Note: Descriptions are shown in the official language in which they were submitted.
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Optical coupling element
Description
The invention relates to an optical coupling element that is manufactured in
one piece
molding technology using polymer waveguide cladding material, with at least
one guide
groove each for an optical waveguide and an optical fiber.
Coupling elements of this type can be used, for example, in optical
communications
technology or in sensor systems.
The transmission of signals and data in communications systems and in sensor
systems is increasingly being done on an optical basis. Instead of electrical
connections, optical waveguides are used to establish optical connections
which, when
properly configured, form an optical network. To construct such a network, a
number of
different components are required in large numbers and at the lowest possible
prices.
These components include, for example, plugs and splices, signal splitters and
signal
branches, wavelength division multiplexers (WDM) and switches.
Optical waveguides are composed of a waveguide core and a waveguide cladding
and
are generally made of glass or plastic. The transport of the optical signal
thereby takes
place essentially in the core of the optical waveguide. One or more optical
modes are
guided, depending on the transmission wavelength and size or refractive index
of the
optical waveguide. In particular in the sensor system sector and for the
transmission
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of data over long-distances, a single-mode transmission is used, which
requires the use
of single-mode optical waveguides. Single-mode optical waveguides of this
type, at the
common wavelengths (0.4 - 1.6 Nm), have core diameters in the range of 2 - 10
pm.
On account of the small core dimensions, particularly strict requirements are
set for the
connections of single-mode optical waveguides with each other or with optical
components. For applications in optical communications technology, it is
thereby
necessary for the position of the fibers to maintain an accuracy of ~ 1 Nm in
the lateral
direction and ~ 0.5° for the angular orientation. Such tolerances can
be achieved, for
fiber ribbon plugs, for example, which are flat cables that have a plurality
of optical
waveguides, by manufacturing the plug with positioning structures for the
optical
waveguides using the injection molding method and a high-precision mold
manufactured using micro circuitry techniques (H.-D. Bauer, L. Weber, W.
Ehrfeld:
"LIGA for Applications for Fibre Optics: High Precision Fibre Ribbon Ferrule",
MST
News 10 (1994), p. 18-19].
Integrated optical components are increasingly being used for the passive and
active
processing of optical signals. For this purpose, an optical waveguide system
is
integrated into a substrate that performs a specific function (signal
branching, switching
etc.). Optical waveguides can be coupled to the component while staying within
the
above referenced tolerances, for example, by an active or semi-active assembly
process of the components in which the position of the fibers is varied as a
function of
the measurement of the incident light and the light measured at the output,
and thereby
optimized to a minimal loss. The cost and complexity of such a manufacturing
process
are relatively high.
W
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A very economical solution for the manufacture of such components lies in the
use of
polymer materials that are processed in a molding process such as injection
molding,
injection stamping, hot stamping, reaction molding etc. In this context, it is
particularly
advantageous, in addition to the wave-conducting areas, to also integrate
fiber guide
areas into the component, in which the fibers need only be inserted, without
any need
for further adjustment. Such a fiber coupling is also called a self-adjusting
or passive
fiber-chip coupling.
The use of LIGA technology is particularly favorable for the manufacture of
the
components. This technology includes the three process steps lithography,
electroplating and molding. In the first step, a resist is deposited on a
substrate and
printed through a suitable mask, e.g. with synchrotron radiation. After the
development,
galvanic metal is deposited in the areas removed from the resist, as a result
of which a
mold insert is formed as a negative of the original structure. This mold
insert is used in a
molding process (e.g. injection molding) for the manufacture of moldings, e.g.
moldings
made of plastic. Using LIGA technology, moldings can be manufactured with a
very
high precision (< 1 Nm). A detailed description of the LIGA technology is
presented,
among other places, in: W. Ehrfeld, M. Abraham, U. Ehrfeld, M. Lacher, H.
Lehr:
"Materials for LIGA products", Micro-Electrical Mechanical Systems: An
investigation of
microstructures, sensors, actuators, ... / edited by W. Benechcke, published
by The
Institute of Electrical and Electronic Engineers, Piscataway, NJ; IEEE Press,
1994.
In addition to a high coupling efficiency, an additional requirement is
generally that the
reflection loss must be as high (sic - low?] as possible. In other words, the
smallest
possible fraction of the light must be reflected back into the incoming fiber.
Such a
reflection must be avoided because it interferes with the transmissian laser
and thus
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can lead to a higher noise level. The value for the reflection loss to be
achieved for
applications in the telecommunications sector is less than -55 dB. (see
Bellcore:
"Generic Requirements for Fibre Optic Branching Components, GR1209, ISSUE 1.
November 1994"; Belcore, Morristown, 1994).
DE 42 12 208 describes a method for the manufacture of optical polymer
components,
in which a microstructure body is manufactured using Si-micromechanics and
excimer
laser processing. From this microstructure body, by galvano forming, a mold
insert is
produced which is used for molding with a polymer material. The microstructure
body
thereby has a V-shaped fiber receptacle and a groove for the waveguides.
US 5,343,544 describes an integrated optical fiber coupler that consists of a
substrate
with fiber positioning grooves and possibly channels. The optical fibers and
the optical
waveguides are separated from each other by a groove that is filled with core
material
of the optical waveguide. This coupling element can be manufactured using a
mold that
has been galvanically formed from a master.
DE 42 08 278 A1 describes an integrated optical component. In this integrated
optical
component that comprises a silicon substrate, the glass fibers run in an
etched, V-
shaped groove. The extraction of the light waves is performed by an optical
waveguide
made of an optical polymer by pulse coupling. If the cross section surface of
the optical
waveguide is very much smaller than the cross section surface of the glass
fibers, the
prior art patent teaches that a second optical waveguide is located between
the glass
fiber and the first optical waveguide. In that case, the second optical
waveguide has a
slightly lower refractive index than the first optical waveguide.
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US 4,865,407 describes a coupling element made of lithium niobate. This
material is
difficult to process and cannot be used for economical mass production. On the
lithium
niobate substrate, a thin titanium film is deposited, lithographically
structured and
thermally diffused into the substrate material. On the end opposite the fiber
end, the film
thickness is varied so that the refractive index of the resulting waveguide
decreases
gradually. As a result of the decreasing refraction index, the light guided in
the
waveguide is deflected into the substrate material and focused. The boundary
surface
of the fiber receptacle structure lies in the area of the focus. Because the
light is
deflected into the substrate material, the fiber core must lie lower than the
waveguide.
An additional disadvantage is that the boundary surface is not realized
vertically with
respect to the axis of the fiber, but is inclined so that it can receive the
diagonally
incident light emitted from the waveguide. The fiber end is also
correspondingly cut at
an angle. One significant disadvantage of this arrangement is that such a
diagonal
boundary surface, which has an undercut, cannot be manufactured using molding
techniques, but requires complex, time-consuming and expensive precision
machining
operations. Likewise, the beveling of the end of the fiber is a complex, time-
consuming
and expensive step.
EP 0 560 043 B1 describes passive integrated optical components with a molding
made
of polymer material that have optical waveguide structures and fiber guide
structures. In
this case, the grooves are rectangular grooves, into which the fibers are
inserted into
the grooves and optical waveguide core material is introduced. As a result of
the
different diameters of the optical waveguide and the optical fibers, the
grooves have
different depths. Consequently, at the point of contact where the end of the
fiber meets
the optical waveguide, there must be a step. Similar stepped structures are
described in
"Precision components for optical fibre connections fabricated by the LIGA
process", by
Arnd Rogner, SPIE., Vol. 1973, pp. 94-100, and EP 0 324 492. Web-like
structures are
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used only where exclusively waveguide structures are connected to each other.
The manufacturing methods generally provide that a substrate is provided with
a step,
the height of which can be precisely achieved, for example, by diamond
milling. Then
the stepped substrate is provided with a resist. During the subsequent
synchrotron
irradiation, a mask is used, the waveguide area and fiber area of which each
have the
shape of a rectangular aperture This mask is oriented with respect to the step
of the
substrate so that the fiber area and the waveguide area on the mask overlap
with the
later fiber area and the waveguide area on the coated substrate That means
that the
transition from the fiber area and the waveguide area of the mask must lie on
the step,
which requires a complex positioning of the mask. In the next step, the
printing and
development of the resist are done using galvano forming; a mold insert is
manufactured which forms the negative. The mold insert is molded to create the
final
molding by means of hot stamping or injection molding. This molding has the
corresponding groove, into which the optical fiber is inserted or into which
the
waveguide material is cast.
A disadvantage of this manufacturing process is that the step must be realized
with
great precision. In particular, the substrate may not have any burrs on the
edge of the
step, because the burr represents an undercut and can lead to disruptions
during the
galvano forming and the molding. In this case, the proper function of the
coupling of
optical waveguides and optical fibers can no longer be performed. Depending on
the
manufacturing technique used for the substrate and the step, such a burr can
be
avoided only with a relatively great deal of effort and expense. For example,
the burr
always occurs during fly-cut milling if the cut of the milling cutter runs
from the area of
the step over the edge.
.r'.
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An additional disadvantage of such a groove structure is that during the
filling of the
waveguide core material into the optical waveguide structure, the material can
flow into
the guide structure for the glass fiber, so that the fiber embedding material
and the
waveguide core material cannot be selected independently of each other.
Finally, the fibers must be provided with a diagonal terminal surface to
minimize
reflections.
The object of the invention is to create a coupling element that is easy to
manufacture,
has a structure in the coupling area that does not adversely affect the
coupling, and in
which the optical waveguide material and embedding material can be selected
independently of each other.
The invention teaches a coupling device in which a web made of waveguide
cladding
material is located as an integral component of the coupling element in the
coupling
area between the guide groove of the optical waveguide and the guide groove of
the
optical fiber.
The starting point for the invention was the knowledge that optical fibers and
optical
waveguides cannot be connected to each other directly, but must be offset by
some
distance from each other in the axial direction. It was discovered that in
contrast to a
lateral offset or an angular error, an axial offset has only a slight effect
on the coupling
loss. One surprising feature of this discovery was that this low caupling loss
could be
achieved in both single-mode and multi-mode technology if the dimensions and
refractive indices of the optical waveguide are appropriately selected.
Because the optical fibers and the optical waveguides do not abut each other
directly,
but are separated from each other by a web, during the manufacturing process,
the
v z
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critical edge of the stepped substrate, e.g. the above-mentioned burr, is
covered by the
web, as a result of which a significantly simpler manufacturing process
becomes
possible.
The separation of the optical waveguide and the optical fiber by the web also
offers
greater flexibility in the selection of the waveguide core material and the
embedding
material for the optical fiber
For example, in couplers of the prior art, the embedding material had to be
the same as
the waveguide core material. As a result, a reflection occurs at the boundary
surface
between the optical fiber and its embedding material, so that the reflected
portion must
be deflected by a relatively complex diagonal cutting of the optical fiber.
With the
coupling element claimed by the invention, the embedding material can be
selected so
that the refractive index need only be appropriately the same as that of the
fiber core
material. Any remaining reflection at the boundary surface between the fiber
embedding
material and the web (waveguide cladding material) can be deflected by a
diagonally
cut surface of the web.
The web thus has the advantage, among other things, that one or both surfaces
of the
web can have a structure which makes it unnecessary to process the extremity
of the
optical fiber, for example.
An additional advantage that results from the use of two different materials
is that the
fiber embedding material can be selected in terms of its adhesive and strength
properties, and the waveguide core material in terms of its optical
transparency. Any
penetration of the materials in question into the other guide groove is
prevented by the
web.
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The surface of the web (light entry surface) facing the guide groove of the
optical fiber
preferably has an inclined diagonal surface that encloses an angle cp of
between 2° and
10° with a line perpendicular to the longitudinal axis of said guide
groove. The
perpendicular line preferably lies in the plane of the optical coupling
element, so that the
inclined surface can be seen in an overhead view of the coupling element. The
width
and height of the inclined surface are preferably equal to or greater than the
diameter of
the core of the optical fiber that the inclined surface faces. The reflection
back into the
optical fiber is eliminated by the selection of an appropriate angle for the
inclined
surface. Thus there is no need to bevel the terminal surface of the optical
fiber.
Independently of or in addition to the configuration of the light entry
surface, the surface
of the web (light exit surface) that faces the guide groove of the optical
waveguide can
form an angle ~ of between 0.1 ° and 2° with a line
perpendicular to the longitudinal axis
of the guide groove of the optical fiber. That means that the two longitudinal
axes of the
guide grooves of the optical fiber and the optical waveguide also enclose the
angle ~.
This configuration leads to a bent arrangement, whereby the angle ~ is set to
the angle
cp, taking into consideration the refractive indexes of the materials used,
thereby
achieving an optimal coupling efficiency.
The guide structure for the optical waveguide is advantageously widened toward
the
light exit surface, so that it becomes possible to transmit the light cone
realized by the
web completely into the core of the optical waveguide.
In an additional realization, there can be a plurality of guide grooves for
optical
waveguides and optical fibers next to one another. In this embodiment, there
is a
common web at a right angle to the guide grooves, whereby both the guide
grooves for
v z
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the optical fibers and the guide grooves for the optical waveguides are
connected to
one another by means of at least one transverse groove that is oriented
parallel to the
web. Such transverse grooves increase the stability and mechanical strength of
the
mold insert used during the manufacture and are used to limit shrinkage during
the
molding.
The guide grooves for the optical waveguide or waveguides as well as the guide
grooves for the optical fiber or fibers and the web are preferably fabricated
in one piece
from polymer material using molding processes.
The invention teaches that it is advantageous to select the web thickness less
than 100
Nm, and preferably less than 50 pm.
The invention is explained in greater detail below with reference to the
exemplary
embodiments illustrated in the accompanying drawings.
Figure 1 is a view of the coupling element in perspective,
Figure 2 is a view, in perspective, of a substrate with resist and mask for
the
printing,
Figure 3 is an overhead view of the coupling element illustrated in Figure 1
to show
the beam path,
Figure 4 is a view in perspective of an additional embodiment of a coupling
element, and
Figures 5a are a sketch and a diagram respectively to explain the reflection
loss as a
and 5b function of the axial offset.
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Figure 1 shows a coupling element 20 with guide grooves in the farm of a fiber
groove
21 and a waveguide groove 22. The fiber grooves 21 and waveguide grooves 22
are
not directly in contact with each other, but are separated from each other by
a web 23.
The width of the web zs (See Figure 5a) is in the range of 5 - 100 Nm. The
waveguide
grooves 22 widen in the direction of the web 23, which is designated the taper
25, as a
result of which the optical field that widens as it passes through the web 23
can be
better received by the waveguide core, which is also explained in greater
detail with
reference to Figure 3.
The web 23, on its surface 40 (light entry surface) facing the fiber groove
21, has an
inclined surface 24, which is inclined, for example, at an angle of cp =
8° with respect to
the line 60 (See Figure 3) perpendicular to the longitudinal axis 61 of the
fiber guide
structure. As illustrated in Figure 3, the total light entry surface is formed
by the inclined
surface 24. The inclined surface 24 is inclined in the horizontal direction
and extends
over the entire depth of the groove 21. As a result of this inclined surface
24, light
reflected by the light entry surface 40 does not run back into the optical
fiber. This
feature is particularly advantageous if the refractive index of the material
used to embed
the optical fiber is approximately equal to that of the core material of the
optical fibers.
The coupling element 20 is preferably manufactured by means of a combination
of
LIGA and precision machining operations, and is described below with respect
to the
manufacture of a coupling between a standard single-mode fiber (fiber diameter
125
pm, fiber core diameter approximately 8 Nm) and a square single-mode waveguide
with
an edge length 8 Nm, in conjunction with Figure 2. The starting point for the
manufacture is the substrate 1 with a step 4, the height of which is very
precisely set,
for example, by diamond milling cutters to ho = 58.5 ~ 1 Nm. Then this
substrate is
coated with resist 3 and the height of the resist is thereby adjusted by
polishing so that
r~.
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in the low fiber area 5 it is h2 = 66.5 pm and in the flat waveguide area 7 h~
= 8 Nm.
During the subsequent synchrotron irradiation, a mask 6 is used which has a
waveguide
area 9 and a fiber area 8 which are separated from each other by a web 28. The
mask
has structures for waveguide grooves 27 in the form of a rectangular opening
with d~ _
8 pm and for fiber grooves 26 in the form of a rectangular opening with d2 =
125 Nm.
This mask is oriented with respect to the substrate step 4 so that the fiber
area 8 and
the waveguide area 9 on the mask 6 coincide with the fiber area 5 and the
waveguide
area 7 on the coated substrate. The next step is the irradiation and
development of the
resist, as a result of which fiber grooves 21' and waveguide grooves 22' are
formed,
which are separated from one another by the web 23'. One advantage achieved by
the
formation of the web 23' is that the burr 9 that is inevitably formed during
the
manufacture falls on the step 4 of the substrate 1 in the area of the web 23',
and is thus
completely covered by the resist 3. In this case, the end of the fiber groove
21' is the
image of the absorber structure provided on the mask 6, and does not result
from the
lateral surface 2 of the substrate step 4. The manufacturing process is
thereby
significantly simplified, and manufacturing-related problems on the substrate
1 cannot
influence the coupling of the optical fibers and optical waveguides. As is
known from
LIGA technology, the next step is the manufacture of a mold insert by the
galvano
forming and its molding by hot stamping or injection molding. The optical
fiber is then
inserted into the fiber groove 21 of the coupling element 20 (See Figure 1 ),
and the
waveguide material is introduced into the waveguide grooves 22.
Using this manufacturing process, an accuracy of the orientation of the
waveguide
groove 22 with respect to the fiber grooves 21 of less than 1 Nm can be
achieved.
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Figure 3 illustrates the optical function of the coupling in detail. In the
coupling element
20, the figure shows the fiber cladding material 35, the fiber core 36, the
waveguide
cladding material 37 and the waveguide core 38. The entire coupling element 20
is
made of waveguide cladding material. The end of the waveguide facing the web
23
widens in the form of the taper 38b. The inclined surface 24 of the web 23 is
also in
front of the fiber end surface 42. The space between the fiber end and the
inclined
surface 24 is filled with a fiber embedding material 41. Exiting from the
fiber core 36 is
the light cone 39 which is refracted on the beveled surface 24 and then
widens. This
light cone is completely received by the waveguide 38 that widens an the front
end. The
adjacent taper 38b causes the field to be transformed back to its original
width.
In accordance with Snell's law of refraction, the magnitude of the refractive
angle ~ is a
function of the angle cp and of the refractive indexes of the fiber embedding
material and
waveguide material. It is particularly advantageous to orient the waveguide
core 38 at
an angle so that its longitudinal axis 62 coincides with that of the refracted
light cone 39.
That means that the light exit surface 44 of the web 23 facing the waveguide
38 is also
oriented at an angle. In this manner, an optimal coupling efficiency is
achieved.
With regard to the intensity of the reflection, it is important that the light
from the fiber
core 36 to the waveguide 38 passes a plurality of boundary surfaces. These
surfaces, in
order, are the boundary surface (fiber end surface 42) between the fiber core
36 and
fiber embedding material 41, the inclined boundary surface 43 between the
fiber
embedding material 41 and the waveguide cladding 37, and the boundary surface
(light
exit surface 44) between the waveguide cladding 37 and the waveguide core 38.
Theoretically, the reflection on the individual boundary surfaces is greater,
the greater
the difference between the refractive indexes on both sides of the boundary
surface.
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Because the materials for the waveguide cladding 37 and core 38 are generally
selected on the basis of their optical transparency and their processing
characteristics,
their refractive indexes are already defined. A typical material for the
waveguide
cladding, for example, is PMMA, which has a refractive index of 1.49, which
differs
significantly from the refractive index of the optical fiber cladding, which
is 1.45. It is
particularly advantageous to use a material for the fiber embedding material
41 that is
different from the material used for the waveguide core 38 and advantageously
has the
same refractive index as the fiber core 36. Consequently, practically no
backward
reflections will occur on the corresponding boundary surface (fiber end
surface) 42. The
same is true for the light exit surface 44, because the refractive indexes of
the
waveguide core and waveguide cladding are generally close to each other. The
principal portion of the reflection therefore occurs on the boundary surface
43 and the
light cone 45 is formed. The light cone is inclined by the angle 2cp as a
function of the
law of reflection with respect to the fiber end surface. In that case, it is
particularly
advantageous to select the angle cp large enough (e.g. cp = 4°), so
that the light reflected
back can no longer be received by the fiber core and therefore there is no
reflection
back into the optical fiber.
An additional advantageous configuration of the invention is illustrated in
Figure 4. The
coupling element 30, which is shown in perspective, has a plurality of fiber
grooves that
lye next to one another, of which the fiber grooves 21 and 31 are visible. The
two fiber
grooves 21 and 31 are connected to each other by a groove 32 that runs in the
transverse direction. The same is true for the waveguide grooves 22 and 33
that lie next
to each other and are connected to each other by the likewise transverse
groove 34.
This configuration has the advantage that it significantly simplifies
assembly, because
the core material, when it is introduced into the guide structure, can be
distributed more
effectively and thereby reduces the risk of unfilled spaces. This advantage
becomes
particularly important when two different materials are used for the waveguide
core and
to embed the optical fibers. An additional advantage is that as a result of
the transverse
W z
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grooves 32 and 34, a higher stability and mechanical strength of the mold
insert is
achieved, and that the transverse grooves 32 and 34 limit shrinkage during the
molding
process.
Figures 5a and 5b show the fiber waveguide coupling losses as a function of
the axial
offset zS. The geometry on which the calculation is based is illustrated in
Figure 5a.
Optical fibers 46 with fiber core 47 and wavve guides with cladding 48 and
core 49 are
separated from each other by the distance zs. Between the fiber and waveguide
end lies
the waveguide cladding material 50 which has the refractive index 1.49. The
results are
calculated in Figure 5b for three typical applications.
(i) shows the coupling of a square step index waveguide with an edge length of
55 Nm
and a numerical aperture of 0.219 into a gradient index fiber with a core
diameter of
62.5 Nm and a numerical aperture of 0.275.
(ii) shows the coupling in the reverse direction, i.e. from the gradient index
fiber into the
waveguide. In both cases, the calculations were based on a wavelength of 850
mm,
which is typically used for multi-mode systems in the field of short-distance
connections.
The increase in the loss in both cases is slow, and for zs = 100 Nm, for
example, is less
than 2.5 dB.
(iii) shows the coupling of a single-mode fiber with a core diameter of 9 Nm
and a
numerical aperture of 0.1 into a square step index waveguide with a core
diameter of 8
Nm and a numerical aperture of 0.1. For the coupling in the reverse direction,
we get the
same result. The calculation was based on a wavelength of 1300 nm, which is
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typically used in the field of telecommunications for long-distance
connections. The loss
increases slowly, and for zs = 100 Nm, for example, is less than 0.75 dB.
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Nomenclature
1 Substrate
2 Lateral surface
3 Resist
4 Step
Fiber area (substrate)
6 Mask
7 Waveguide area (substrate)
8 Fiber area (mask)
9 Waveguide area (mask)
20 Coupling element
21 Fiber groove
21' Fiber groove
22 Waveguide groove
22' Waveguide groove
23 Web
23" Web
24 Diagonal surface
25 Taper
26 Fiber groove structure
27 Optical waveguide structure
28 Web
29 Burr
30 Coupling element
31 Fiber groove
32 Transverse groove
33 Waveguide groove
34 Transverse groove
35 Fiber cladding
36 Fiber core
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37 Waveguide cladding
38 Waveguide core
38b Taper
39 Light cone
40 Light entry surface
41 Fiber embedding material
42 Fiber end surface
43 Boundary surface
44 Light exit surface
45 Light cone
46 Optical fiber
47 Fiber core
48 Optical waveguide cladding
49 Optical waveguide core
50 Optical waveguide cladding material
60 Perpendicular line
61 Longitudinal axis
62 Longitudinal axis
_.
