Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL BANDPASS FILTERS BASED ON ELECTRO-OPTICALLY INDUCED WAVEGUIDE GRATINGS
WITH PI-SHIFT
s FIELD OF THE INVENTION
The present invention relates to optical filters and more particularly
concerns reconfigurable and multi-functional pi-shifted filters based on
electro-
optically induced waveguide gratings.
io
BACKGROUND OF THE INVENTION
For many applications, the transmission characteristics of a fiber grating are
is really the wrong way around: it is a band-stop rather than a band-pass. For
example, tuning a radio enables the selection of a channel, not the rejection
of it
from a broad frequency spectrum. However, traditional fiber gratings, short
period
(Bragg) as well as long period ones, work quite in reverse, and therefore
cannot
be easily used for channel selection.
2o Several band-pass filter designs using fiber gratings have been
constructed.
The combination of FBGs and an optical circulator can turn a reflection-type
filter
into a transmission-type filter, but optical circulators can be costly and
cause
serious additional losses.
Attempts have been made to design a band-pass filter from a single grating.
2s One solution suggested in the prior art is to introduce a n-shift in the
middle of the
grating. FIG. 1 (PRIOR ART) demonstrates an example of rr-shift in harmonic
distribution. It is believed that this idea was first proposed in 1976.
A rr-shift in a grating may be introduced in several ways. Post-processing of
the uniform grating in a certain region creates a permanent phase-shifted
region
30 (rr-shift). This occurs because an extra exposition to ultraviolet (UV)
light changes
the refractive index in that region, creating an additional phase step.
However,
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post-processing may be difficult to execute in practice, especially in short
gratings.
A better procedure is to use phase-shifted phase masks to introduce the
desired
n-shift in a grating. All these techniques are however time consuming and once
phase shift is introduced, little can be done to change its position,
magnitude or
s eliminate it at all.
There is therefore a need for pi-shifted optical gratings that are easier to
make and more versatile in their application than prior art devices.
io SUMMARY OF THE INVENTION
It is an object of the invention to provide a device for selectively inducing
a
~-shifted filter. In accordance with the invention, this object is achieved
with a
waveguide having at least one selectively actuated ~-shifted grating therein,
is comprising:
a core and a cladding, wherein said core or cladding is made of an electro-
optic material;
a plurality of electrodes on one side of said waveguide;
at least one electrode on another side of said waveguide opposite said one
2o side;
means for selectively applying a voltage pattern to said electrodes so that
said pattern induces at least one ~-shifted grating when said pattern is
applied.
In accordance with an aspect of the present invention, there is provided a
2s pi-shifted optical grating device based on electro-optically (EO) induced
waveguide
gratings. Preferably, the electro-optically induced gratings are of the type
shown in
FIG. 2, but the scope of the invention is not limited thereto. To induce a rr-
shift in
such a grating, the applied voltage polarity for a portion of the electrode
fingers is
simply reversed. In this manner, the ~-shift(s) can be conveniently induced or
3o removed at will in any portion of the grating.
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Referring to FIGs. 3a, 3b, 3c and 3d there are shown examples of
structures illustrating the principles of the present invention. FIG. 3a shows
the
central portion of an EO grating without any ~-shift, while FIG. 3b shows the
same
structure in the center of which the electrodes polarities have been reversed
to
s introduce the ~-shift. FIGs. 3b and 3c show a similar before and after
scheme, with
the difference that in the former case constant and variable components of
electric
field are induced inside the waveguide with a variable with periodicity I,
whereas in
the latter case only a variable component of the electric field distribution
with
periodicity 21 is created.
to Further advantages and features of the present invention will be better
understood upon reading of preferred embodiments with reference to the
appended drawings.
15 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a uniform grating and one with a
pi-shift;
Figures 2a) and 2b) are a representation of two preferred embodiments of
2o the present invention, a) where both top and bottom electrodes are discrete
and b)
where the bottom electrode is continuous;
Figures 3a)-3d) are representations of the introduction of a ~c-shift into EO
superimposed gratings, (a,b) where the central part of the structures is
without the
~-shift and (c, d) where the central part of the structures is with the ~-
shift;
2s Figures 4a) and 4b) show the transmission spectra of the EO-induced LPG
without ~-shift (solid) and with ~c-shift (dashed) for a) equal xL-product
value 0.5~
and b) for KL-product value 0.5~ for the uniform grating and KL-product value
0.706 for the ~-shifted grating;
Figure 5 shows the transmission spectra for the ~-shifted grating with ~cL-
3o product value 0.706 (solid) and oL-product value 2.118 (dashed);
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Figures 6a) and 6b) show two different schemes of the electric potential
application to the IDE for effective coupling between the modes with different
symmetry;
Figure 7 shows the transmission spectra for different positions of ~-shift
s within the grating: ~=0 (KL=0.7060 solid line; the dotted line is for
0/L=0.1
(KL=0.7410; the dashed line is for 0/L=0.2 (~cL=0.8640; and the bold dotted
line is
for 4/L=0.285 (KL=0.882~c);
Figure 8 shows the transmission spectra of the grating with one (KL=0.706,
solid line), two (xL=0.76, dotted line), three (~cL=0.77, dashed line), four
to (KL=0.78, dot-dashed line) and five (oL=0.79, bold line) symmetrically
positioned
~-shifts plotted versus the normalized wavelengths;
Figures 9a), 9b) and 9c) show the layout of EO reconfigurable grating-filter
structure: (a) single grating; (b) grating with single ~-shift; and (c)
multiple ~-shifted
gratings;
is Figure 10 shows the transmission spectra of the gratings with five
symmetrical ~-shifts: ~cL=1.385; L~=L6=2.125L; (solid line) and KL=0.79;
L~=L6=0.5L; (dashed line);
Figure 11 shows the creation of a Mach-Zender interferometric filter by
grounding M IDE finger pairs in the middle of the structure;
2o Figures 12a) to 12f) show the transmission spectra of the MZ filter with M
grounded IDE finger pairs (solid line) for the grating with the period 21 as
against
the spectrum with the uniform grating (dashed line) with the same coupling
coefficient and the number of activated IDE fingers;
Figures 13a)-13d) show the electrode structure and the potential application
2s scheme to induce two superimposed gratings (a) with 1 and 21 periods; (b)
the
same gratings with ~-shifts; (c) ~-shifted configurations for dV--0; and (d)
dV=-2Vo.
Figures 14a) and 14b) show the transmission spectra for the two
superimposed gratings of Figure 15 without (dashed line) and with ~c-shift
(solid
line) and (b) demonstration how contribution for the ~=shift superimposed
gratings
3o can be controlled through dV voltage.
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Figure 15 shows the reflection spectra of EO induced induced Bragg
gratings with ~c-shift (solid line) and without ~-shift (dashed line) for (a)
~cL=2 and
(b) KL=6.
Figure 16 shows the reflection spectra for different positions of ~c-shift
within
s the EO induced Bragg grating for KL=2 for (a) ~/L=0; (b) 0/L=0.08; and (c)
D/L=0.22.
Figure 17 shows the reflection spectra of the EO induced Bragg gratings
with one (solid); two (dotted); three (dashed); four (dot-dashed); and five
(bold-dotted) symmetrical ~-shift for oL=4. ,
to Figure 18 shows the reflection spectra of the Fabry-Perot type EO induced
grating filter for (a) the sub-gratings of a fixed length L/2 = (N-M)/2; and
(b) for the
sub-gratings with variable length L/2 = (N-M)/2 for M grounded IDE fingers and
the
center: M=500 (solid); M=600 (dotted); M=700 (dash); and M=800 (dot dash).
Figure 19 shows the reflection spectra for a Fabry-Perot tunable filter
Is composed from EO induced Bragg gratings with M disabled (grounded) IDE
fingers where each sub-grating consists of N/2=1000 enabled IDE fingers with
(a)
M=500 (solid), M=1500 (dash); and (b) M=2500 solid and M=3500 (dash).
Figures 20 a) and b) show the electrode structure and potential application
scheme to provide a ~-shift according to another preferred embodiment of the
2o invention, where (a) the electrodes are interdigitalized and symmetrical
and (b) the
bottom electrode is solid.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
2s
1. Introduction
An optical fiber (waveguide) grating is generally used as a filter for
selecting
an optical signal at a specific wavelengths) from multiple wavelengths
3o propagating along a core. The optical grating can eliminate or reflect
light at a
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specific wavelength by inducing a periodic change in the refractive index of a
waveguide. The optical grating is categorized into short (Bragg) period
gratings
(FBG) and long period gratings (LPG).
A FBG reflect light at a specific wavelength in the filtering process, whereas
s a LPG or transmission grating removes light without reflection by converting
the
optical signal component which must be removed from the core mode into the
cladding mode. We will start our description with LPGs which includes a
plurality of
refractive index perturbations spaced along the waveguide by a predetermined
distance that ranges from tens of microns to several hundreds of microns.
io The present invention concerns the inducement of gratings into an electro-
optic sensitive medium in an optical waveguide structure, but it should be
recognized that the present invention can also find application in optical
fibers.
In essence, the present invention is directed to a method and apparatus for
inducing the ~-shift into a waveguide, the waveguide comprising a core and a
is cladding (Fig. 2), preferably mounted on a substrate (on a bottom portion
thereof)
and a superstrate (top portion thereof). It will also be apparent to a person
skilled
in the art that the words "top" and "bottom" are for ease of comprehension
only.
A plurality of electrodes 9 is placed on one side of the top cladding. In the
case illustrated in Fig. 2, the electrodes 9 are placed on the top cladding on
the
2o side of the superstrate and will be referred to a "top electrodes" for ease
of
description.
At least one electrode 11 is placed on the other side of the core (i.e. in
Fig.
2, on the side of the substrate). However, the present invention also
contemplates
using a plurality of electrodes 11 on the other side, where the electrodes 9,
11 are
2s symmetrical about a longitudinal axis. The at least one electrode (Fig. 2b)
and the
plurality of electrodes (Fig. 2a) will be hereinafter referred to as "bottom
electrodes".
In a planar waveguide, the grating can be induced in top or bottom cladding,
i.e. the top cladding can be electro-optic, and the core and the bottom
cladding are
3o not, or the bottom cladding can be made from an electro-optic material and
the
core and the top cladding are from non-electro-optic one(s), or finally the
core can
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be electro-optic, and the top and bottom claddings are not. However, unlike a
planar waveguide, for a fiber, or a circular waveguide, it is impossible to
make
distinction between the top and bottom claddings and so the grating can only
be
included in the core or the cladding.
s In a preferred embodiment, the top and bottom electrodes are interdigitated
electrodes (IDE).
2. Reconfigurable Band-Pass Filters on the Basis of EO Induced Long Period
~o Gratings
2.1. Single Tr-shift structure
The solid curves in Fig.4 give an example of transmission spectra (solid) of
Is the electro-optically (EO) induced waveguide grating without rr-shift
(Fig.3b) and
with rr-shift, when fingers of interdigitated electrode (IDE) along the second
half of
the structure length are inversely biased in respect to the first half (see
Fig.3d).
Because the grating is induced electro-optically, it is always possible to
switch
between two types of spectra presented in Fig.4. The grating also can be
switched
20 OFF if the electric potential difference between the electrodes is equal to
zero and
behaves as a low-loss waveguide. It is well known that 100% out-coupling from
the core mode to the cladding mode takes place for KL = 0.5rr, where L is the
grating full length, and K is the coupling coefficient. For the rr-shifted
grating this
value is not enough to achieve full rejection in the two loss dips (see
Fig.4a). To
2s get 100% losses in these dips, as in Fig.4b, KL- value should be increased
to
0.706rr, which can be done in our design just by increasing difference of
potential,
Vo, 1.4 times or by increasing the grating length L through activating
additional
number of IDE fingers. Therefore special attenuation in the dips around the
band-
pass gap can be controlled electronically.
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The spectrum with band-pass gap also can be electronically switched to
about three times wider bandwidth, as it can be seen from Fig.S, by increasing
the
K L product three times, K(Vo)L = 3x0.706rr. Of course, it has to be
appropriate
dynamic range of the device for adjusting the K L - product. To provide the
s dynamic range the structure design should be optimized for proper external
electric field distribution to maximize the overlap integral for core-cladding
mode
interaction. For example, the electric potential application pattern in Fig.6a
is more
suitable for coupling the fundamental core mode into the odd (asymmetric)
cladding modes, whereas the configuration of the electric field from IDE in
Fig.6b
io is more effective for interaction between the fundamental mode and even
(symmetric) cladding modes.
All transmission spectra in Fig.4 and 5 are for the n-shift in the middle of
the
EO induced grating. However with proper designed electronic interface, the
position of the rr-shift can be controlled electronically, moving it to right
or left side
is and dividing the grating into two sections, the first section having a
length of L-d
and the second section having a length L+d, where d is the distance of the rr-
shift
from the center. As we can see in Fig.7, by moving the rr-shift toward the one
end
of the grating, the two dips will move closer to each other and finally merge
into
one dip at d/L 0.285. In Fig.7 the solid curve is for d=0 (rcL = 0.706n); the
dotted
2o curve is for d/L=0.1 (KL = 0.741n); the dashed curve is for dlL =0.2 (rcL =
0.864rr);
and finally the bold dotted curve is for d/L =0.285 (KL = 0.882rr). This kind
of
tunable filtering behavior can be used for variable optical attenuation.
2s 2.2. Multiple rr-shifts
As we have shown, rr-shifted EO induced grating produces transmission
gap in the stop-band which can be switched ON and OFF. In many applications,
it
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is desirable to control the bandwidth of the transmission gap, and to obtain a
flatter
response in the band-pass region. This can be achieved with a cascade of Tr-
shifts
sandwiched between sub-gratings. This concept was used for filter design with
the
help of ultraviolet imprinted sort-period (Bragg) cascaded gratings. Depending
on
s the length of the sub-gratings, many peaks may appear or they may coalesce
into
one.
In the proposed design of EO induced waveguide grating multiple rr-shifts
can be introduced or removed as easy as a single ~rr-shift that makes this
grating
truly reconfigurable filter. Fig.8 demonstrates how the width of the band-pass
gap
io can be changed by introducing several, generally M-1 (M, 3), symmetrical n-
shifts
form one to five, where we can see transmission spectra of a long period
grating
with one (KL = 0.706n, solid), two rcL = 0.76rr, dot), three (rcL = 0.77n,
dash), four
(KL = 0.78rr, dot-dash) and five (KL = 0.79rr, bold) symmetrically positioned
rr-
shifts. It is assumed that the positions of the rr-shifts (or positions of the
bias
is voltage inversion) are symmetric with respect to the center (see Fig.9(c)),
i.e. sub-
grating (or portions with the same voltage polarity) lengths are L~ =LM =Lo ur
and
L2 = L3 = ... = L M_ ~ = L, N.; L, N = 2Lo ur and L = 2Lour +(M-2)L, N . The
signs + and
- in Fig.9 denote positive and negative voltage at the two pairs of electrodes
to
provide the electric field without constant spatial component, i.e. zero "dc"
coupling
2o coefficients.
It is worthwhile noting that the bias voltage Vo has to be adjusted when
transmission is reconfigured between different band-pass windows in Fig.B. The
bias voltage should maintain proper values of the KL product that corresponds
to
the filter rejection level -35 dB. The product value is changed from KL =
0.5rr for
2s the grating without n-shift and correspondingly without the band-pass gap,
to KL =
0.79 for the grating with five n-shifts. When all the IDE fingers are
activated, the
bias voltage is the only parameter to adjust. For the above example it has to
be
increased 1.58 times.
The proposed design provides a broad range of different spectra that can
3o be easily reconfigurable between one another provided a good computer
controlled electronic interface to apply a proper electric potential
distribution to the
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IDE fingers. Fig.10 gives another example of two spectra for the grating with
five
symmetric ~rr-shifts, where for the solid curve ~eL = 1.385rr; L~ = L6
=2.125L; , and
the dashed curve is the same as in Fig.8 for five rr-shift structure, i.e.
.rcL = 0.79~rr;
L~=L6=0.5L;.
s
2 3 Mach-Zehnder band-pass filters
As we already mentioned, the coupling in our EO induced grating does not
to occur without the presence of the voltage at IDE fingers. That creates
another
opportunity to split our superimposed gratings into two sections by disabling
(grounding) a number of IDE fingers in the middle of the structure. Two
sequential
long period gratings with a space between them act as a Mach-Zehnder (MZ)
interferometer for the range of wavelengths for which coupling is enabled. The
first
is grating (the section in our case) couples part of the core mode intensity
into the
cladding mode, and the second grating (section) recombines them. Due to the
phase difference accumulated by propagation through the core and the cladding
respectively, they will interfere, constructively or destructively, depending
on the
wavelength and the space between the gratings (sections) leading to a periodic
2o transmission spectrum. By varying the space length, coupling coefficients,
the
structure of the transmission spectrum can be changed.
In Fig.11 the central part of the structure is shown where several pairs of
IDE fingers (from one to M) are grounded creating separation ~=IM in the
middle of
the structure. Control of the balance between LP and ~ = L- 2LP allows easy
2s alteration of the stop-band.
In Fig.12(a-f) the transmission spectra are shown for the grating with
periodicity 21 (Fig.3b,d) with correspondingly M =1, 2, 151, 152, 1000, and
1001
grounded finger pairs. In the simulation we maintained the constant number of
electrodes under the potential in the both section. In this situation we
should have
3o an appropriate number of disabled (grounded) electrodes on the right and
left ends
of the structure, which are activated as the electrodes in the middle part are
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disabled. All spectra are plotted as against the spectrum of the uniform
grating
(without IDE finger grounding, dashed curve). As we can see, grounding one or
any small odd number of electrodes (Fig.12a) gives us exactly the same effect
as
reversing the voltage polarity (~rr-shift) described in the first section,
whereas
s grounding of small amount of even number of electrodes practically does not
change the spectrum at all (Fig.12b). Increasing the amount of grounded
electrodes in the middle can be used to at least partially suppress the side
lobes
as it can be seen comparing spectra in Fig.12a and Fig.12c. To produce the
spectra with multiple gaps the distance between the sections has to be
to comparable with the section length (Fig.12e and 12d).
In our design, side-lobe suppression can be also easily done by modulating
the voltage Vo along the grating length using Gaussian, raised-cosine or any
other
apodization profiles.
In the case of potential application pattern in Fig.3a, Vo modulation results
is in changing the constant component of the electric field along the grating
length.
This change of constant component causes the change in EO-induced value of
average refractive index that has the same effect as a grating chirp
introduction.
Therefore the grating also can be controlled in terms of its phase group delay
or
dispersion.
We analyzed the MZ filtering characteristics separately for the two different
potential application schemes (Fig.3a and 3b). However it is obvious that
these
two schemes can be used together to control electronically their individual
contributions. The combined electric potential scheme is presented in Fig.13a
in its
uniform distribution (without rr-shift) and with rr-shift in Fig.l3b. For
clarity the two
2s initial potential application schemes are shown as particular cases in
Fig.13c and
13d. These two initial schemes are realized when aV = 0 and aV =-2Vo . For -
2Vo
< dV <0 the both periodic distributions are present and their contributions
can be
controlled electronically by dV voltage. This design can be useful for LPG
where
the beating length between the core fundamental mode and the i-th cladding
mode
3o is close to double value of the beating length between the core fundamental
mode
and the j-th cladding modes. Fig.14a demonstrates the double-dip spectra
(dash)
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of the superimposed gratings and appearance of band-pass gaps in the middle of
the dips (solid) when the n-shift is introduced by the electric field
reversing and it
also demonstrates control over transmission losses of the rr-shifted gratings
by dV
-voltage in Fig.14b.
s
3. Reconfigurable Filters on the Basis of EO Induced Bragg Gratings
The same principle of EO induced grating can be used for contra
io propagating interaction of guided modes that reflects light at a specific
wavelength
based on Bragg diffraction. This type of gratings requires submicron
periodicity /1
estimated by simple formula: /1 = as l(2n), where >rB is the Bragg resonance
wavelength, and n is the average refractive index of the waveguide core
material.
For infrared telecommunication wavelengths and for silica type materials this
is period is about half a micron. It means that our IDE structure should have
a period
1 equal /1 for the potential application scheme in Fig.3a or 1 = /1/2, if we
are
considering application scheme, shown in Fig.3b. It is not a simple task to
do,
nevertheless a number of sensor and microbiological applications have already
demonstrated that submicron IDE structures are feasible, and the rapid advance
in
2o nanoscale technology promises to make this type microfabrication a routine
task in
the nearest future. However the nature of a non-uniform electrostatic field is
that it
decays rapidly with' distance away from the IDE. The spatially variable field
components are essentially washed out at a distance from the IDE equal to the
IDE period. It imposes restriction on the wavelength thickness that should not
be
2s larger than 1 pm. This size of waveguide is not uncommon for semiconductor-
based waveguide, where the high difference do between cladding and core
indices forces to use very thin waveguide to maintain single-mode operation.
Still
such thin waveguides create problems in fiber coupling that prompts to use
complex design techniques to avoid substantial coupling losses.
3o There is a solution allowing to use IDE with longer period, and as result
with
thicker waveguide. It is to use higher spatial harmonic of the periodical
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electrostatic field distribution instead of the fundamental one. However
higher
spatial harmonics decrease rapidly in their magnitude with the harmonic order
m,
especially for the application scheme in Fig.3a.
The situation is even tougher for the potential application scheme in Fig.3b,
s where IDE period should be a half of the Bragg grating period. However this
situation can be different if we use EO material with quadratic (Kerry effect
instead
of linear (Pockets) one. This potential configuration creates an electric
field in the
waveguide that can be described the following Fourier series:
l0 E~ (x~ Z~ _ ~~ A~ (Z~ ~os~ ~x ~ + Aa (~~ ~os~ 3 ~ x + As (~~ Los 5 ~ x
+.... +
For a linear EO material, the refractive index change is proportional to the
normal component of the electric field, E~ (x, z), therefore the fundamental
spatial
harmonic has the period of 21. However for a quadratic EO material the
refractive
Is index change is proportional to E~ (x, z) squared. As a result we will get
the first
two spatial harmonics with the wave numbers:
_3~c _~c _ _~~c _3~c _~ __ _4~c
l Z l ' Z +l l '
2o i.e. the periods I and Il2 and with magnitudes proportional to A~(z)A2(z).
Presently
there are not too many quadratic EO materials with strong enough Kerr effect.
One
of the actively explored materials is lead modified lead zirconate titanate
(PLZT),
with EO coefficient about 10-~~ m2/V2 however it has high intrinsic refractive
index
(about 2.3 - 2.4) that requires Bragg grating periodicity 1.55 pm/(2x2.3)~
0.32 -
2s 0.34 pm. The good candidate for this application might be isotropic polymer
dispersed liquid crystals (PDLC) with intrinsic refractive index close to 1.6
and very
high EO coefficient 2 10-x' m2/Vz for 1.5 pm wavelength.
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3.1. Single rr-shift
The solid curves in Fig.15a and 15b show us reflection spectra of the EO-
induced Bragg gratings with a single ~rr-shift in the middle for different
values of KL
s -product (KL = 2 for Fig.15a and reL = 6 for Fig.15b), whereas the dashed
curves
represent spectra without ~r-shift. As we can see, the rr-shift opens very
narrow
transmission gap with a Lorentzian line shape. This gap can be switched ON and
OFF in our design or the spectrum itself can be reshaped by changing coupling
coefficient K(Vp), or through grating length variation by enabling or
disabling the
io IDE fingers.
Moving the position of the rr-shift from the center to the left or right side
creates similar effect of gradual change in transmission within the gap from
100%
for d = 0 to 0% for d=0.25, as it can be seen in Fig.16.
is
3.2. Multiple rr-shifts
In the same way as in the case of LPG, multiple rr-shifts can be a powerful
technique to control of the reflection spectrum. An example is presented in
Fig.17,
2o where the spectra are presented for one, two, three, four and five
symmetrical rr-
shifts similar to the structure for LPG in Fig.9. The spectra were calculated
for the
structures where length of the two outer sub-gratings Lour are half of the
length of
inner sub-gratings L, N for the structures with two or more rr-shifts. We can
see
how strongly band-pass can be controlled by multiple inversion of the voltages
on
2s the IDE fingers. The ripple factor which is increased with a number of ~rr-
shifts, can
be substantially reduced by controlling the L,NILc~T ratio.
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3.3. Fabry-Perot filter
For the Bragg type EO induced grating grounding some fingers inside the
structure creates a Fabry-Perot (FP) type cavity with its specific type of
spectrum.
s By changing a number of the grounded IDE fingers, we can control the grating
length L = Ll2 +U2 and the distance between sub-gratings that in turn allows
us
easy alteration of the stop-band and the free space range (FSR). There are two
options here: 1) the total number of enable IDE fingers in each sub-grating,
N/2, is
kept constant, i.e. disabling (enabling) a certain amount of IDE fingers in
the sub-
io gratings from their inner cavity ends we simultaneously enable (disable)
the same
amount of IDE fingers in the sub-gratings from their outer ends; and 2) the
total
number of enabled IDE fingers in each sub-grating, (N-M)/2, increases
(decreases) as we enable (disable) the IDE fingers of the sub-gratings from
their
inner (cavity) ends. Fig.18 demonstrates the reflection spectra for the first
option
is (Fig.18a) and the second one (Fig.18b) for the number of disabled
(grounded) IDE
fingers M = 500 (solid), M=600 (dot), M=700 (dash) and M=800 (dot-dash) for N
=2000 and ic=5000 m -~.
In Fig.19 we can see the reflection spectra for the first option when M =500
(Fig.19a solid), M=1500 (Fig.19a, dash), M=2500 (Fig.19b, solid), M=3500
(Fig.19b, dash), where the number of band-pass gaps is growing from two for
M=500 to eight for M=3500. These two examples are calculated for identical sub-
grating, however one should understand that the design can be used to induce
dissimilar sub-grating it terms of their length or coupling coefficients.
2s
4. Alternative electro-optic materials.
So far we discussed electro-optic materials (isotropic or anisotropic) which
are uniform in their intrinsic state. Periodical distribution of the
refractive index can
3o be induced through external spatially periodical stimulus (such as external
periodical electric field). However there is a class of artificially
synthesized
CA 02488510 2004-12-03
WO 03/102675 PCT/CA03/00840
16
materials, holographic polymer dispersed liquid crystals (H-PDLC), where the
material already possesses spatial periodicity of its refractive index. The N-
PDLC
material comprises a transparent polymer material populated by periodical
distribution of liquid crystal micro-droplets. Such droplet distribution forms
s holographic fringes, or, in the case of a waveguide, it can be short or long
period
gratings. Typically, the H-PDLC has two optical states corresponding to the
electrical stimulus being ON or OFF, these being equivalent respectively to
the
grating being disabled or activated. In its normal or rest state the liquid
crystal
droplets tend to be randomly aligned. When the external electric field is
applied the
io droplets tend to re-orient such that that liquid crystal molecules become
aligned
with the direction of the applied electric field. This property is widely
known in the
art and is used to switch ON and OFF the hologram or waveguide grating(s).
We propose to use a structured electrode to selectively disable a fringe or a
number of fringes within H-PDLC waveguide grating by applying an electric
Is potential to a finger pair or a group of finger pairs keeping the rest of
electrode
grounded. This allows us to dynamically split the grating into arbitrary
amount of
subgratings (or Fabry-Perot resonators) with the same transmission spectrum
manipulation freedom over the transmission spectrum as was described above,
and shown in Fig. 20.
2o Of course, numerous modifications could be made to the embodiments
described above without departing from the scope of the present invention.
