Waveguide sheet and methods for manufacturing the same

This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/006,110, filed on Dec. 19, 2007; U.S. Provisional Patent Application No. 61/064,384, filed on Mar. 3, 2008; U.S. Provisional Patent Application No. 61/127,095, filed on May 9, 2008; U.S. Provisional Patent Application No. 61/076,427, filed on Jun. 27, 2008; and U.S. Provisional Patent Application No. 61/135,098/, filed on Jul. 16, 2008. The entire disclosure of each of these applications is incorporated by reference herein.

In various embodiments, the present invention relates to optics, and in particular to optical waveguides.

The technology to transmit and guide light through optical systems exploits a physical phenomenon in which light is confined within a material surrounded by other materials with lower refractive index. Such optical systems are generally referred to as optical waveguides, and are employed to direct, diffuse, and/or polarize light in many applications, e.g., optical communication and illumination.

When a ray of light moves within a transparent substrate and strikes one of its internal surfaces at a certain angle, the ray of light is either reflected from the surface or refracted into the open air in contact with the substrate. The condition according to which the light is reflected or refracted is determined by Snell\'s law, which relates the impinging angle, the refracting angle (in the case of refraction) and the refractive indices of both the substrate and the air. Broadly speaking, depending on the wavelength of the light, for a sufficiently large impinging angle (above the “critical angle”) no refraction occurs, and the energy of the light is trapped within the substrate. In other words, the light is reflected from the internal surface as if from a mirror. Under these conditions, total internal reflection is said to take place.

Many optical systems operate according to the principle of total internal reflection. Optical fiber represents one such system. Optical fibers are transparent, flexible rods of glass or plastic, basically composed of a core and cladding. The core is the inner part of the fiber, through which light is guided, while the cladding surrounds it completely. The refractive index of the core is higher than that of the cladding, so that light in the core impinging the boundary with the cladding at an angle equal to or exceeding the critical angle is confined in the core by total internal reflection. Thus, geometric optics may be used to derive the largest angle at which total internal reflection occurs. An important parameter of every optical fiber (or any other light-transmitting optical system) is known as the “numerical aperture,” which is defined as the sine of the largest incident light ray angle that is successfully transmitted through the optical fiber, multiplied by the index of refraction of the medium from which the light ray enters the optical fiber.

Another optical system designed for guiding light is the graded-index optical fiber, in which the light ray is guided by refraction rather than by total internal reflection. In this optical fiber, the refractive index decreases gradually from the center outwards along the radial direction, and finally drops to the same value as the cladding at the edge of the core. As the refractive index does not change abruptly at the boundary between the core and the cladding, there is no total internal reflection. However, the refraction nonetheless bends the guided light rays back into the center of the core while the light passes through layers with lower refractive indices.

Another type of optical system is based on photonic materials, where light is confined within a bandgap material surrounding the light. In this type of optical system, also known as a photonic material waveguide, the light is confined in the vicinity of a low-index region. One example of a photonic material waveguide is a silica fiber having an array of small air holes throughout its length.

International Patent Application Publication No. WO2004/053531, the entire contents of which are hereby incorporated by reference, discloses a waveguide for propagating and emitting light. The waveguide is made of a flexible, multilayer waveguide material in which the refractive index of one layer is larger than the refractive index of the other layers to allow propagation of light via total internal reflection. One layer of the waveguide material comprises one or more impurities which scatter the light to thereby emit a portion thereof through the surface of the waveguide material.

Impurities for light scattering are also employed in light diffusers (also known as light-scattering films or diffusing films), which diffuse light from a source in order to attain a uniform luminance. For example, in a liquid crystal display device a light diffuser is placed between the light source or light reflector and the liquid crystal panel so as to diffuse the illuminating light, allowing the device to be used as a plane or flat light source as well as enhancing the luminance on the front side of the device.

Conventional illumination apparatuses capable of emitting diffused light with uniform luminance are complicated to manufacture and too large for many applications. They tend to be unitary and large rather than small and scalable. Additionally, such apparatuses often exhibit insufficient color mixing and diffusion to emit light with a high degree of color and luminance uniformity.

The foregoing limitations of conventional illumination apparatuses are herein addressed by utilizing a waveguide that incorporates in-coupling, propagation, and out-coupling regions and/or that is easily manufactured as a group of aligned core structures.

Generally, embodiments of the invention propagate and diffuse light until it exits though a surface of the waveguide device or a portion thereof. The light path may involve two right angles: in various embodiments, light is absorbed into the structure through the bottom surface of one portion the waveguide (e.g., the in-coupling region) and is emitted from a top surface of a second portion of the waveguide (e.g., the out-coupling region). These waveguide portions have substantially no overlap; they may be separated, for example, by a propagation region from which light is not emitted.

In various embodiments, light entering a waveguide\'s in-coupling region is substantially retained within the waveguide until it is emitted from the out-coupling region. The different emission and retention behavior of the various waveguide portions may be obtained using different concentrations of scattering particles; for example, the propagation region may be devoid of scattering particles altogether in order to keep light confined therein.

Embodiments of the invention successfully provide an optical waveguide device that may be tiled or overlapped. As further detailed herein, the optical properties of the waveguide may be tailored to the requirements of particular applications.

The design of waveguide-based light structures in accordance with the invention also facilitates convenient manufacture. The light structure may, for example, be assembled by joining a plurality of core structures, each of which has a different concentration of scattering particles (or no scattering particles at all). Forming the joined core structures may be accomplished by, e.g., co-injection molding, coextrusion, coating, lamination, bonding, and/or welding.

In an aspect, embodiments of the invention feature an illumination structure including a substantially non-fiber waveguide and a discrete light source disposed proximate a bottom surface of a first portion of the waveguide. Light is absorbed into the illumination structure through the bottom surface of the first portion and is emitted from a top surface of a second portion of the waveguide; the second portion has substantially no overlap with the first portion of the waveguide. The first and second portions of the waveguide may be spaced apart from each other. In general, light is emitted only from the second portion of the waveguide.

In another aspect, embodiments of the invention feature a substantially non-fiber waveguide and a discrete light source disposed proximate a bottom surface of a first portion of the waveguide. A propagation direction, within a second portion of the waveguide, of light from the discrete light source is substantially perpendicular to an in-coupling direction of the light. The propagation direction of the light may be substantially perpendicular to an out-coupling direction of the light in a third portion of the waveguide. The illumination structure may include a phosphor material for converting some of the light to a different wavelength, the converted light mixing with unconverted light to form mixed light spectrally different from both the unconverted light and the converted light. An out-coupling direction of the mixed light may be substantially perpendicular to the propagation direction of light from the discrete light source.