Optical waveguides – Planar optical waveguide
Reexamination Certificate
1999-04-19
2001-03-27
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S014000, C385S015000, C385S123000, C385S130000, C385S131000
Reexamination Certificate
active
06208791
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns means of improving the optical efficiency of optically excited light emitting structures such as pixels in flat-panel displays.
BACKGROUND OF THE INVENTION
Many commercial emissive display devices generate visible light using electron beam or ultraviolet radiation incident upon a phosphor, such as in cathode ray tube (CRT) or AC plasma visual displays. A less well known display technology, typified in Bischel et al. U.S. Pat. No. 5,544,268, incorporated herein by reference, uses optical waveguides to convey light from a light source onto a display screen. Waveguide-based flat panel displays generally utilize planar and/or channel waveguides. They typically include several parallel channel waveguides to be formed on a substrate. Optical switches are located either in or on the channel waveguides at predetermined matrix locations across the display screen. Optical energy injected into the channel waveguides is extracted at these predetermined positions by the optical switches and directed toward pixel structures which may, in certain embodiments described in the '268 patent, include re-radiators to emit light from the pixel structure towards a viewer. Such reradiators can include out-of-plane reflectors, scattering materials, or luminescent materials which emit at a wavelength which may differ from the wavelength of the input optical energy. Metal reflectors on or near the visible light emitting pixels are used in a variety of ways in different display architectures to redirect visible light emitted by phosphors into a preferred direction to achieve enhanced brightness at the viewer location. For example, Thomas U.S. Pat. No. 5,097,175, incorporated herein by reference, describes a pixel structure for CRT displays, where a material that emits visible light upon excitation with an electron beam is deposited on a transparent substrate in the form of a parabolic shaped cell that is coated with a reflective metal layer to redirect visible light emitted inside the cell through the substrate toward a viewer.
In another example of the use of reflectors to direct light for a visual display, Murata U.S. Pat. No. 5,055,737, incorporated herein by reference, describes a luminescent screen which contains a material that emits visible light when excited by light incident from the viewing direction. This screen contains a reflective structure that redirects light from the emitting material, that otherwise would propagate in undesired directions, back toward the viewer, thereby enhancing brightness.
Thus the conventional function of reflectors used in displays is to direct the light generated in the pixel toward the viewer. Such reflectors do not serve to enhance the efficiency of conversion to visible light of pump energy such as that from an electron beam in a CRT or from the ultra-violet light in a plasma display. Optical performance, including the conversion efficiency, brightness, and chromaticity, of display pixels containing certain optically activated luminescent materials such as phosphors, glasses, or crystals would benefit from increasing the amount of absorbed pump radiation. Therefore a different kind of reflective pixel structure is needed which confines the pump radiation while allowing for the emission of generated light.
SUMMARY OF THE INVENTION
The present invention provides a means of increasing the efficiency of the light conversion processes of re-radiating materials in integrated structures. Roughly described, this is achieved by using a reflective coating deposited on or near the light emitting material to achieve spatial confinement of the input light delivered by an integrated waveguide. The fraction of the input light that contributes to the generation of useful output light is thereby increased. It will be apparent to those skilled in the art that the structures described herein pertain not only to pixels in information displays but more generally to any reflectively enclosed light emitting structure that is optically excited by light from an integrated optical waveguide.
In one embodiment, the optical performance, including conversion efficiency, of an upconversion phosphor contained in such a device is improved by the enhanced absorption of infrared pump light resulting from multiple passes through the absorber by reflection of otherwise unabsorbed pump light off the reflective coating of the confinement structure. Apertures in the reflective coating of the confinement structure allow for the visible light generated in the upconversion process to emerge from the pixel structure and to propagate in a preferred direction, such as toward a viewer or sensor. The efficiency of visible light generation upon infrared excitation of an upconversion phosphor material generally increases as the infrared power absorbed per unit volume is increased, because the probability of non-radiative energy transfer processes involved in the generation of the visible light increases as the average distance between excited optically active ions decreases.
At a fundamental materials level, the invention enables the use of re-radiator materials that have small absorption coefficients for input light due to a small concentration of active dopant ions that absorb light. It is well known that the conversion efficiency of such phosphors often decreases as the concentration of absorbing ions is increased (see “Luminescent Materials”, by G. Blasse and B. C. Grabmaier, 1994, incorporated herein by reference). The use of reflectors on a pixel designed to increase the absorption of input light helps ensure utility for such materials that may have low absorption but high conversion efficiency.
The invention provides a means for increasing the brightness of small emissive structures such as pixels in information displays which are excited using light delivered by integrated optical waveguides. The advantage of the invention is significant in the example of an information display where the dimensions of the pixel feature are constrained according to display resolution requirements and the pixel cannot be increased to an arbitrarily large size to achieve larger single pass absorption of input light.
Furthermore, in the case of upconversion phosphor grains in an optically transparent polymer binder, the mixture may comprise phosphor as perhaps only 5% by volume of the total volume enclosed by a pit/mound structure. Confinement of the input light by the reflective surfaces increases the total input light energy that is absorbed per upconversion phosphor particle. The resultant increase in excitation density within the phosphors can provide a higher efficiency of conversion of infrared to visible light within the phosphor grains.
An additional advantage gained from the use of reflectors that confine the pump light delivered to a pixel is that the variation of efficiency with pump light wavelength of a wavelength-converting phosphor, for example, can be reduced. In such a structure, for example, a NaYF
4
upconversion phosphor doped with ytterbium and erbium ions having an absorption peak around 977 nm may be pumped at a variety of wavelengths in, say, the 960 nm to 990 nm range and produce comparable green light emission intensity despite a significant variation in the absorption coefficient of the phosphor over that wavelength range. This increase in wavelength tolerance relaxes the specification of lasers used with such a device and can improve device performance uniformity as the wavelengths of pump lasers tend to vary with temperature.
This invention will be better understood upon reference to the following detailed description in connection with the accompanying drawings.
REFERENCES:
patent: 5009483 (1991-04-01), Rockwell, III
patent: 5055737 (1991-10-01), Murata et al.
patent: 5097175 (1992-03-01), Thomas
patent: 5182787 (1993-01-01), Blonder et al.
patent: 5290730 (1994-03-01), McFarlane et al.
patent: 5544268 (1996-08-01), Bischel et al.
patent: 5647036 (1997-07-01), Deacon et al.
Shionoya, S., et al., eds., “Phosphor Handbook”, CRC Press (1999),
Bischel William K.
Cockroft Nigel J.
Deacon David A.G.
Field Simon J.
Hehlen Markus P.
Fliesler Dubb Meyer & Lovejoy LLP
Gemfire Corporation
Palmer Phan T. H.
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