Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-05-28
2004-12-07
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S033000, C385S035000, C385S901000, C362S518000, C362S346000
Reexamination Certificate
active
06829412
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to waveguides that collect and condense light from a light source, transforming the area and divergence angle of the light from their input to their output with minimum loss of brightness.
2. Description of the Related Art
The objective of systems that collect, condense, and couple electromagnetic radiation into a target such as a standard waveguide, e.g. a single fiber or fiber bundle, or output electromagnetic radiation to the input of a projection engine, is to maximize the brightness of the electromagnetic radiation at the target. There are several common systems for collecting and condensing light from a lamp for such illumination and projection applications.
One optical collection and condensing systems, U.S. patent application Ser. No. 09/604,921, the disclosure of which is incorporated by reference, provides a dual-paraboloid reflector system. This optical collection and condensing system, as illustrated in FIG.
1
(
a
), uses two generally symmetric paraboloid reflectors
10
,
11
that are positioned so that light reflected from the first reflector
10
is received in a corresponding section of the second reflector
11
. In particular, light emitted from a light source
12
, such as an arc lamp, is collected by the first parabolic reflector
10
and collimated along the optical axis toward the second reflector
11
. The second reflector
11
receives the collimated beam of light and focuses this light at the target
13
positioned at the focal point.
The optical system of FIG.
1
(
a
) may employ a retro-reflector
14
in conjunction with the first paraboloid reflector
10
to capture radiation emitted by the light source
12
in a direction away from the first paraboloid reflector
10
and reflect the captured radiation back through the light source
12
. In particular, the retro-reflector
14
has a generally spherical shape with a focus located substantially near the light source
12
(i.e., at the focal point of the first paraboloid reflector) toward the first paraboloid reflector to thereby increase the intensity of the collimated rays reflected therefrom.
In FIG.
1
(
a
) is shown light paths for three different rays (a, b, and c) emitted from the light source
12
when viewed in a direction normal to the lamp axis. The light output from a lamp subtends an angle of about 90° around an axis normal to the lamp, as indicated by rays a and c in FIG.
1
(
a
).
The light output from a lamp subtends a cone angle of nearly 180°, on the other hand, when viewed in a direction parallel to the lamp axis, as indicated by rays a′ and c′ in FIG.
1
(
b
).
One shortcoming of the above described on-axis, dual-paraboloid optical system is that a large angle is produced between rays a and c, and rays a′ and c′, at a target. As a result, the rays strike the target
13
at a high angle of incidence relative to the target surface. Thus, the numerical aperture (NA) at the input of the target
13
may be very large, sometimes as high as 1.0, while the area upon which the light is focused is small. A large numerical aperture combined with a small area may be unsuitable for the optical components to which light from the system may be coupled. If a different, e.g. smaller, numerical aperture is desired, some means of transforming the area and divergence angle of the light with minimum loss of brightness may be incorporated into the device.
Representative means of transforming input areas and divergence angles of light are lenses and tapered optical waveguides, also known as tapered light pipes (TLP). While lenses provide an efficient means of transforming input areas and divergence angles of light, they require a certain amount of space in which to operate. Also, they are not well adapted to large numerical apertures. Consequently, tapered light pipes are often used instead of lenses. Tapered light pipes, however, must be relatively lengthy to transform light efficiently.
In U.S. application Ser. No. 09/669,841, the disclosure of which is incorporated by reference, a dual ellipsoidal reflector system is described as providing 1:1 magnification for small light source target. This optical collection and condensing system, as illustrated in
FIG. 2
, uses two generally symmetric ellipsoid reflectors
20
,
21
that are positioned so that light reflected from the first reflector
20
is received in a corresponding section of the second reflector
21
. In particular, light emitted from the light source
22
is collected by the first elliptical reflector
20
and focused onto the optical axis
25
toward the second reflector
21
. The second reflector
21
receives the focused beam of light and refocuses this light at the target
23
positioned at the focal point.
As may be seen in
FIG. 2
, the dual-ellipsoid system suffers from the same disadvantage as the dual-paraboloid system in that a large angle is produced between ray a and ray c at the target. As a result, ray a and ray c also strike the target at large angles of incidence relative to the target surface, requiring further transformation of the input area and divergence angle of the light.
Another embodiment of the dual-ellipsoid system may be seen in FIG.
3
. This dual-ellipsoid system suffers from the same disadvantage as the abovementioned dual-paraboloid and dual-ellipsoid systems in that a large angle is produced between ray a and ray c at the target. Here too, ray a and ray c strike the target at large angles of incidence, requiring further transformation of the input area and divergence angle of the light.
In practice, light with such a large NA may be transformed such that the NA is smaller and the area is larger following the brightness principle. The transformation may be performed with, e.g. a tapered light pipe.
A standard long tapered light pipe
40
a
with a flat input surface
41
a
for use with the above systems is shown in FIG.
4
(
a
). A standard short tapered light pipe
40
b
with a flat input surface
41
b
for use with the above systems is shown in FIG.
4
(
b
). Both the long and the short tapered light pipes may be used to transform light having a small area d
1
and large numerical aperture NA
1
at the input
41
to a larger area d
2
and smaller numerical aperture NA
2
at the output
42
. If light
43
impinges the tapered light pipe
40
at large angles of incidence
44
as shown in
FIG. 4
, the tapering of the light pipe
40
will transform the large input angles
44
into smaller output angles
45
. The degree to which the angles are transformed will depend on the degree of taper. For ideal tapered light pipes, brightness is conserved. Consequently, for an ideal tapered light pipe, the product of the numerical aperture NA
1
and the area d
1
of the light at the input
41
will be equal to the product of the numerical aperture NA
2
and the area d
2
of the light at the output
42
. To wit:
d
1
*NA
1
=d
2
*NA
2 (1)
In actual implementation, optimizations need to be performed such that the optimized dimensions may deviate from the ideal configurations.
The output angles
45
are designed for a specific system by matching the tapered light pipe to an output device. In designing a tapered light pipe, three of the variables will often be known, and the fourth can be calculated. In one example, a tapered light pipe of length 75.0 mm was designed with d
1
=3.02 mm, NA
1
=0.7, and d
2
=9.0 mm. The output numerical aperture NA
2
is thus predicted to be 0.23. Upon fabricating the tapered light pipe, however, the actual numerical aperture at the output was found to be 0.26, larger than the predicted 0.23. Such a large numerical aperture will result in a loss of coupling efficiency in subsequent optical elements. But if the input area is reduced to reduce the numerical aperture at the output, less light will be coupled into the tapered light pipe in the first place, reducing the overall collection efficiency of the system.
The reason the numerical aperture at the out
Palmer Phan T. H.
Rothwell, Figg Ernst & Manbeck, PC
Wavien, Inc.
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