Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to an optical device for transmitting electromagnetic
radiation, said device having two planar waveguides wherein light is
propagated' in the longitudinal direction of the said planar waveguides, and
comprising a section, which is interposed between the two planar
s waveguides, for changing polarization state
The invention moreover relates to a method of manufacturing a section for
changing polarization state in an optical device, said device comprising an
integrated.planar waveguide for transmitting electromagnetic radiation.
'
Optical planar waveguides for changing the polarization state of light are
well-known. These are produced e.g. by affecting the material of which the
waveguide is made by electrical or magnetic fields. This is possible only for
crystalline materials. Other methods use acoustic or thermal signals which
act through the photoelastic effect. Here, crystalline as well as amorphous
materials may be used.
The anisotropic effects that can be achieved in the above-mentioned
elements by using external signals are modest, and relatively long extents
2 o are therefore required to achieve effective interaction between the
external
field and the light in the guide. Typically, 10 mm - 20 mm waveguides are
required. ,
An optical fiber polarization controller is known from WO 9853352. This
2s device employs wave plates made of short sections of a birefringent optical
fiber. The optical fiber polarization controller controls the polarization
state of
' input light by twisting or rotating birefringent fiber slices connected to
conventional single mode fibers. It is a disadvantage of WO 9853352, that
the polarization controller has to be manufactured by connecting several
3 o different types of optical fibers, which requires delicate handling of
short
section of birefringent fibers.
The object of the invention is to make it possible to achieve a controlled
interaction beiween the light and a strongly anisotropically designed
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waveguide, allowing a complete change in the polarization state to be
achieved.
The object of the invention is achieved in that the section for changing
polarization state is double-refracting and independently rotatable about the
longitudinal direction relatively to the planar waveguides. It should be men-
tioned in this connection that typically at least two 50 - 100 p,m long
sections
are required to achieve the complete change in the polarization state.
1o As stated in claim 2, the section for changing polarization state is a
waveguide section with a rectangular cross-section, or a waveguide section
having at least one additional layer applied on one side of it giving rise to
internal stress in the structure. So e.g. by forming the waveguide with a
rectangular cross-section, there will be a difference between the longitudinal
s5 coefficients of propagation of the TE and TM wave types. A great difference
in the longitudinal coefficients of propagation may also be achieved by
introducing thin layers of material with a high index of refraction on one or
two opposite sides of a waveguide of rectangular or square cross-section.
Another possibility is to place a thin high index layer in the centre of
2o waveguide core. In these cases, the thin layers give rise to internal
stress in
the structure. This stress occurs because of the difference in thermal
coefficients of expansion of the thin layers and the surrounding glass.
As stated in claim 3, a metal layer or other absorbant material is applied to
25 one of the sides of the section for changing polarization state.
Hereby, the polarization state oscillating in the plane to which the absorbing
layer is applied, will not be affected by it, while the state oscillating
orthogonal
to the absorbing layer will be attenuated. The absorbing layer may e.g. be
applied to the rotatable section by a lift-off technique.
For performing the rotations of the optical waveguide according to the
invention, it is expedient if, as stated in claim 4, it is arranged on an
actuator
mechanism, and additionally, as stated in claim 5, that the actuator
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mechanism consists of at least one flap with a sheet on whose upper side the
section for changing polarization state is mounted.
It is stated in claim 6 that the flap is formed as an electrode which is
provided
s with an insulating film on its underside, and which, together with a gold
electrode on a glass substrate, causes the flap to move.
Expediently, as stated in claim 7, the flap is formed in a silicon substrate.
For
suitable background doping, this will exhibit electrical conductivity and may
~ o hereby b'e used directly as an electrode.
For use in the application of the optical waveguide according to the invention
when switching ~ from one random polarization state to another random
polarization, state, it is an advantage if, as stated in claim 8, the
waveguide
15 contains several sections which may be rotated individually. This may e.g.
be
for a single compensator, where at least two sections are arranged in series
along an, axis which is parallel with the direction of light propagation. For
use
in the application of the optical waveguide according to the invention for
compensating the polarization mode dispersion (PMD), it is an advantage to
2 o arrange several sections which may be rotated individually, in parallel
between a wavelength demultiplexer and a wavelength multiplexer. Hereby,
the individual wavelength channels in a wavelength division multiplexing
signal (WDM) may be corrected individually.
2s For use as an optical insulator, it is expedient, as stated in claim 9,
that at
least two sections are rotated 45° relatively to each other about a
common
axis defined by the direction of light propagation.
Expedient embodiments of the optical waveguide are defined in the
3 o dependent claims in general.
As mentioned, the invention also relates to a method.
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This method is defined in claim 10 and is characterized by comprising the
steps of:
a) forming on the front side of a silicon substrate two planar waveguides
s consisting of a core surrounded by a glass cladding, and a core section
which is interposed between the two planar waveguides and
surrounded by the glass cladding, where the core section is formed
either with a rectangular cross section or where the core section has an
additional high density material applied to at least one side of it,
to
b) applying a mask to the rear side of the silicon substrate and etching a
silicon sheet out of a part of the substrate which is disposed below the
optical waveguide,
15 C) applying a further mask to the rear side of the silicon substrate and
etching at least one flap out of a part of the substrate which is disposed
in extension of the silicon sheet,
d) applying a mask to the front side of the silicon substrate and then
2 o releasing the silicon sheet and the flap,
e) applying an insulating film to the flap,
f) applying to a glass substrate a metal pattern which forms an electrode,
g) joining the silicon substrate with the glass substrate so that the free end
of the flap is joined with the glass substrate.
Expedient embodiments of the method according to the invention are defined
3 o in claims 11 and 12.
In a simple embodiment of the invention, the actual rotation of the waveguide
section is achieved by twisting the waveguide cross-section in the transition
between sheet and substrate. Such a structure will require a relatively long
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transitional member between substrate and sheet to allow significant rotation
of the member without causing ruptures in it. To achieve considerably shorter
transitional members for the section, claim 11 provides an expedient method
of manufacturing a rotary bearing for the structure. This rotary bearing is
manufactured by replacing item a) in the method defined in claim 10 by a
method comprising the steps of:
a1) applying a lower glass cladding and a first glass core to the front side
of the substrate,
a2) applying a mask to the front side of the substrate and forming a part of
the core by an etch,
a3) applying a sacrificial layer to the front side of the substrate,
a4) applying a second mask to the front side of the substrate and
removing a part of the sacrificial layer by an etch,
a5) applying a second glass core to the front side,
a6) ~ applying a third mask to the front side of the substrate and forming the
last part of the core by an etch,
a7) - heat-treating the glass core,
a8) applying a further sacrificial layer to the front side,
a9) applying a fourth mask to the front side and removing excess
sacrificial layer by an etch,
a10) applying an upper glass cladding to the front side of the substrate.
These rotary bearings are released during the etching in step d) when sheet
and flap are released.
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In an expedient embodiment, as defined in claim 12, the sacrificial layer is
formed in amorphous or polycrystalline silicon or in silicon nitride. The
requirement to be met by the sacrificial layer is that it must be capable of
re-
sisting the etch of glass core and cladding, while being removable without the
glass core and cladding being etched.
The invention will now be explained more fully with reference to an example
shown in the drawing, in which
to
fig. 1 schematically shows the basic principle of the invention,
fig. 2 shows the structure of a component with a planar optical waveguide
with controllable rotation,
Z5
fig. 3 is a cross-sectional view of the component of fig. 2 in a first working
position,
fig. 4 shows the component of fig. 3 in a second working position, and
figs 5a) - 5f) show the method of manufacturing a rotary bearing.
I n fig. 1, 1 a, 1 b designate planar waveguides which have a common axis
along which the light propagates. A section 2 is interposed between the two
2s planar waveguides, intended for changing the polarization state, which may
e.g. be the direction of polarization of a wave (not shown) from the left-hand
side of the figure in the planar waveguide 1 a to the planar waveguide 1 b
shown at the right-hand side of the figure, or vice versa.
3 o In an expedient embodiment of the invention, the section 2 is separated
from
the planar waveguides 1 a, 1 b and can thus rotate about its axis, without the
planar waveguides being affected by physical forces. The method of
manufacturing rotary bearings for such a structure will be explained more
fully in connection with fig. 5.
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In a simpler embodiment, two transitional members will be provided between
the section' 2 and the planar waveguides 1a and 1b. These transitional
members, designated 5a and 5b in fig. 2, are subjected to a twisting which
s does not give rise to any significant double refraction and is thus without
importance to the optical function of the component. These transitional
members, however, are of importance in connection with the size of the
component.
~ o As is generally known, the section 2 may be designed with a polarizing or
double-refracting effect. In general, as is also well-known, the section has a
core which is surrounded by a cladding. The principle of waveguide effect
may be total iritemal reflection or reflection against a photonic crystal
structure. The size of the core may be dimensioned for both single mode and
is multimode wave propagation.
In fig. 2, the section 2 is shown as a double-refracting section 3 with a core
which has a rectangular cross-section and is incorporated in a rotating
mechanism, as will be explained more fully below, cf. also figs. 3 and 4,
2 o where, however, the section 3 is shown with a core of square cross-
section.
On a silicon substrate 7, an optical waveguide of glass is formed with a core
11 and a cladding 10. A mask is applied to the opposite side of the silicon
substrate 'l, and then a sheet 4 is provided by etching.
Then, a further mask covering the sheet is applied, following which a flap 6
is
provided in extension of the sheet 4 by etching.
Still a further mask is applied to the front side of the silicon substrate,
and
3 o etching is carried out, following which the entire sheet 4 and parts of
the flap
6 are released from the silicon substrate, as shown at 12 and 13 in fg. 3. An
insulating film is applied to the underside of the flap, which may be done
e.g.
by a thermal oxidation of the silicon substrate.
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An electrode part, preferably of gold 9, is deposited on a glass substrate 8.
Finally, the silicon substrate 7 and the glass substrate 8 are joined at the
locations shown by the reference numerals 14, 15 and 16 in fig. 3.
The manufacture of a rotary bearing will be described below with reference to
figs. 5a) - 5f). This method introduces a rotary bearing in the waveguide from
the above method.
to In fig. 5a), a lower glass cladding 20 and a first glass core 21 are
applied to
the front side of the substrate. A mask is applied to the front side of the
substrate, and a part of the core is formed by an etch 22. A sacr~cial layer
is
applied to the front side of the substrate 23, and a second mask is applied to
the front side of the substrate, and a part of the sacrificial layer is
removed by
an etch. A second glass core is applied to the front side, and a third mask is
applied to the front side, following which the last part of the core is formed
by
an etch 24. The glass core is heat treated to achieve a rounded profile for
the
rotary member, following which another sacrificial layer is applied to the
front
side 25. A fourth mask is applied to the front side, and excess sacrifice
layer
2 o is removed by an etch. An upper glass cladding is applied to the front
side of
the substrate 26. The rotary bearing is released during the same etch as
releases the sheet 4 and the flap, as outlined in fig. 5f). Here, sheet and
flap
are released by a glass etch, which is then followed by an etch of the
sacr~cial layer. As a result, the rotary bearing consists of a short piece of
glass core 24 which is embedded in the glass claddings 20 and 26.
The function of the above-mentioned, manufactured component is as follows:
The effect of applying a voltage to the electrode will be illustrated with the
3 o core 11 shown in fig. 3 as the basis. This voltage causes the flap 6 to be
attracted by the electrode, whereby the sheet 4 with waveguide core 17
rotates, as shown in fig. 4. It should be stressed ~in this connection that in
the
rotation the waveguide with core 11 and cladding 10 maintains the position of
its longitudinal axis along which the light propagates. Hereby, the light
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travelling along the waveguide 11 experiences a changed orientation of the
cross-section of the waveguide 17 when the flap f is attracted. This gives
rise to changed coupling to the TE and TM wave types, respectively, in the
waveguide core 17 from the core 11.
'
Example 1
The use of the section described above, if combined with a plurality of the
same sections in series along an axis which is parallel with the direction of
light propagation, makes it possible to provide a polarization controller.
io
In an embodiment of such a controller, two separate half wave and quarter
wave sections are combined to an integrated system, in which the individual
optical axes may be rotated ~45° independently of each other. The
rotation of
t45° is achieved by attaching an actuator mechanism to each side of a
1 s waveguide section. The advantage of using such a polarization controller
is
that it is possible to change the direction of polarization and the state of
the
light with just a very low loss of power.
Another embodiment with a smaller rotation of the optical axes may be
2 o achieved by increasing the number of sections, which must alternately
exhibit
a positive and a negative phase delay and be rotated to the right and to the
left, respectively, relatively to the direction of light propagation.
Example 2
25 A variable analyzer may be manufactured by arranging a polarizer as the
rotatable wave guiding section. Rotation of the polarizer produces a variable
analyzer.
Example 3
3 o An optical .insulator may be manufactured by arranging a quarter wave
section between two polarizers, the axes of these having been rotated
45°
mutually.
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The optical insulator is manufactured by applying to the first waveguide
section a metal layer which attenuates the propagation of the TM wave type,
so that just the TE wave type is transmitted through the first section. The TE
wave is the linear polarization state which oscillates in the plane of the
s substrate. The second section is formed so as to give rise to a phase delay
of
~/2, and such that the optical axis is rotated 45° relatively to the
plane of the
substrate. After the second section, the light will be linearly polarized with
a
direction rotated 45° relatively to the output of the first section.
The axis of the
polarizer in the third section has likewise been rotated 45° relatively
to the
Zo axis of the first section, thereby allowing unobstructed passage of the
light
through it in the guiding direction.
For light reflected back to the optical insulator in the blocking direction,
the
course is as follows. Passage of section 3 means that the light is linearly
15 polarized with a rotation of 45° relatively to the plane of the
substrate. This
light is rotated another 45° by passage through section 2, following
which it is
linearly polarized in a direction rotated 90° relatively to the plane
of the
substrate. This corresponds to a pure TM wave. This polarization state is
attenuated by passage of the first section, whereby no light passes through
2 o section 1 in the blocking direction.
Example 4
An optical modulator may be provided by combining a polarization controller,
as described above, with a polarizer. By switching between e.g. the TE wave
2 s type and the TM wave type, only the TE wave type will be transmitted
through a polarizer with an axis in the TE direction. As a substitute for the
polarizer, a polarization-sensitive directional coupler may be used in another
embodiment.
3 o A further example of a modulator is two polarizers which are rotated
independently to the right and to the left, respectively. Passage of tight is
allowed when their axes are parallel, while the light is extinguished when
their axes are perpendicular to each other. This modulator may also be used
as a variable optical attenuator.
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Although the invention has been explained in connection with speck
examples and embodiments, nothing prevents further embodiments from
being manufactured within the scope defined by the claims.
1t might e.g. be an actuator on both sides of a waveguide section, thereby
allowing, rotation of the section clockwise as well as counterclockwise (e.g.
1 o Since the waveguide core is interrupted at the two bearings between the
waveguide sections, it will be necessary to add a material which adapts the
indices of refraction between the sections such that no reflections occur at
these transitions. This material might e.g. be an oil with an index of
refraction
which corresponds to the index of the glass cladding. Another possibility of
Zs suppressing reflections at the rotary bearing is to terminate it with an
angled
face toward the waveguide or to form the bearing with a tip toward the
waveguide.
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