Note: Descriptions are shown in the official language in which they were submitted.
CA 02380808 2005-12-05
FIBER DEVICES USING GRIN FIBER LENSES
BACKGROUND OF THE INVENTION
s Field of the Invention
This invention relates to optical devices and graded refractive index lenses.
Discussion of the Related Art
A graded refractive index (GRIN) lenses has a refractive index whose value
varies with radial distance from the aXis of the lens. The non-trivial
variation in
to refractive index causes light refraction and gives the GRIN lens focusing
capabilities that
are similar to those of an ordinary lens. Therefore, many optical devices
employ GRIN
or ordinary lenses interchangeably.
Optical devices use lenses to focus, collimate, or expand light beams. Figure
1
shows a fiber device 10 in which a GRIN fiber lens 11 is fused to a terminal
end 12 of an
15 optical fiber 13. The GRIN fiber lens 11 expands the light beam emitted by
the optical
fiber 13. The GRIN fiber lens 11 improves the optical coupling between optical
fiber 13
and fiber device 15 as compared to the coupling that would otherwise exist
between the
optical fiber 13 and fiber device 15 due to diffraction. The GRIN fiber lens
11 reduces
diffraction losses when the optical fiber 13 is optically coupled to another
optical fiber.
2o Since the diameter of a light beam varies along the axis of a GRIN lens,
the
diameter variations provide a measure of the lens' length. The length over
which the
variations in the beam diameter make two complete cycles is known as the pitch
of the
lens. Typically, lengths of GRIN lens are referred to in multiples of the
pitch length,
e.g.,'/Z pitch or'/4 pitch.
25 BRIEF SUMMARY OF THE INVENTION
Certain exemplary embodiments can provide an apparatus for mode converting,
comprising first and second optical waveguides; and a GRIN fiber lens attached
to both
the first and second waveguides; and wherein one end of the GRIN fiber lens is
attached
directly to an end of the first optical waveguide and an opposite end of the
GRIN fiber
30 lens is attached directly to an end of the second optical waveguide; and
wherein the first
waveguide has a fundamental propagation mode of different size than the second
waveguide.
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la
Certain exemplary embodiments can provide an apparatus for mode converting,
comprising first and second optical fibers; and a GRIN fiber lens attached to
both the
first and the second optical fibers; and wherein one end of the GRIN fiber
lens is
attached directly to an end of the first optical fiber and an opposite end of
the GRIN fiber
lens is attached directly to an end of the second optical fiber; and wherein
the lens has a
magnification configured to convert the size of a fundamental propagation mode
of the
first fiber into the different size of a fundamental propagation mode of the
second fiber.,
Certain exemplary embodiments can provide an apparatus for mode converting,
to comprising first and second optical fibers; and a GRIN fiber lens attached
to both the
first and the second optical fibers; and wherein one end of the GRIN fiber
lens is
attached directly to an end of the first optical fiber and an opposite end of
the GRIN fiber
lens is attached directly to an end of the second optical fiber; and wherein
each fiber has
a core and a cladding and a discontinuity in refractive index across an
interface between
the core and cladding, the discontinuities being different across the
interfaces of the first
and second fibers.
One apparatus embodying principles of the inventions is a mode converter that
reduces losses when waveguides with different propagation modes are end-
coupled. In
optical fibers, the forms of the propagation modes depend on the radial
dependence of
2o the refractive index.
The mode converter includes first and second optical waveguides and a GRIN
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fiber lens. The GRIN fiber lens is attached to both the first and the second
_ waveguides.
Another apparatus embodying principles of the inventions end-couples to at
least three optical fibers through an optical element. Rather than lenses with
curved
refractive surfaces, the apparatus uses fiber GRIN lenses attached to the
fiber ends for
collimation/focusing of light.
Another device embodying principles of the inventions includes a fiber array
an optical element, and a waveguide. in this device, GRIN fiber lenses are
attached to
the fibers of the array. The optical element directs light between ones of the
fibers
and the waveguide
- BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a cross-sectional view of a f ber device that uses a conventional
GRIN fiber lens to end-couple two optical fibers;
Figure 2 is a cross-sectional view of a fiber device in which an optical fiber
is
fused to an embodiment of a GRIN fiber lens;-
Figure 3A shows radial profiles .of germanium dopant densities in a
conventional GRIN fiber lens and a new GRIN fiber lens; ° .
Figure 3B shows radial profiles of refractive indexes for the GRIN fiber
lenses
of Figure 3A;
2 0 Figures 4A and 4B illustrate beam.collimation in fiber devices with new
and
conventional GRIN fiber lenses, respectively;
Figure 5 is ~a flow chart 'for a method of fabricating the fiber device of
Figure
2.
Figure 6A is a cross-sectional view of a mode converter;
2 5 Figure 6B is a cross-sectional view of a mode converter that uses a
compound
GRIN fiber lens;
Figure 7A is a top view of a 1x2 micro-optical router;
Figure 7B is a top view of another topology for a 1x2 micro-optical router;
Figure 7C is a top view of a device that optically couples three optical
fibers;
3 0 Figure 8 is a cross-sectional view of a 1xN micro-optical router;
Figure 9 is a top view of an NxM micro-optical muter; and
Figure 10 is a cross-sectional view of an optical fiber with an in-line
optical
device.
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In the Figures, like reference numbers refer to functionally similar features.
DETAILED DESCRIPTION OF THE EMBODIMENTS
1. Grin Fiber Lenses
Figure 2 shows an optical fiber device 16 in which an optical fiber 17 is end-
s coupled to a GRIN fiber lens 18, e.g., fused or glued to the fiber 17. The
GRIN fiber
lens 18 and optical fiber 17 are co-axial and have similar or equal outer
diameters
whose values are in the range of about 100 microns (gym) to about 135 ltm;
e.g., 125
p,m. The GRIN fiber lens 18 collimates a light beam 19 emitted from the end of
the
optical fiber 17 thereby decreasing the numerical aperture below that of a
bare optical
fiber. The GRIN fiber lens 18 is also able to focus an incident light beam
into the end
of the optical fiber 17.
Exemplary optical fibers 17 include single-mode and mufti-mode fibers.
Exemplary GRIN fiber lenses 18 have refractive indexes whose radial profiles
differ significantly from those of conventional GRIN fiber lenses. The new
radial
15 profiles enable decreased numerical apertures and increased Rayleigh ranges
for fiber
device 16 as compared to values of the same quantities in conventional fiber
device
i0 of Figure 1. The decreased numerical aperture implies that an appropriate
length
GRIN fiber lens 18 would cause less diffraction and a lower power density in
emitted
light beam 19 than in the light beam 14 emitted by conventional fiber device
10. The
2 0 increased Rayleigh range implies that emitted beam 19 is better collimated
than the
beam 14. The improved properties of the emitted beam 19 facilitate transverse
alignments required to end-couple the fiber device 16 to another fiber device
(not
shown).
In some embodiments of fiber device 16, GRIN fiber lens 18 has an end face
21 that is angle cleaved to reduce back reflections of light into optical
fiber 17. In
particular, a normal vector to the end-face 21 is preferably cleaved at an
angle 1 ° - 2 °
or less with respect to the axis of the GRIN fiber lens 18. This cleave angle
is smaller
than a typical cleave angle of about 8 ° used to lowerreflections from
its end face
back into the optical fiber (not shown). The beam expansion provided by the
GRIN
3 0 fiber lens 18 lowers the amount of angle cleave needed to produce an
equivalent
reduction in back reflections into the fiber 17.
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The new GRIN fiber lens 18 has a circular core 22 and an annular cladding 24
that surrounds the core 22. In the core 22, the refractive index varies with
the radial
distance from the axis of the GRIN fiber lens i8. In the cladding 24, the
refractive
index is constant and has a lower value than in the core 22. The GRIN fiber
lens has
an outer diameter of about 125 Vim.- The outer diameter is the same as that of
conventional GRIN fiber lens l l shown in Figure 1.~ But, the new and
conventional
GRIN fiber lenses 11, i8 have different radial refractive index profiles due
to
differences in density distributions of dopant atoms in their cores. Exemplary
dopants
include germanium (Ge), aluminum (Al), phosphorus (P), and fluorine {F).
Figure 3A shows radial profiles 26 and 27 of Ge-dopant densities in
conventional GRIN fiber lens 11 and new GRIN fiber lens 18, respectively. In
the
core 22 of the new GRIN fiber lens 18, the Ge-dopant density has a radial
profile that
is largest on the central axis and curved concave downwards. The profile does
not
have an axial density dip, i:e., unlike some conventional GRIN fiber lenses
(not
shown). The curvature of the radial profile of the Ge-dopant has a smaller
average
magnitude in the core 22 of the new GRIN fiber lens 18 than in the core of
conventional GRIN fiber lens 11. In the claddings of both the new and
conventional
GRIN fiber lenses 18, 11, the Ge-dopant densities are lower than in the fiber
cores
and are constant with respect to radial distance from the fiber axes.
2 0 The boundaries between core and cladding, i.e., at radial distances of R~
and
R~', are characterized by abrupt changes in the Ge-dopant densities and/or
radial
gradients of the densities. The core diameter is larger in the new GRIN fiber
lens 18
than in conventional GRIN fiber lens l l, i:e., R~' > R~. Increasing the core
diameter
increases the Rayleigh range of fiber device 16 when a GRIN fiber lens 18 of
2 5 appropriate length is used therein. Exemplary embodiments of the GRIN
fiber lens 18
have an outer diameter of about 125 Vim. and a core 22 with a diameter of
about 85
pm, preferably 100 pm or more, and more preferably 105 pm or more. In some
GRIN fiber lenses 18, cladding is absent so that the core has a diameter of
about 125
pm.
3 0 Figure 3B shows refractive index profiles 28 and 29 that correspond to the
Ge-
dopant density profiles 2b and 27 of GRIN fiber lenses 11 and 18,
respectively. The
radial profiles 28, 29 are concave down in the core 22.
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The radial profiles 28, 29 also show that the new GRIN fiber lens 18 has a
refractive index whose radial profile has a significantly more gentle
variation than in
the conventional GRIN fiber lens l l: A parameter "g" measures the radial
curvature
of the refractive index profile in the core of a GRIN fiber lens. In
particular, the
parameter g is defined as:
_l d2P(r)
g _- _ no dry I._o
Here, "r" is radial distance for the axis of the GRIN fiber lens, no is the
value of the
refractive index on the axis of the GRIN fiber lens, and P(r) is the value of
the
refractive index at the distance "r" from the axis ofxhe fiber lens.
The GRIN fiber lens 18 has a refractive index profile that has a gentler
radial
variation over the lens' core. Refractive index profiles of the GRIN fiber
lens 18
typically; have radial curvatures that are smaller in magnitude than those
disclosed in
Table 1 of "Analysis and Evaluation of Graded-Index Fiber-Lenses", Journal of
Lightwave Technology, Vol. LT-5, No. 9 (Sept 1987), pages 1156-1164, by W. L.
Emkey et al (EMKEY), which is incorporated by reference herein in its
entirety.
Typically, magnitudes of the radial curvature of refractive index profile for
embodiments of the GRIN fiber lens 18 are, at least, twice as small as valves
for the
same quantity that are disclosed in EMKEY. Exemplary GRIN fiber lens 18 have a
"g" that is less than 1.7~x 106 prri z, preferable less than about 0.9 x 10-6
~,rri 2 and
2 0 more preferably less than about 5.0 x 10'~ ~.m 2. For 125 ~.m - diameter
GRIN fiber
lenses 18, values of "g" are selected from the range 1.7 x 10'~ prri 2 to 5.0
x 10-~ ~.rri 2
and preferably in the range 0.9 x 10~ ~,rri 2 to 5.0 x 10'' ~.m:2 to provide
good beam
collimation.
Exemplary GRIN fiber lens 18 have core index profiles that vary
2 5 approximately quadratically in the distance from the lens axis. But, other
embodiments of the GRIN fiber lens 18 have non-quadratic index profiles.
Referring again to Figure 2, the new GRIN fiber lens 18 has a wider core 22
than the conventional GRTN fiber lens 11. The wider core 22 and the smaller
value of
the parameter "g" enable the new GRIN fiber lens 18 of appropriate length to
produce
3 0 a beam with a wider cross section and a lower energy density when used as
a beam
collimator.
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6
Figures 4A and 4B show light beams 31, 32 emitted by new and conventional
fiber devices 16 ; 10' of the types shown in Figures 1' and 2. The fiber
devices 16 ; 10'
have GRIN fiber lenses 18 ;11' with equal pitches, e.g., 5/16 pitch, but
different
refractive index profiles. The newprofile in the lens 18'significantly
increases the
Rayleigh range, RR, of the fiber device 16' above the Rayleigh range, RR', of
the
conventional device 10'. The increased Rayleigh range results from a more
gradual
beam expansion in the GRIN fiber lens 18' as compared to the beam expansion in
the
conventional GRIN fiber lens 11. In particular, Figures 4A and 4B show that
making
the radial curvature in refractive index of a GRIN fiber lens smaller than in
'
conventional GRIN fiber lenses significantly reduces the divergence of the
emitted
beam for a given pitch.
The Rayleigh range determines the distance range over which an optical
device can couple to a fiber device without substantial losses. The larger
Rayleigh
range in the new fiber device 16' makes a larger set of distances available
for end-
coupling to such a device than are available for the conventional fiber device
10'.
GRIN lenses of equal pitch ordinarily have equal products of g~~ times the
lens-length. Since the new GRIN fiber lenses 18 have smaller g-values, the new
GRIN fiber lenses I8 are ordinarily longer than conventional GRIN fiber lenses
11 of
equal pitch. The longer lengths make the new GRIN fiber lenses 18 easier to
handle,
2 0 align, and fuse to optical fibers than the conventional GRIN fiber lenses
11. The
increased lengths also reduce cbllimation errors associated with cleaving
errors that
occur during production of the new GRIN fiber lenses 18.
Figure 5 is a flow chart for a method 100 of fabricating a GRIN fiber lens of
doped silica-glass through modified chemical vapor deposition (MCVD). MCVD
construction of optical fibers is described in U.S. Patent 4,909,816
and4,217,027
The fabrication method 100 includes forming an improved GRIN preform and then,
using the improved GRIN preform to make the GRIN fiber lenses, e.g., GRIN
fiber
lenses 18 of Figure 2.
To form the GRIN preform, layers of silica-glass are deposited inside a silica-
3 0 glass cladding tube by MCVD (step 102). During the MCVD, a time-varying
partial
pressure of dopant gases is bled into the gas mixture used to deposit silica-
glass .on the
inside of the cladding tube. Exemplary dopants include Ge, Al, P, and F.
Introduction of one or more of these dopants into the silica-glass changes the
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refractive index of the glass. The partial pressure of dopant gas is varied
during the
MCVD process to produce a non-trivial radial profile of dopant atoms in the'
final
silica-glass preform.
The radial profile in dopant atoms produces a selected radially graded
refractive index in the final preform. Exemplary profiles for the dopant
density and
the refractive index have profiles with concave downward or negative radial
curvature. Often, the index profile varies as the square of the distance from
the
preform's axis in the core of the preform, e.g., profiles 27, 29 of Figures 3A
and.3B.
Other radial profiles may be obtained by suitably altering the time-variation
of the
partial pressure of dopant atoms during the MCVD. Non-quadratic profiles in
GRIN
fibers are capable of reshaping of 'light beams therein-as is known to those
of skill in
the art.
The method 100 includes using the tube produced by the internal deposition to
form the rod-like preform. To form the rod-like preform, heat is applied to
partially
collapse the tube of doped silica-glass (step 104). In one embodiment, the
heating
includes making repeated passes of the tube through a hot zone of a' furnace.
The
heating is stopped prior to totally blocking the axial channel in the tube
with glass.
After partially collapsing the tube, a silica-glass etchant mixture is passed
through the axial channel to remove several layers of glass from the axis of
the tube
2 0 (step 106). An exemplary gaseous etchant mixture includes CZF~, 02, and
CIZ. Other
gaseous etchant mixtures include HF: The removed layers have lower dopant
concentrations than adjacent outer layers of silica-glass, because dopants
vaporize and
are lost through the tube's axial canal during the heating used to collapse
the tube. If
these layers with lower dopant densities were not removed, the final preform
would
have an axial dip in dopant~density and a corresponding axial dip in
refractive index.
The axial dip in refractive, index interfered the operation of some
conventional GRIN
fiber lenses.
After the etching removal of several central layers of glass, the tube is
externally heated to finish its collapse to a rod-like preform of doped
silica; glass (step
3 0 108).
After cooling the preform, etchants are applied to the outer surface to remove
a selected thickness of cladding tube from the outside of the preform (step
110).
Removing a portion of the cladding tube enables-subsequent drawing of glass
fibers
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with less or no cladding, e.g., see profiles 27 and 29 in Figures 3A and 3B.
These
thin-clad or non-clad fibers are advantageous for GRIN fiber lenses, because
such
fibers enable an optical beam to expand over a larger portion of the cross
section of
the final GRIN fiber. Spreading the beam over a larger cross section decreases
the
associated numerical aperture and decreases power densities so that defects on
the end
surface of the lens or on the target of the emitted beam are less likely to
cause
component damage.
Fabrication of GRIN fiber lenses also includes using a standard fiber drawing
furnace to draw GRIN fiber from the graded-index preform (step 112). After
cooling;
one end of the drawn GRIN fiber is fused to one end of a standard fiber, i.e.,
a fiber
with a non-graded index core (step 114). To fuse the GRIN and standard fibers,
the
ends of the two fibers are heated with an electrical arc or a tungsten
filament in an
argon environment while the ends are appropriately aligned and positioned
adjacent
each other.
Finally, the GRIN fiber is cleaved to produce an optical lens with a desired
length (step 116). The final attached GRIN fiber lenses has a pitch of 1/e,
~/z, or any
other desired length and is fused to the fiber on which it functions as a beam
collimator and expander.
To reduce reflections from the face of the final fiber device back into the
fiber,
2 0 the cleaving is often performed along a direction that is not
perpendicular to the axis
of the GRIN fiber. In a non-GRIN optical fiber, cleaving the fiber's end face
at an 8
degree angle with irespect to a direction perpendicular to the fiber's axis
significantly
reduces back reflections. For a GRIN fiber lens, this cleaving angle can be
reduced to
less than 8 degrees from a direction perpendicular to the lens axis to achieve
the same
reduction in back reflections into an attached optical fiber; e:g., a
preferred cleave
angle is about 0.5 -2 degrees.
The method 100 produces GRIN fiber lenses, e.g. lens 18 of Figure 2, that
have lower refractive powers per unit length than conventional GRIN fiber
lenses,
e.g., lens 11 of Figure 1. Thus, the new GRIN fiber lenses are significantly
longer
3 0 than conventional GRIN fiber lenses having the same optical power. The
longer
lenses collimate light better and are easier to manipulate during device
construction.
Exemplary GRIN fiber lenses with low radial dopant gradients have full pitch
lengths
of about 2, 3, or 4-20 mm.
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The GRIN fiber lens 18 of Figure 2 can also be made by vapor axial
deposition (VAD); outer vapor deposition (OVD), and sol-gel processes that are
known to those of skill in the art. Such processes are also able to avoid
creating an
axial dip in refractive index in the final GRIN fiber lens.
2. Fiber Devices That Use Grin Fiber Lenses
Various embodiments provide optical fiber devices that are described below.
The various devices described can use either conventional GRIN fiber lenses,
e.g.,
lens 11 of Figure 1, or new GRiN fiber lenses; e.g., lens 18 of Figure 2.
Figure 6A shows a mode converter 40 that couples a pair of optical fibers 36,
38 having different fundamental or higher propagating modes, In some
embodiments,
the optical fibers 36, 38 have cores of different diameters or have refractive
index
jumps of different sizes across core-cladding boundaries. In the mode
converter 40,
GRIN fiber lens 43 is attached to the ends of the optical fibers 36, 38. In
exemplary
mode converters 40, the GRIN fiber lens 43 is either fused directly to the
optical
fibers 36; 38 or joined to the fibers 36; 38 by a glue layer (not shown) whose
thickness is not greater than the width of the cores of fibers 36, 38.
Since optical fibers 36, 38 have different core diameters and/or refractive
index jumps, the fibers 36, 38 have propagating modes, e.g., fundamental
modes, with
different sizes. Herein, the size of a propagating mode is defined as the
mode's full-
2 0 diameter between half-maximum amplitude values. Due to the different sizes
of the
propagating modes, coupling the optical fibers 36, 38 directly would produce a
significant coupling loss of optical energy, i.e., a splice loss.
To reduce splice losses, GRIN fiber lens 43 is positioned between optical
fibers 36, 38 and is selected to expand the narrower propagating mode of
optical fiber
2 5 36 to have a larger diameter that equals that of the propagating mode of
ttte optical
fiber 38. Designing the GRIN fiber lens 43 to produce the appropriate size
conversion entails selecting an appropriate lens length. One of skill in the
art would
know how to select the length of GRIN fiber lens 43 based the amount of
magnification needed to convert the size of the propagating mode of one fiber
36 into
3 0 that of the propagating mode of the other fiber 38.
In other embodiments, the mode converter 34 couples a waveguide other than
an optical fir to optical fiber. 38.
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Figure 6B shows a specific embodiment 34' of the mode converter 34 of
Figure 6A. In the mode converter 34', GRIN fiber lens 43' is a compound lens
made
of a sequence of GRIN fiber lens elements 43A, 43B. The first element 43A is
fused
directly to the end of optical fiber 36, and the last element 43B is fused
directly to the
end of optical fiber 38: Exemplary GRIN elements 43A and 43B are fused
together
and have different refractive index profiles and lengths. The lengths and
index
profiles of the two lens elements 43A, 43B are selected to better optically
couple the
fibers 36, 38. In some embodiments, the first GRIN element 43A expands the
light
beam emitted by fiber 36, and the second element 43B focuses the beam waist to
the
size of the propagating mode in the fiber 38.
Figure ZA shows a 1x2 micro-optical routes 46. The routes 46 includes an
input optical fiber 48, output optical f hers 50, 52, and a movable reflector
54 for
directing light from the input fiber 48 to a selected one of the output fibers
50, 52:
The terminal ends of the optical fibers 48, 50; 52 are fused to GRIN fiber
lenses 49,
49 ; 49", e.g., identical GRIN fiber lenses. The GRIN fiber lens 49 functions
to
collimate or focus the emitted light beam from fiber 48. The GRIN fiber lenses
49 ;
49" function to collect light and couple the collected light into the
associated optical
fibers'50, 52. The output optical fibers 50, 52 are located so that the waist
of the
beam emitted by -the input optical fiber 48 is at the midpoint of the oprical
path
2 0 between the input and output optical fibers 48, 50, 52. The reflecting
surface of
reflector 54 is located at the beam waist to within about a Rayleigh range
when
positioned to reflect light to the output optical fiber 50.
To select a routing; reflector 54 is moved in or out of the path of the light
beam emitted by optical Eber 48. The reflector 54 is fixed to a micro-electro-
2 5 mechanical (MEM) device 56 that moves the reflector 54 in and out of the
beam's
optical path in response to electrical signals applied to the MEM device 56.
The GRIN fiber lenses 49, 49 ; 49" improve beam collimation and collectiow
so that terminal ends 58, 60, 62 can be separated by distances xhat are large
enough to
enable insertion and removal of reflector 54 in routing region 64. In
embodiments of
30 routes 46 based on the new GRIN'fiber lenses 18 of Figures 2, 3A-3B, and
4A, better
beam cpllimation enables distances between terminal ends 58, 60, 62 to be as
large as
about 9 mm: For these large inter-fiber distances, the GRIN fiber lenses 49,
49 ; 49"
reduce optical coupling losses to less than about 0.5 decibels (dB) and
preferably to
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less than about 0.2 dB - 0.05 dB. However, larger inter-fiber spaces involve
more
serious fiber device alignment issues.
In some embodiments, the micro-muter 46 has an overall' size, S; that is much
smaller than the overall size of an analogous router in which the GRIN fiber
lenses
49, 49 ; 49" are replaced by conventional lenses with curved refractive
surfaces. The
lenses with curved refractive surfaces have larger diameters than the GRIN
fiber
lenses 49, 49 ; 49". The larger lens diameters require positioning the ends of
the input
and output fibers at larger separations in such a muter than in the micro-
router 46.
The lenses.with curved refractive surfaces also typically produce larger
diameter
collimated~beams in the routing region than the fused GRIN fiber lenses 49 of
micro-
router 46. The larger beam diameters necessitate a larger reflective surface
on the
routering reflector of the muter whose' lenses have curved refractive surfaces
than
would be needed on the reflector 56 of the micro-router 46.
In some embodiments of micro-router 46, the distance, S, characteristic of
separations between GRIN lenses 49, 49 ; 49" has a value in the range of about
1-3
times the fiber diameter to about 1-3 times the Rayleigh range; e.g., less
than about 1
mm. In these embodiments, the small size of the region 64 between the lenses
49, 49',
49" is achieved in part, because diameters of the attached GRIN fiber lenses
49, 49',
49" are small and in part, because the reflective surface on reflector 54 has
a small
2 0 beam acceptance window. The acceptance window for reflecting the input
beam can
be less than the fiber diameter, because the GRIN fiber lens 49 produces a
beam waist
that is smaller than the diameter of fiber 48. Both the small diameter GRIN
fiber
lenses 49, 49 ; 49" and the smallness of reflector 54 enable the router 46 to
be much
smaller than routers that use lenses with curved refractive surfaces.
2 5 Figure 7B shows an alternate embodiment 46' of the muter 46 shown in
Figure
'7A. In muter 46', the fibers 48, 50; 52 are adjacent and located in a linear
array 68. A
single rotatable reflector 56', e.g., a MEMS controlled reflector, selectively
routes
light from the fiber 48 to either the fiber 50 or the fiber 52: In .some
embodiments, the
axes the fibers 50 and 52 are slightly tilted with respect to the axis of the
fiber 48 to
3 0 insure that light from the reflector 56' parallel to the axis of the
fibers 50, 52.
Arranging the fibers 48, 50; 52 in array 68 makes the width of the router 46'
roughly equal to the width, W, of the array 68. The small diameters and' fine
'
collimation of GRIN fiber lenses 49; 49',49" enable packing the fibers 48, 50,
52
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Reed 26-3 12
closely in the array 68. Thus, embodiments of the router 46 can have a width,
W, that
is much smaller than the width of a similar-form router in which lenses with
curved
refractive surfaces replace the GRIN fiber lenses 49, 49 ; 49".
Figure 7C shows an embodiment of an optical device 46" that couples three
optical fibers 48; 50, 52 based on light polarization, light wavelength, or
relative fiber
position. The optical fibers 48, 50; 52 have attached GRIN fiber lenses 49;
49'; 49"
that collimate and collect light. The device 46" includes an optical element
54' that
transmits light between the optical fibers 48, 50, 52, e.g., in a manner that
depends on
polarization or wavelength. In various embodiments, optical device 54'
includes a
polarizing beamsplitter, a grating, an optical circulator, or a wavelength
selective
reflector such as a Bragg grating.
Figure 8 shows a 1xN micro-optical muter 70 that includes an input optical
fiber 72, an output-array 73 of N output optical fibers 741-74N, and a
reflector 76. The
optical fibers 72, 741-74N are single-mode fibers to which terminal GRIN fiber
lenses
77o-77r, have been fused. The light beam 78 from the input optical fiber 72
intersects
the reflector 76 near the waist of the beam 78, i.e., within' a Rayleigh
range.
Exemplary reflectors 76 include mirrors that move or rotate and diffraction
gratings that reflect light in a wavelength dependent manner. For example, the
muter
may be a spectrally sensitive demultiplexer for a wavelength division
multiplexed
2 0 network.
The GRIN fiber lenses 77a-77N expand and collimate the light beam 78 of the
input optical fiber 72 and focus the light beam 78 into the output optical
fibers 74,-
74N. Due to the GRIN fiber lenses 77o-77N, the output array 73 of optical
fibers 741-
74N and input optical fiber 72 can be separated by an optical path that is
long enough
2 5 to enable insertion of bulk reflector 76 into the path without significant
coupling
losses. For the router 70 coupling losses are typically less than about 0.5 dB-
0.2 dB
and preferably less than about 0.1 dB.
In micro-optical router 70, GRIN fiber lens 77o focuses the beam from fiber 72
onto a reflective acceptance window on the reflector 76. Perpendicular to
direction D,
3 0 the diameter of the acceptance window is less than the fiber diameter.
Also, the use
of the GRIN fiber lenses 77o-77N enables an increased fiber packing density in
the
array 73 without interference between light beams reflected towards different
ones of
the fibers 74i-74N. Finally, the°use of GRIN fiber lens 77a enables the
acceptance
CA 02380808 2002-04-08
Reed 26-3 13
window and overall size of reflector 76 to be smaller than that of the
reflector that
would otherwise be needed in a muter using lenses curved refractive surfaces
(not
shown). Thus, using the GRIN fiber lenses 770-77N enables greater
miniaturization in
micro-routes 70 than in a fiber routes based on lenses with curved refractive
surfaces.
Other embodiments use the GRIN fiber lens 18 of Figure 2 to construct Nxl
routers (not shown) by methods that would be obvious to one of skill in the
art in light
of the above-disclosure. For example, a 2x1 routes can be constructed by
exchanging
designations of input and output for fibers 48, 50, 52 in 1x2 micro-routes 46
of Figure
7A.
Figure 9 is a top view of an NxM optical routes 80. The routes 90 includes an
array 81 of N input optical fibers, 821-82N, and an array 83 of M output
optical fibers,
841-84M. The fibers 821-82N, 841-84M have GRIN fiber lenses 851-85N, 861-86M
fused
to terminal ends thereof. The GRIN fiber lenses 851-85N, 861-86M provide beam
collimation and collection functions analogous those previously described in
relation
to GRIN fiber lenses 49, 49 ; 49" of Figure 7A. Between the input and output
fibers
821-82N , 841-84M are banks 87F, 87R of fixed and routing reflectors, 88F1-
88~, 89R1=
89~. Exemplary reflectors 8981-89RN include wavelength-selective reflectors,
e.g.,
gratings, and wavelength insensitive reflectors. Properly aligning the
reflectors 88Ri-
88k1,1 routes light from individual ones of the input fibers 821-82N to
selected ones of
2 0 the output fibers, 841-84M. The reflectors 88R1-88RN are operated by MEMs
devices
891-89N and have acceptance windows for input beams whose diameters are
smaller
than the inter-fiber spacing, IFS, of array 81.
By using attached GRIN fiber lenses 851-85N, 861-86M the fiber packing
densities in the arrays 81, 83 can be increased above fiber packing densities
of an
2 5 NxM fiber routes in which lenses with curved refractive surfaces (not
shown) replace
the GRIN fiber lenses 851-85H, 861-86M of Figure 9. Similarly, sizes of
reflective
surfaces of reflectors 88F1-88~; 89R1-89H1~1 in the routes 80 are smaller than
sizes of
reflective surfaces of reflectors in roofers based on lenses with curved
refractive
surfaces, because the beam diameters produced by the GRIN fiber lenses 851-85N
are
3 0 small. Both effects enable the new NxM to be smaller than an NxM routes
based on
lenses with curved refractive surfaces.
Figure 10 shows a micro-optical device 90 that is located in-line between ends
91; 93 of optical fibers 92, 94: Exemplary micro-optical devices 90 include
CA 02380808 2002-04-08
Reed 26-3 14
wavelength-sensitive add/drop modules, polarizers, polarization rotators, one-
way
optical isolators, and controllable optical attenuators. The ends 91, 93 of
the optical
fibers 92, 94 are fused to GRIN fiber lenses 96, 98. The GRIN fiber lens 96
collimates light emitted by the optical fiber 92. The GRIN fiber lens 98
focuses
received light into the optical fiber 94. The micro-optical device 90 has an
approximate thickness, d, that is not greater than the_Rayleigh range
associated with
the GRIN fiber lenses 96, 98. For such a thickness, the GRIN fiber lenses 96,
98
reduce diffraction-related coupling losses.
Other embodiments of the invention will be apparent to those skilled in the
art
in light of the specification, drawings, and claims of this application.
