Slim waveguide coupling apparatus and method

#Embodiments of the invention generally relate to illumination panels, and, more specifically, to coupling of light sources to waveguides.

#Thin, planar illumination systems are desirable for many applications such as, for example, low-profile back-illuminated displays. FIG. 1 illustrates such an illumination system 100 fabricated by coupling a discrete light source, e.g., a light-emitting diode (“LED”) 102 to a narrow face 104 of a waveguide 106. Generally, a waveguide 106 having a refractive index of N=1.5 and an LED 102 having Lambertian output characteristics, combined as in the illumination system 100, have a theoretical maximum coupling efficiency limit of 85%. That is, at most 85% of the light emitted by the LED 102 will be trapped within the waveguide 106, and the remaining portion of the emitted light will be lost.

#This coupling inefficiency may be attributed to the constraints inherent in the side-emitting LED design of the illumination system 100. While thinner waveguides are desirable, the thickness t of the waveguide must be larger than the width d of the LED in order to achieve coupling efficiencies approaching 85%. Relatively high coupling efficiencies (e.g., greater than approximately 70%) are difficult to obtain for cases where the thickness t of the waveguide is smaller than the width d of the LED. Thus, as waveguides become thinner, the coupling efficiency of the waveguide decreases and more light is lost. The coupling inefficiency may even set a practical lower bound on the thickness of the waveguide 106. In addition, many side-emitting illumination systems utilize specially engineered LED and waveguide structures in order to increase the coupling efficiency. These structures not only add to the complexity and cost of the illumination system 100 but also increase its thickness.

#In the drawings, like reference characters generally refer to the same parts throughout the different views. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

#FIG. 1 is a cross-sectional view of a prior-art side-mounted illumination system;

#FIG. 2 is a cross-sectional view of an illumination system featuring a parabolic mirror;

#Described herein are methods and systems for coupling light from a light source to a waveguide. Embodiments of the invention apply to two different cases: (1) when a thickness, t, of the waveguide is approximately equal to or larger than the width, d, of the light source and (2) when t is less than d. In cases where t is approximately equal to or larger than d, a curved, semi-curved, broken-line, or single-line top mirror redirects light that strikes it into confined modes (i.e., propagation paths confined by the waveguide). As utilized herein, with reference to the two-dimensional cross-sectional view of the mirror element, “curved” refers to a mirror with a curved shape, “semi-curved” refers to a mirror with a curved segment and straight segments, “broken-line” refers to a mirror having several discrete straight segments that approximate a curved segment, and “single-line” refers to a mirror consisting of a straight segment. The mirror is positioned such that light propagating at an angle less than necessary for TIR (and which therefore will not be confined within the waveguide) strikes the mirror. Thus, the mirror shape and position may enable the redirection of light from unconfined modes into confined modes. Any light already in a confined mode that does strike the mirror may remain in a confined mode after reflection.

#FIG. 2 illustrates one example of a parabolic mirror 202 that is set around a point A (the top-right corner 204 of an LED 206), such that the light rays emitted from the point A toward the region of the mirror bounded by points M and D (“region M-D”) are reflected back into the waveguide 208 at an angle equal to (or larger than) the critical angle θT for total internal reflection. Thus, all of the light rays from the portion of the top surface 210 of the LED 206 bounded by points A and B (“region A-B”) may be reflected at angles larger than θT so as to be confined. The light rays from the region bounded by points B and C (“region B-C”) may undergo one or multiple reflections at the mirror such that their final propagating angle is also greater than θT.

#Light rays that do not strike the mirror are, by definition, already confined (i.e., propagating at an angle greater than θT) because the light ray from point A to point M and the light ray from point A to point D propagate at an angle θT. The light ray from point A to point M propagates perpendicularly to the surface of the mirror 202 at point M such that the light is reflected back toward point A. At point D, the mirror 202 is substantially parallel to the plane of the waveguide 208, and the light ray propagating from point A at an angle θT is reflected into the waveguide 208 at an angle θT (pursuant to Snell's Law). The light rays may act similarly if one or more cladding layers (not shown) are added to the waveguide 208.