i.e. according to the numerical aperture of the fiber.
  • the height of the cone is rather high and a portion of emitted light may be reflected several times at the conical reflecting surface before impinging on the fiber.
  • the devices and the method according to the present invention are primarily characterized in what will be presented in the characterizing part of the independent claims. Further details related to the invention are presented in the dependent claims.
  • the devices and the method according to the present invention are primarily characterized in that the height of the reflecting surface with respect to the level of the active surface of the light emitting diode is smaller than or equal to the diameter of said active surface multiplied by 1 ,5, said light emitting diode comprising a resonant cavity.
  • said reflecting surface has the form of a mantle of a truncated cone, the half-cone angle of said cone being selected to minimize the divergence of the emitted beam.
  • Resonant cavity light emitting diodes with a large active surface and high beam divergence may be used in a device according to the present invention. Consequently, the power of a low-divergence beam emitted from the device according to the present invention may be substantially higher than the power of a beam emitted from a prior art device, said devices having the same predetermined height.
  • a device according to the present invention may be substantially smaller than a prior art device, said devices having the same optical power.
  • the device according to the present invention is suitable for use in miniature systems.
  • the height of the reflecting surface is low, it may be easily implemented by various stamping, molding or machining techniques.
  • the device is implemented by coating a hollow conical reflector with a metal layer and/or with a dielectric layer. The low height facilitates the coating of said hollow reflector e.g. by vacuum coating techniques.
  • Fig. 1 shows schematically a resonant cavity light emitting diode
  • Fig. 2 shows schematically an optical beam with high divergence emitted from a resonant cavity light emitting diode
  • Fig. 3 shows, by way of example, angular distributions of intensity emitted by two resonant cavity light emitting diodes
  • Fig. 4 shows schematically an embodiment of the present invention, said embodiment being based on a conical reflecting surface
  • Fig. 5 shows schematically the dimensions of the reflecting surface and of the resonant cavity light emitting diode
  • Fig. 6a shows, by way of example, an angular distribution of optical power density emitted by a resonant cavity light emitting diode
  • Fig. 6b shows normalized optical power of a beam emitted into a conical solid angle by a resonant cavity light emitting diode according to Fig. 6a, as a function of half-cone angle, said solid angle being defined by said half-cone angle,
  • Fig 7a shows, by way of example, an angular distribution of optical power density emitted by a light emitting device according to the present invention
  • Fig. 7b shows normalized optical power of a beam emitted into a conical solid angle by a light emitting light emitting device according to Fig. 7a, as a function of half-cone angle, said solid angle being defined by said half-cone angle,
  • Fig. 8 shows schematically a light emitting device comprising a conical shell coated with a reflective layer
  • Fig. 9 shows schematically a light emitting device comprising an asymmetric conical reflector
  • Fig. 10 shows schematically a light emitting device comprising a slab with a conical opening, the interior of said conical opening being coated with a reflective layer,
  • Fig.11 shows schematically a light emitting device comprising a transparent conical body
  • Fig. 12 shows schematically a combination of an optical waveguide and a light emitting device according to the present invention
  • Fig. 13 shows schematically a combination of an optical waveguide and a light emitting device, a tapered end of said waveguide being used as a reflector,
  • Fig. 14 shows schematically a light emitting device comprising a reflector with two different cone angles
  • Fig. 15 shows schematically a light emitting device comprising a reflector with two-dimensional curvature
  • Fig. 16 shows schematically a display unit comprising a plurality of light emitting devices according to the present invention.
  • a resonant cavity light emitting diode 10 is a device which comprises a solid state medium 4 implemented between at least two mirrors 5,6.
  • the solid state medium emits light based on radiative electron- hole recombination in at least one p-n junction.
  • the mirrors 5,6 are at least partially reflecting and define a resonant cavity 7 between them.
  • Light emitted from the solid state medium 4 is reflected back and forth between said mirrors 5,6, enhancing light emission.
  • Light escapes through the output mirror 5 forming an incoherent light beam BB1 emitted from the resonant cavity light emitting diode.
  • One or more layers of the solid state medium 4 may also act as mirrors 5,6.
  • the resonant cavity light emitting diode 10 may also emit light at wavelengths invisible to human eyes.
  • the emitted light rays LL constitute the emitted beam BB1 , which propagates, in average, in direction SZ and exhibits high divergence.
  • the direction SZ is the averaged direction of all light rays LL constituting the emitted beam BB1. Typically, the direction SZ is perpendicular to the mirrors 5,6.
  • the beam BB1 is transmitted through the output mirror 5.
  • the near- field diameter NFD1 of the beam BB1 defines a circular portion on the output mirror 5, said portion being called herein as the active surface 11.
  • the near-field diameter NFD1 of the beam BB1 is defined to be the diameter of a circle, which encloses exactly 95% of the optical power of the beam BB1 at the level of the output mirror 5.
  • the optical axis N1 is parallel to the direction of propagation SZ of the beam BB1 and intersects the center of the active surface 11.
  • ⁇ denotes an angle between a light ray LL and the direction SZ.
  • Fig. 3 shows, by way of example, angular distributions of intensity INT emitted by two resonant cavity light emitting diodes 10.
  • the distributions SL, DL are associated with different cavity lengths, but with the same driving current, approximately 5 milliamperes, coupled to the light emitting diodes 10.
  • the distributions SL, DL are shown as polar plots.
  • the direction 0° is defined to be in the direction SZ.
  • the distributions SL and DL are far-field distributions.
  • the distribution marked by SL exhibits a maximum which is associated with the direction 0°, i.e. the direction SZ.
  • the distribution marked with DL exhibits a maximum which is associated approximately with the direction 41 ,5°, and there is a local intensity minimum at the direction 0°.
  • the angular distribution of the intensity of the emitted beam BB1 may also exhibit a local minimum in the direction SZ.
  • the optical power associated with the distribution DL is greater than two times the optical power associated with the distribution SL.
  • the maximum optical power emitted by a resonant cavity light emitting diode 10 is associated with an angular intensity distribution, which exhibits a local minimum in the direction SZ, the distribution DL being an example.
  • the light emitting device 100 comprises at least a resonant cavity light emitting diode 10 and a reflecting surface 20.
  • the reflecting surface 20 is adapted to direct light emitted by the light emitting diode 10 substantially towards the direction SZ.
  • the light rays LL emitted by the light emitting diode 10 and the rays reflected from the surface 20 constitute the beam BB2 emitted from the light emitting device 100.
  • the circular active surface 11 of the resonant cavity light emitting diode 10 has a diameter DIAL
  • the diameter DIA1 is equal to the near-field diameter NFD1 (Fig. 1 ) of the beam BB1.
  • the reflecting surface 20 has a height H1 with respect to the output plane of the mirror 5 of the resonant cavity light emitting diode 10, i.e. with respect to the active surface 11.
  • the angle ⁇ between the reflecting surface and the direction SZ is substantially greater than 0°, i.e. the reflecting surface 20 is inclined with respect to the optical axis N1.
  • Fig. 6a shows, by way of example, an angular distribution of optical power density (dP/d ⁇ ) emitted by a resonant cavity light emitting diode 10 (Fig. 2).
  • the dimension of the optical power density is herein Watts per degrees (W/°).
  • the abscissa value associated with the maximum power density is marked by ⁇ MAX .
  • the angular distribution of optical power density shown in Fig. 6a was calculated based on the angular distribution of intensity DL shown in Fig. 3, assuming axial symmetry.
  • the angle ⁇ x associated with the maximum of the power density is typically slightly greater than the angle associated with the maximum intensity.
  • Fig 6b shows normalized optical power P/P MAX emitted by the resonant cavity light emitting diode 10 (Fig. 2) into a conical solid angle having a half-cone angle ⁇ with respect to the direction SZ.
  • the values have been normalized by dividing the power values P by the total optical power P MAX of the beam BB1.
  • the light emitting diode 10 is the same as in Fig. 6a.
  • the curve shown in Fig. 6a is the derivative of the curve shown in Fig. 6b.
  • Fig. 7a shows, by way of example, an angular distribution of optical power density (dP/d ⁇ ) emitted by a light emitting device 100, said light emitting device 100 comprising a conical reflecting surface 20 (Figs. 4 and 5).
  • the half-cone angle ⁇ is 22,5° and the height H1 of the reflecting surface 20 is 0,8 times the diameter DIA1 of the active surface 11.
  • the light emitting diode 10 is the same as in Fig. 6a.
  • the half-cone angle ⁇ of the reflecting surface is selected according to the equation:
  • an optimum angle ⁇ between the reflecting surface 20 and the direction SZ is substantially equal to an angle ⁇ MAX divided by two, said angle ⁇ MAX being associated with a maximum angular optical power density emitted by the light emitting diode.
  • manufacturing tolerances should be taken into consideration, and the optimum angle ⁇ between the reflecting surface 20 and the direction SZ is preferably within ⁇ MAX /2 ⁇ 3°.
  • the angle ⁇ MAX associated with the maximum power density emitted by a resonant cavity light emitting diode 10 is typically in the range 30° to 50°.
  • the half-cone angle ⁇ of the reflecting surface is advantageously in the range 15° to 25°.
  • the angular distribution of power density depends on the refractive index of the light-transmitting substance, e.g. glass.
  • the angles ⁇ MAX and ⁇ in equation (1) refer to the situation in the light-transmitting substance.
  • Fig. 7b shows normalized optical power P/P MAX emitted by the light emitting device 100 into a conical solid angle having a half-cone angle ⁇ with respect to the direction SZ (Figs. 4 and 5).
  • the values have been normalized by dividing the power values P by the total optical power P MAX of the beam BB1.
  • the light emitting diode 10 and the reflecting surface 20 are the same as in Fig. 7a.
  • the curve shown in Fig. 7a is the derivative of the curve shown in Fig. 7b.
  • the boundaries of the beams BB1 , BB2 must be specified.
  • the divergence of the beam BB1 is 94° when the boundary of the beam BB2 is set according to 50% of the total optical power.
  • the divergence of the beam BB2 is 36° when the boundary of the beam BB2 is set according to 50% of the total optical power.
  • the half-cone angle ⁇ of the reflecting surface is approximately 22,5 degrees (Fig. 5).
  • the height H1 of the reflecting surface 20 is selected to be smaller than or equal to 1 ,5 times the diameter DIA1 of the active surface 11 , then substantially no light rays LL emitted from the active surface 11 are reflected more than once at the reflecting surface 20, considering only light rays LL traveling in a plane including the optical axis N1.
  • the height H1 of the reflecting surface 20 is advantageously greater than or equal to the 0,5 times the diameter DIA1 of the active surface 11. Otherwise light rays LL emitted at an angle of 45 degrees from the central region of the active surface 11 are not reflected.
  • the light emitting device 100 may comprise a conical shell 21.
  • the inside of the shell 21 may be coated with a reflective coating 20a.
  • the shell 21 may be transparent and the exterior may be coated with a reflective coating 20b.
  • the shell 21 may be implemented by molding or stamping plastic, glass ceramics or metal.
  • the shell 21 may also be machined by drilling, turning and/or grinding.
  • the shell 21 may be coated with a reflective layer by vacuum deposition.
  • the reflective layer may be implemented using aluminum, oxide-coated aluminum, rhodium, or dielectric multilayer coatings.
  • the reflecting surface 20 comprises electrically conductive material
  • electrical contact with the active surface 11 of the light emitting diode 10 may be prevented with a proper spacing between the reflecting surface 20 and the active surface 11.
  • the light emitting device 100 may comprise an asymmetric reflecting surface 20, e.g. a portion of a conical reflecting surface. This embodiment may be advantageous when available space is very limited, but maximum efficiency is not required.
  • the reflecting surface 20 may be implemented by forming a conical opening in a slab 22, and by coating the interior of the conical opening with a reflecting surface 20.
  • the reflecting surface 20 may be based on total internal reflection in a transparent conical body 24.
  • the reflecting surface 20 may be covered with a protective coating. If the refractive indices and/or the orientation of the reflecting surface 20 with respect to the emitted light rays do not fulfill the criterion required for total internal reflection, the surface 20 may be coated with an additional reflective layer.
  • Reflecting surfaces 20 based on total internal reflection are preferred over reflectors based on metallic surfaces, because reflection losses are negligible in case of total internal reflection.
  • a space between the body 24 and the light emitting diode 10 may be provided with suitable protrusions or spacers.
  • the light emitting device 100 may be used to couple light into the core 201 of an optical waveguide 200. It is advantageous to implement a bevel and guide surfaces to enable easy insertion and positioning of the waveguide 200 into the device 100. A portion of the cladding 202 of the waveguide 200 may be used as a fixing surface. The light emitting device 100 and the optical waveguide 200 constitute together a combination 400.
  • a portion of the optical waveguide 200 may be tapered.
  • the reflecting surface 20 may be implemented using the tapered portion.
  • the operation of the combination 400 may be based on total internal reflection, without additional coatings.
  • the tapered portion may be coated with additional reflective and/or protective layers.
  • the tapered portion may comprise a cladding layer (not shown) which has lower refractive index than the core 201 of the waveguide 200.
  • the tapered portion may be implemented by molding, by grinding, by turning, by heating and pulling or by joining an additional conical body to the end of the waveguide 200, for example. It is advantageous that the tapered portion is short, as it is stronger than a long thin tapered fiber.
  • the straight portion of the waveguide 200 comprises a cylindrical reflecting surface, i.e. the core-cladding interface. However, as the core-cladding interface is substantially parallel to the direction SZ, the angle ⁇ between the light rays and the direction SZ is not reduced in reflections at the cylindrical portion. The cylindrical portion does not contribute to the height H1 of the reflecting surface 20 of the light emitting device 100.
  • the device 100 may comprise several conical surfaces stacked on top of each other, which surfaces have different cone angles, i.e. different half cone angles ⁇ .
  • the height H1 refers to the combined height of the stacked reflecting surfaces 20, with respect to the level of the active surface 11.
  • the reflecting surface 20 may also exhibit a curved form in two dimensions.
  • the form may be parabolic, ellipsoidal or spherical.
  • a display unit 500 may be implemented using an array of light emitting devices 100.
  • a plurality of light emitting devices 100 may be implemented in a single plate.
  • the low reflecting surface 20 provides good efficiency for concentrating light emitted from the resonant cavity light emitting diode 10.
  • the size of the device 100 may be reduced and/or the optical power increased.
  • the use of the device 100 according to the present invention is advantageous when a concentrated high-intensity beam should be generated in a low-cost small-volume device.
  • the use of the reflecting surface 20 reduces light intensity in unwanted directions, i.e. in directions deviating significantly from the direction SZ. This helps to reduce cross coupling between optical signal channels, dazzling (glare) of detectors or human observers, or radiative heating of components.
  • the devices 100, 400, 500 may be used to implement fiber optic transmitters used in telecommunications, headlights and stoplights of vehicles, navigation lights, traffic lights, emergency beacons in vehicles and buildings, light torches, illuminating devices attached to garments, beacons attached to life-vests or other life-saving equipment, visual displays, screens and data projectors, for example.

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  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Manufacturing & Machinery (AREA)
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Abstract

The invention relates to the reduction of the divergence of a light beam (BB1) emitted by a resonant cavity light emitting diode (10), said beam (BB1) being emitted in a direction (SZ). Light emitted by the resonant cavity light emitting diode (10) is directed substantially towards said direction (SZ) by a reflecting surface (20). According to the present invention, the height (H1) of said reflecting surface (20) with respect to the level of the active surface (11) of said resonant cavity light emitting diode (10) is smaller than or equal to the diameter (DIA1) of said active surface (11) of said resonant cavity light emitting diode (10) multiplied by a factor of 1,5. Advantageously, the reflecting surface (20) has the form of a truncated cone.

Description

LIGHT EMITTING DEVICE AND METHOD FOR DIRECTING LIGHT
The present invention relates to a light emitting device comprising a light emitting diode and a reflecting surface, said light emitting diode being adapted to emit a diverging light beam into a direction, and said reflecting surface being adapted to direct light emitted from said light emitting diode substantially into said direction. The present invention relates also to a method to direct light using said reflecting surface.
Further, the present invention relates to a combination of an optical waveguide and said light emitting device. Yet, the present invention relates to a display unit comprising said light emitting device.
BACKGROUND OF THE INVENTION
Light emitting diodes emit a beam with high divergence. This is a drawback in applications in which high optical power should be concentrated into a predetermined direction. On the other hand, light emitted to unwanted directions may cause dazzling (glare) of human observers or cross coupling between separate signal channels, for example. The divergence of the beam may be reduced using optical elements such as lenses, reflectors or combinations thereof.
Japanese patent publication JP10221574 discloses a method to couple light from a light source to an optical fiber using a conical reflecting surface. According to the teaching, the half-cone angle of the reflecting surface is advantageously selected according to the acceptance angle of the optical fiber, i.e. according to the numerical aperture of the fiber.
Consequently, the height of the cone is rather high and a portion of emitted light may be reflected several times at the conical reflecting surface before impinging on the fiber.
SUMMARY OF THE INVENTION It is an object of the present invention to reduce the divergence of a light beam emitted by a light emitting diode. It is a further object of the present invention to simplify the implementation of the required divergence reducing means.
To attain these objects, the devices and the method according to the present invention are primarily characterized in what will be presented in the characterizing part of the independent claims. Further details related to the invention are presented in the dependent claims.
To attain these objects, the devices and the method according to the present invention are primarily characterized in that the height of the reflecting surface with respect to the level of the active surface of the light emitting diode is smaller than or equal to the diameter of said active surface multiplied by 1 ,5, said light emitting diode comprising a resonant cavity.
Preferably, said reflecting surface has the form of a mantle of a truncated cone, the half-cone angle of said cone being selected to minimize the divergence of the emitted beam.
It has been noticed that when using a resonant cavity light emitting diode as a light source, even a low reflector is effective in reducing the divergence of the emitted beam.
Resonant cavity light emitting diodes with a large active surface and high beam divergence may be used in a device according to the present invention. Consequently, the power of a low-divergence beam emitted from the device according to the present invention may be substantially higher than the power of a beam emitted from a prior art device, said devices having the same predetermined height.
Alternatively, a device according to the present invention may be substantially smaller than a prior art device, said devices having the same optical power. Thus, the device according to the present invention is suitable for use in miniature systems. As the height of the reflecting surface is low, it may be easily implemented by various stamping, molding or machining techniques. In certain embodiments the device is implemented by coating a hollow conical reflector with a metal layer and/or with a dielectric layer. The low height facilitates the coating of said hollow reflector e.g. by vacuum coating techniques.
A majority of the light rays of light are reflected only once at the reflecting surface. Thus, the reflective losses may be significantly reduced when compared with prior art devices.
The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which
Fig. 1 shows schematically a resonant cavity light emitting diode,
Fig. 2 shows schematically an optical beam with high divergence emitted from a resonant cavity light emitting diode,
Fig. 3 shows, by way of example, angular distributions of intensity emitted by two resonant cavity light emitting diodes,
Fig. 4 shows schematically an embodiment of the present invention, said embodiment being based on a conical reflecting surface, Fig. 5 shows schematically the dimensions of the reflecting surface and of the resonant cavity light emitting diode,
Fig. 6a shows, by way of example, an angular distribution of optical power density emitted by a resonant cavity light emitting diode,
Fig. 6b shows normalized optical power of a beam emitted into a conical solid angle by a resonant cavity light emitting diode according to Fig. 6a, as a function of half-cone angle, said solid angle being defined by said half-cone angle,
Fig 7a shows, by way of example, an angular distribution of optical power density emitted by a light emitting device according to the present invention,
Fig. 7b shows normalized optical power of a beam emitted into a conical solid angle by a light emitting light emitting device according to Fig. 7a, as a function of half-cone angle, said solid angle being defined by said half-cone angle,
Fig. 8 shows schematically a light emitting device comprising a conical shell coated with a reflective layer,
Fig. 9 shows schematically a light emitting device comprising an asymmetric conical reflector,
Fig. 10 shows schematically a light emitting device comprising a slab with a conical opening, the interior of said conical opening being coated with a reflective layer,
Fig.11 shows schematically a light emitting device comprising a transparent conical body, Fig. 12 shows schematically a combination of an optical waveguide and a light emitting device according to the present invention,
Fig. 13 shows schematically a combination of an optical waveguide and a light emitting device, a tapered end of said waveguide being used as a reflector,
Fig. 14 shows schematically a light emitting device comprising a reflector with two different cone angles,
Fig. 15 shows schematically a light emitting device comprising a reflector with two-dimensional curvature, and
Fig. 16 shows schematically a display unit comprising a plurality of light emitting devices according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Fig. 1 , a resonant cavity light emitting diode 10, called also as the RCLED, is a device which comprises a solid state medium 4 implemented between at least two mirrors 5,6. The solid state medium emits light based on radiative electron- hole recombination in at least one p-n junction. The mirrors 5,6 are at least partially reflecting and define a resonant cavity 7 between them. Light emitted from the solid state medium 4 is reflected back and forth between said mirrors 5,6, enhancing light emission. Light escapes through the output mirror 5 forming an incoherent light beam BB1 emitted from the resonant cavity light emitting diode. One or more layers of the solid state medium 4 may also act as mirrors 5,6. The resonant cavity light emitting diode 10 may also emit light at wavelengths invisible to human eyes.
The emitted light rays LL constitute the emitted beam BB1 , which propagates, in average, in direction SZ and exhibits high divergence.
The direction SZ is the averaged direction of all light rays LL constituting the emitted beam BB1. Typically, the direction SZ is perpendicular to the mirrors 5,6.
The beam BB1 is transmitted through the output mirror 5. The near- field diameter NFD1 of the beam BB1 defines a circular portion on the output mirror 5, said portion being called herein as the active surface 11. The near-field diameter NFD1 of the beam BB1 is defined to be the diameter of a circle, which encloses exactly 95% of the optical power of the beam BB1 at the level of the output mirror 5.
Referring to Fig. 2, the optical axis N1 is parallel to the direction of propagation SZ of the beam BB1 and intersects the center of the active surface 11. α denotes an angle between a light ray LL and the direction SZ.
Fig. 3 shows, by way of example, angular distributions of intensity INT emitted by two resonant cavity light emitting diodes 10. The distributions SL, DL, are associated with different cavity lengths, but with the same driving current, approximately 5 milliamperes, coupled to the light emitting diodes 10. The distributions SL, DL are shown as polar plots. The direction 0° is defined to be in the direction SZ. The distributions SL and DL are far-field distributions. The distribution marked by SL exhibits a maximum which is associated with the direction 0°, i.e. the direction SZ. The distribution marked with DL exhibits a maximum which is associated approximately with the direction 41 ,5°, and there is a local intensity minimum at the direction 0°. Thus, the angular distribution of the intensity of the emitted beam BB1 may also exhibit a local minimum in the direction SZ.
In this example the optical power associated with the distribution DL is greater than two times the optical power associated with the distribution SL. In general, the maximum optical power emitted by a resonant cavity light emitting diode 10 is associated with an angular intensity distribution, which exhibits a local minimum in the direction SZ, the distribution DL being an example. Referring to Fig. 4, the light emitting device 100 according to the present invention comprises at least a resonant cavity light emitting diode 10 and a reflecting surface 20. The reflecting surface 20 is adapted to direct light emitted by the light emitting diode 10 substantially towards the direction SZ. The light rays LL emitted by the light emitting diode 10 and the rays reflected from the surface 20 constitute the beam BB2 emitted from the light emitting device 100.
Referring to Fig. 5, the circular active surface 11 of the resonant cavity light emitting diode 10 has a diameter DIAL The diameter DIA1 is equal to the near-field diameter NFD1 (Fig. 1 ) of the beam BB1.
The reflecting surface 20 has a height H1 with respect to the output plane of the mirror 5 of the resonant cavity light emitting diode 10, i.e. with respect to the active surface 11. The angle β between the reflecting surface and the direction SZ is substantially greater than 0°, i.e. the reflecting surface 20 is inclined with respect to the optical axis N1.
Fig. 6a shows, by way of example, an angular distribution of optical power density (dP/dα) emitted by a resonant cavity light emitting diode 10 (Fig. 2). The dimension of the optical power density is herein Watts per degrees (W/°). The abscissa value associated with the maximum power density is marked by αMAX. In this case the maximum power density is directed in directions having α = 45°. Said directions constitute a conical surface around the direction SZ.
The angular distribution of optical power density shown in Fig. 6a was calculated based on the angular distribution of intensity DL shown in Fig. 3, assuming axial symmetry. The angle α^x associated with the maximum of the power density is typically slightly greater than the angle associated with the maximum intensity.
Fig 6b shows normalized optical power P/PMAX emitted by the resonant cavity light emitting diode 10 (Fig. 2) into a conical solid angle having a half-cone angle α with respect to the direction SZ. The values have been normalized by dividing the power values P by the total optical power PMAX of the beam BB1. The light emitting diode 10 is the same as in Fig. 6a. The curve shown in Fig. 6a is the derivative of the curve shown in Fig. 6b.
50% of the optical power is emitted into a conical solid angle having a half-cone angle α = 47°.
Fig. 7a shows, by way of example, an angular distribution of optical power density (dP/dα) emitted by a light emitting device 100, said light emitting device 100 comprising a conical reflecting surface 20 (Figs. 4 and 5). The half-cone angle β is 22,5° and the height H1 of the reflecting surface 20 is 0,8 times the diameter DIA1 of the active surface 11. The light emitting diode 10 is the same as in Fig. 6a.
Advantageously, the half-cone angle β of the reflecting surface is selected according to the equation:
β = α^_ (1 )
In other words, an optimum angle β between the reflecting surface 20 and the direction SZ is substantially equal to an angle αMAX divided by two, said angle αMAX being associated with a maximum angular optical power density emitted by the light emitting diode. In the practical implementation, manufacturing tolerances should be taken into consideration, and the optimum angle β between the reflecting surface 20 and the direction SZ is preferably within αMAX/2 ± 3°.
The angle αMAX associated with the maximum power density emitted by a resonant cavity light emitting diode 10 is typically in the range 30° to 50°. Thus, the half-cone angle β of the reflecting surface is advantageously in the range 15° to 25°.
The angular distribution of power density depends on the refractive index of the light-transmitting substance, e.g. glass. Thus, the angles αMAX and β in equation (1) refer to the situation in the light-transmitting substance.
Fig. 7b shows normalized optical power P/PMAX emitted by the light emitting device 100 into a conical solid angle having a half-cone angle α with respect to the direction SZ (Figs. 4 and 5). The values have been normalized by dividing the power values P by the total optical power PMAX of the beam BB1. The light emitting diode 10 and the reflecting surface 20 are the same as in Fig. 7a. The curve shown in Fig. 7a is the derivative of the curve shown in Fig. 7b.
50% of the optical power is emitted into a conical solid angle having a half-cone angle α = 18°. 38% of the optical power is emitted into a solid angle having a half-cone angle α = 12°.
In order to define the divergence of the beams BB1 , BB2 (Figs 2 and 4), the boundaries of the beams BB1 , BB2 must be specified. In case of Fig. 6b, the divergence of the beam BB1 is 94° when the boundary of the beam BB2 is set according to 50% of the total optical power. In case of Fig. 7b, the divergence of the beam BB2 is 36° when the boundary of the beam BB2 is set according to 50% of the total optical power. Thus, the use of the reflecting surface 20 concentrates the emitted light beam BB2 significantly when compared with the original beam BB1.
As the angle αMAX associated with maximum power density is typically approximately 45 degrees, the half-cone angle β of the reflecting surface is approximately 22,5 degrees (Fig. 5). When the height H1 of the reflecting surface 20 is selected to be smaller than or equal to 1 ,5 times the diameter DIA1 of the active surface 11 , then substantially no light rays LL emitted from the active surface 11 are reflected more than once at the reflecting surface 20, considering only light rays LL traveling in a plane including the optical axis N1. Thus, reflection losses in case of a metallic reflecting surface 20 are minimized. The height H1 of the reflecting surface 20 is advantageously greater than or equal to the 0,5 times the diameter DIA1 of the active surface 11. Otherwise light rays LL emitted at an angle of 45 degrees from the central region of the active surface 11 are not reflected.
Referring to Fig. 8, the light emitting device 100 may comprise a conical shell 21. The inside of the shell 21 may be coated with a reflective coating 20a. Alternatively, the shell 21 may be transparent and the exterior may be coated with a reflective coating 20b.
The shell 21 may be implemented by molding or stamping plastic, glass ceramics or metal. The shell 21 may also be machined by drilling, turning and/or grinding. The shell 21 may be coated with a reflective layer by vacuum deposition. The reflective layer may be implemented using aluminum, oxide-coated aluminum, rhodium, or dielectric multilayer coatings.
When the reflecting surface 20 comprises electrically conductive material, electrical contact with the active surface 11 of the light emitting diode 10 may be prevented with a proper spacing between the reflecting surface 20 and the active surface 11.
Referring to Fig. 9, the light emitting device 100 may comprise an asymmetric reflecting surface 20, e.g. a portion of a conical reflecting surface. This embodiment may be advantageous when available space is very limited, but maximum efficiency is not required.
Referring to Fig. 10, the reflecting surface 20 may be implemented by forming a conical opening in a slab 22, and by coating the interior of the conical opening with a reflecting surface 20.
Referring to Fig. 11 , the reflecting surface 20 may be based on total internal reflection in a transparent conical body 24. The reflecting surface 20 may be covered with a protective coating. If the refractive indices and/or the orientation of the reflecting surface 20 with respect to the emitted light rays do not fulfill the criterion required for total internal reflection, the surface 20 may be coated with an additional reflective layer.
Reflecting surfaces 20 based on total internal reflection are preferred over reflectors based on metallic surfaces, because reflection losses are negligible in case of total internal reflection.
Physical contact between the body 24 and the active surface 11 may be harmful because it may disturb the proper operation of the cavity mirror 5 (Rg. 1) of the light emitting diode 10. A space between the body 24 and the light emitting diode 10 may be provided with suitable protrusions or spacers.
Referring to Fig. 12, the light emitting device 100 may be used to couple light into the core 201 of an optical waveguide 200. It is advantageous to implement a bevel and guide surfaces to enable easy insertion and positioning of the waveguide 200 into the device 100. A portion of the cladding 202 of the waveguide 200 may be used as a fixing surface. The light emitting device 100 and the optical waveguide 200 constitute together a combination 400.
Referring to Fig. 13, a portion of the optical waveguide 200 may be tapered. The reflecting surface 20 may be implemented using the tapered portion. The operation of the combination 400 may be based on total internal reflection, without additional coatings. However, the tapered portion may be coated with additional reflective and/or protective layers. Further, the tapered portion may comprise a cladding layer (not shown) which has lower refractive index than the core 201 of the waveguide 200.
The tapered portion may be implemented by molding, by grinding, by turning, by heating and pulling or by joining an additional conical body to the end of the waveguide 200, for example. It is advantageous that the tapered portion is short, as it is stronger than a long thin tapered fiber. The straight portion of the waveguide 200 comprises a cylindrical reflecting surface, i.e. the core-cladding interface. However, as the core-cladding interface is substantially parallel to the direction SZ, the angle α between the light rays and the direction SZ is not reduced in reflections at the cylindrical portion. The cylindrical portion does not contribute to the height H1 of the reflecting surface 20 of the light emitting device 100.
Referring to Fig. 14, the device 100 may comprise several conical surfaces stacked on top of each other, which surfaces have different cone angles, i.e. different half cone angles β. The height H1 refers to the combined height of the stacked reflecting surfaces 20, with respect to the level of the active surface 11.
Referring to Fig. 15, the reflecting surface 20 may also exhibit a curved form in two dimensions. For example, the form may be parabolic, ellipsoidal or spherical.
Referring to Fig. 16, a display unit 500 may be implemented using an array of light emitting devices 100. A plurality of light emitting devices 100 may be implemented in a single plate.
When using a resonant cavity light emitting diode 10, the low reflecting surface 20 provides good efficiency for concentrating light emitted from the resonant cavity light emitting diode 10. When compared with prior art devices with high reflectors or with condensing lenses, the size of the device 100 may be reduced and/or the optical power increased.
The use of the device 100 according to the present invention is advantageous when a concentrated high-intensity beam should be generated in a low-cost small-volume device.
The use of the reflecting surface 20 reduces light intensity in unwanted directions, i.e. in directions deviating significantly from the direction SZ. This helps to reduce cross coupling between optical signal channels, dazzling (glare) of detectors or human observers, or radiative heating of components.
The devices 100, 400, 500 may be used to implement fiber optic transmitters used in telecommunications, headlights and stoplights of vehicles, navigation lights, traffic lights, emergency beacons in vehicles and buildings, light torches, illuminating devices attached to garments, beacons attached to life-vests or other life-saving equipment, visual displays, screens and data projectors, for example.
For the person skilled in the art, it will be clear that modifications and variations of the devices and method according to the present invention are perceivable. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

1. A light emitting device (100) comprising at least a light emitting diode
(10) and a reflecting surface (20), said light emitting diode (10) being adapted to emit a diverging light beam (BB1) in a direction (SZ), and said reflecting surface being adapted to direct light emitted from said light emitting diode (10) substantially towards said direction (SZ), an angle (β) between said reflecting surface (20) and said direction (SZ) being greater than zero degrees, characterized in that said light emitting diode comprises a resonant cavity (7), a near field diameter (NFD1 ) of said light beam (BB1) defining a circular active surface (11 ) on an output mirror (5) of said resonant cavity (7), and the height (H 1) of said reflecti ng surface with respect to the level of said active surface
(11 ) being smaller than or equal to the diameter (DIA1) of said active surface (11 ) of said light emitting diode (10) multiplied by a factor of
1 ,5.
2. The device (100) according to claim 1 , characterized in that said angle (β) between said reflecting surface (20) and said direction (SZ) is in the range 15 to 25 degrees.
3. The device (100) according to claim 1 or 2, characterized in that said reflecting surface (20) is a portion of a conical surface.
4. The device (100) according any of the foregoing claims 1 to 3, characterized in that said device (100) comprises a truncated cone of transparent material.
5. The device (100) according any of the foregoing claims 1 to 4, characterized in that said reflecting surface (20) is based on total internal reflection.
6. The device (100) according to any of the foregoing claims 1 to 4, characterized in that said reflecting surface (20) comprises metal.
7. The device (100) according to any of the foregoing claims 1 to 4, characterized in that said reflecting surface (20) comprises a dielectric coating.
8. A method to direct light, said method comprising at least directing light emitted by a light emitting diode (10) by a reflecting surface (20), said light emitting diode (10) emitting a diverging light beam (BB1) in a direction (SZ), and said reflecting surface (20) directing light emitted from said light emitting diode (10) substantially towards said direction (SZ), an angle (β) between said reflecting surface (20) and said direction (SZ) being greater than zero degrees, characterized in that said light emitting diode comprises a resonant cavity (7), a near field diameter (NFD1 ) of said light beam (BB1) defining a circular active surface (11 ) on an output mirror (5) of said resonant cavity (7), and the height (H1 ) of said reflecting surface with respect to the level of said active surface (11 ) being smaller than or equal to the diameter (DIA1 ) of said active surface (11 ) of said light emitting diode (10) multiplied by a factor of 1 ,5.
9. A combination (400) of a light emitting device (100) and an optical waveguide (200), said light emitting diode (10) being adapted to emit a diverging light beam (BB1) in a direction (SZ), and said reflecting surface being adapted to direct light emitted from said light emitting diode (10) substantially towards said direction (SZ) and into said optical waveguide (200), an angle (β) between said reflecting surface (20) and said direction (SZ) being greater than zero degrees, characterized in that said light emitting diode comprises a resonant cavity (7), a near field diameter (NFD1 ) of said light beam (BB1 ) defining a circular active surface (11 ) on an output mirror (5) of said resonant cavity (7), and the height (H1 ) of said reflecting surface with respect to the level of said active surface (11 ) being smaller than or equal to the diameter (DIA1 ) of said active surface (11 ) of said light emitting diode (10) multiplied by a factor of 1 ,5.
10. The combination (400) according to claim 9, characterized in that an end of said optical waveguide (200) has the form of a truncated cone, said reflecting surface (20) being implemented using said end of said optical waveguide (200).
11. A display unit (500) comprising at least one light emitting device (100), said light emitting device (100) comprising at least a light emitting diode (10) and a reflecting surface (20), said light emitting diode (10) being adapted to emit a diverging light beam (BB1 ) in a direction (SZ), and said reflecting surface being adapted to direct light emitted from said light emitting diode (10) substantially towards said direction (SZ), an angle (β) between said reflecting surface (20) and said direction (SZ) being greater than zero degrees, characterized in that said light emitting diode comprises a resonant cavity (7), a near field diameter (NFD1 ) of said light beam (BB1 ) defining a circular active surface (11 ) on an output mirror (5) of said resonant cavity (7), and the height (H1 ) of said reflecting surface with respect to the level of said active surface (11 ) being smaller than or equal to the diameter (DIA1 ) of said active surface (11 ) of said light emitting diode (10) multiplied by a factor of 1 ,5.
PCT/FI2005/050373 2004-10-26 2005-10-25 Light emitting device and method for directing light WO2006045897A1 (en)

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JP2011048371A (en) * 2009-08-28 2011-03-10 General Electric Co <Ge> Apparatus for coupling between light emitting diode and light guide
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