Optical waveguides – Optical imaging tunnel
Reexamination Certificate
2000-07-10
2001-11-27
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical imaging tunnel
C353S020000
Reexamination Certificate
active
06324330
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to light tunnels, and in particular to light tunnels used to provide a light distribution having a high degree of spatial uniformity.
BACKGROUND OF THE INVENTION
The use of a hollow or solid rectangular (or square) reflective light tunnel or solid glass rod (herein collective referred to as “light tunnels”) is well known as a non-imaging device that can generate a “uniform” intensity distribution from a non-uniform spatial intensity distribution produced by a light source, such as from an arc lamp or laser.
Most light tunnels are designed to provide uniformity of illumination up to about the 95% level (i.e., 5% non-uniformity). However, for certain applications such as photolithography and laser thermal processing (LTP), the spatial uniformity needs to exceed 99% (i.e., less than 1% non-uniformity). To achieve this level of uniformization, the light tunnel must have a length to width ratio on the order of 100:1 to provide the necessary large number of homogenizing reflections (typically more than 4 reflections in each direction).
FIG. 1
shows an example of a non-imaging light tunnel
10
having an optical axis A along the z-direction, a width W=5.7 mm and a length L=500 mm. Light tunnel
10
is solid and may be made of optical glass, such as fused silica having a refractive index n=1.45 in the visible spectrum. With reference also to
FIG. 2
, light tunnel
10
is polished with six mutually perpendicular surfaces S
1
-S
6
, which includes 4sides (S
1
-S
4
), an input end (S
5
) and an output end (S
6
). Surfaces S
1
-S
6
meet at sharp edges and are straight and flat over their extent. Light tunnel
10
having such dimensions is very fragile and difficult to fabricate.
FIG. 3
shows light tunnel
10
having the proportion W/L of {fraction (1/100)} for the sake of illustration. The above dimensions are close to a practical limit of both manufacturability and sensible cost.
In use, light tunnel
10
functions as follows. With reference again to
FIG. 1
, a light beam
20
comprising light rays R (including a straight-through light ray R
T
) from a light source
26
is relayed via an optical relay system
28
and converged onto input end S
5
through an input angle &agr;. The spatial extent of light beam
20
imaged onto input end S
5
is typically somewhat circular with a reasonably symmetric spatial intensity distribution. The spatial uniformity of light beam
20
in the x and y directions is shown schematically in FIG.
4
A. Light beam
20
is preferably defocused so that it just underfills input end S
5
and does not concentrate energy above the Laser Damage Threshold (LDT) of the optical glass from which the light tunnel is fabricated.
A reasonable value of a is found to be about ±10° (half-cone angle), which corresponds to a numerical aperture (in air) of 0.18 or an f/# of f/2.8. Larger numerical apertures present additional problems in managing the output illumination, which has an output angle &agr;, the same as the input angle. Somewhat “faster” f/#s can be used, depending on the complexity and expense the optical system can endure for the downstream relay optics
30
that relays light from output end S
6
through other sections of the optical system (not shown).
It is known in the art that the longer the light tunnel, the greater the spatial uniformity of the illumination formed on output end S
6
because the spatial uniformity increases with a larger number of reflections, and, for a given input f/#, it is possible to have a greater number of reflections with a longer light tunnel. Normally, output end S
6
has cross-sectional dimensions W
x
and W
y
, so that the light tunnel may be square or rectangular. In
FIG. 1
, W
x
=W
y
=W, for the sake of simplicity.
With continuing reference to
FIG. 1
, all light rays R in beam
20
entering input end S
5
of light tunnel
10
exit from output end S
6
, provided that the refractive index of the optical glass n>(2)
½
or 1.414 . . . for the given wavelength of light used. Virtually all optical glasses exceed this value. Hence any light ray that enters input end S
5
at &agr;=90° or less will refract and be guided down the tunnel's length by multiple-reflections from surfaces S
1
-S
4
of light tunnel
10
, and exit at ±&agr; from output end S
6
. Every light ray R will undergo an “even” or “odd” number of reflections, depending on the length L and incident input angle &agr;. Because light rays R in a solid light tunnel
10
undergo Total Internal Reflection (TIR), there is no light loss internally due to reflection. Absorption and scattering in the optical glass and “end” losses due to Fresnel surface reflections are the only losses encountered. Anti-reflection coatings can minimize the latter.
It is possible to construct a rectangular hollow light tunnel
10
by butting together four mirrors. Other than the “internal medium” being air with a refractive index of 1.0, the geometrical behavior is, to first order, identical to that of a solid light tunnel. For a given width W, a hollow light tunnel
10
will be the shortest embodiment for a given number of reflections or “bounces” of rays R from input end S
5
to output end S
6
. That is, a hollow light tunnel
10
will produce the most uniform output distribution in the shortest length L. Even so, the length L must be great, requiring long slender mirrors. The edges that butt to the surface of the adjacent mirror must be sharp. Dirt on the inside reflective surfaces can be a practical problem. The inner surfaces of the mirrors preferably include an optical coating designed for grazing incidence reflection to avoid excessive polarization and selective absorption. A mirror-based hollow light tunnel will not be as efficient as a solid glass light tunnel when reflection losses from the mirrors are compared to TIR of solid glass.
For given values of &agr;, W
x
, W
y
, L and n, the number of reflections that occur on either side of the directly transmitted ray R
T
traveling along axis A is limited by &agr; and will be equal to N
xy
, which is given by:
N
xy
=±Tan(Sin
−1
(1/(
2×n×f/#))) (L/W),
(Eq-1)
where f/#=1/(2 Sin &agr;)=1/(2NA) (Eq-2)
For a circular beam, the total number of reflections is given by N
tot
:
N
tot
=(&pgr;/4)(2N
xy
+1)
2
(Eq-3)
For L=500, W
x
=W
y
=W=5.7, n=1.43, and f-number=f/2.8, N
xy
=±11 reflections either side of the transmitted beam, which gives
N
tot
=415 reflections.
The angular subtense of a single beam is given by:
&Dgr;&agr;=Tan
−1
(nW/L) (Eq-4)
which for &agr;=±10.30 half-cone angle results in each beamlet subtending an angle of 0.95°.
For small angles &agr;, the number of reflections N
xy
, is approximated by:
N
xy
≅(±&agr;)/(nW/L) (Eq-5)
Hence, there will be at output end S
6
an average of 415 superimposed images of input end S
5
. The result is a highly spatially uniform distribution of light I
out
at output end
56
that is sharp-edged and behaves as an ideal light source. This is illustrated in FIG.
4
B.
The problem with the foregoing is that when W needs to be much larger than 5 mm, or L much longer than 500 mm (i.e., about 20 inches), serious fabrication difficulties are encountered. In particular, the light tunnel is slender and fragile and thus difficult to handle and easily damaged in processing (see FIG.
3
). It becomes necessary to increase the width, W, of the tunnel when high intensity light sources, such as lasers, are used. For example, lasers with high energy/pulse characteristics (approximately greater than 5 joule/cm
2
/pulse with a wavelength in the visible range on most conventional glass tunnels) may exceed the damage threshold. However, increasing the width of the light tunnel necessarily requires that the length also increase so as to maintain the number of reflectio
Jones Allston L.
Ullah Akm E.
Ultratech Stepper, Inc.
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