Note: Descriptions are shown in the official language in which they were submitted.
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HIGH-PERFORMANCE ELECTRO-OPTIC INTENSITY MODULATOR USING
POLYMERIC IiAVE6UIDES AND 6RATIN6 NODULATI~1
The present invention provides a high power and high extinction-ratio
electro-optic intensity modulator utilizing grating coupling and poling
effects of nonlinear polymers. It provides an optical signal modulator
for optical communication systems, optical interconnects, and large
scale fiber-optic network systems.
To date, communication and computer systems have played a dominant role
in many fields. At the same time, among various microstructure
optoelectronic technologies, integrated optics represents a promising
approach in these advanced information processing areas. In these
systems, Si-based logic an memory integrated circuitry continues its
evolution toward higher speed and enhanced functionality, with a
resulting decrease in feature size and increase in process complexity.
Future electronic systems will require on-chip signal conversion
between electrical, optical and microwave media to reach the speed and
functionality projections. Thus a radically different alternative
concept exploits the use of photons, instead of electrons, to carry
i nformat i on i n what i s corrrmnon ly referred to as "opt i ca 1 i
nterconnects . "
One implementation of this strategy relies on the integration of
crystal-, semiconductor- or polymer-based optoelectronic interconnects
on a host Si substrate, and thus requires feasible crystal-,
semiconductor- or polymer-based optoelectronic technologies in order to
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produce Si-based photonic modulators for optical waveguide
interconnects.
Although the technologies for some electro-optic (EO) waveguide devices
based on inorganic materials such as crystals and semiconductors have
had a long developing history, the conditions for manufacturing and
processing integrated optical devices are also seriously limited.
While polymers, a new kind or organic nonlinear EO materials, not only
have high EO nonlinearity, but also high thermo-optic (TO) effect, and
have shown a promising future. Polymers generally have potentially
both large EO and TO coefficients, low dielectric constants, improved
thermal and temporal stability, and easy fabrication conditions. The
above physical properties of polymers are very useful in constructing
waveguide-type optical functional devices such as modulators and
switches. A variety of polymer-based modulators aimed at providing
feasible structures with high-extinction ratios have been reported.
The technologies associated with packaging and interfacing with other
devices are also taken as important considerations. These EO
modulators focus mainly on two types: phase modulators and intensity-
modulators.
For many applications, the required distance and bandwidth are within
the operating parameters of both single-mode and multi-mode optical
systems. High extinction-ration modulators are always needed in both
single- and multi-mode fiber-optic communication systems. Therefore,
the structures that can be suitable for both single-mode and multi-mode
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waveguide modulators will have wide applications in industry. As
various fiber-optic communication systems are developed and applied in
the real world, the interesting needs of high capacity in these systems
require both more information channels in a single fiber and higher bit
rate in each channel.
In telecommunications networks, the time-division-multiplexing (TDM)
systems have been successfully used according to the SONET-standards.
Among the high transmission rates for TDM systems, 2.5 Gbits/s is
relatively popular according to OC-48-standard and the new transmission
systems having up to 10 Gbits/s are widely applied according to OC-192-
standard. The wavelength-division-multiplexing (WDM) lightwave system
is the optical communication in the wavelength multiplex mode. Use of
this novel approach WDM has the potential of improving the performance
of the fourth generation lightwave systems by a factor of more than
1000. Recently, research on the devices and techniques for high
capacity WDM systems or dense wavelength division multiplexing (DWDM)
systems having effective network restoration capability, i.e.,
reconfigurable WDM systems, has received much more attention. In
future, the hybrid fiber-optic cormnunication systems including both
ultra-high bit-rate TDM and high capacity WDM (or DWDM) systems, the
routing of optical signals will be performed in optical cross-connects
(OXCs). The ultra-high speed operations of the TDM systems will open
a huge market for a variety of high performance EO waveguide
modulators. The functions and applicability of the WDM systems will be
extended by the reconfigurable structures. Therefore, the single high-
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performance EO waveguide switching devices and the programmable OXCs
using the EO switching cells will have wide applications in fiber-optic
con~nun i cat i on .
In accordance with theoretical study, the coupling efficiency between
two single-mode waveguides can be achieved a high value only at the
critical coupling length, while the total coupling efficiency between
two mufti-mode waveguides cannot achieve a high value at all.
A paper titled "Polymeric optical intensity modulator optimized in
quasi-single mode operation" by W. Hwang et al., published in Appl.
Phys. Lett. 69 (11) (1996), pp. 1520-1522, discusses an EO polymer
waveguide intensity modulator and is incorporated herein by reference.
The present invention provides a high-power and high extinction-ratio
EO intensity modulator based on polymeric waveguides with
unidirectional single and mufti-mode coupling and a modulation
mechanism.
The structures of the EO waveguide intensity modulators according to
the present invention are simple and based on both single-mode and
mufti-mode waveguides.
An EO waveguide device according to the present invention comprises two
waveguide channels. One channel is used for guiding an optical signal,
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called the guiding channel; while the other channel is used for
coupling an optical beam out, called the coupling-out channel. These
two waveguide channels are sandwiched between two cladding layers: an
upper cladding layer and a lower cladding layer. A modulated grating-
s coupler is formed along the outside edge of the coupling-out channel.
This modulated grating-coupler is induced by a grating modulation
effect when a modulating electric field is applied onto the electrodes
having a grating pattern. In fact, the coupling-out channel can couple
the optical beam out only when the modulated grating-coupler is formed
by the grating modulation effect. After an optical signal is input
into the guiding channel from the input end, an optical signal will be
received at the output end if the modulated grating-coupler is not
formed. When the modulated grating-coupler is formed by the grating
modulation effect, the optical beam can be completely coupled out and
no optical signal will be received at the output end of the guiding
channel. Thus a switching effect can be implemented by choosing
alternative states (i.e., the unmodulated state and the modulated
state) and a much higher switching contrast (i.e., the extinction
ratio) can be achieved with the structures based on the present
invention.
In a preferred embodiment according to the present invention, the
guiding channel should be longer than the coupling-out channel. In
such case, the input, the detection and the modulation of optical
signals with these optical waveguide devices can be easily and
efficiently performed.
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BRIEF DESCRIPTION OF THE DRAWINGS
Example embodiments of the invention are described with reference to
the accompanying drawings, in which:
Figure 1 is a perspective view of the structure of the intensity
modulator according to the present invention;
Figure 2(a) is a top view of the modulator shown in Figure 1; and
Figure 2(b) is a vertical cross-section taken along the axis A-A in
Figure 2(a).
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawing figures, the EO waveguide intensity modulator
comprises a substrate 20, a waveguide channel 22 for guiding a light
signal optical beam 34, a waveguide channel 24 for coupling the optical
beam out a lower cladding layer 26, an upper EO cladding layer 28, a
lower modulating electrode 30 and an upper modulating electrode 32. (It
should be noted that the grating inducing electrode 32 is shown here
schematically. As is known to those skilled in the art, the gratings
are a few tens to a few thousand in number within a length of appr. 10-
mm.) The waveguide channel 22 is called the guiding channel and the
waveguide channel 24 is called the coupling-out channel. The input
20 optical beam 34 is coupled into the guiding channel 22, and the output
optical beam 36 can then be controlled by the modulation effect of this
device. This modulation effect is induced when the electric field
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forms an index-modulated grating-coupler in the channel 24 with a
grating pattern of the upper electrode 32 cause the channel 24 to have
a coupling-out effect on the optical beam 34. Thus, in the coupler
based on these two waveguides, only the coupling from the guiding
channel 22 to the coupling-out channel 24 is allowed, while coupling
from the coupling-out channel 24 to the guiding channel 22 is
eliminated, i.e. a unidirectional coupling process is achieved. The
unidirectional coupling process with a modulated grating-coupler can
effectively improve the optical energy transfer efficiency to be
approximately 100fo, thus a high switching extinction ratio can be
implemented with an appropriate modulation voltage. Therefore, if no
modulating field is applied onto the device, the unidirectional
coupling process cannot be formed, so an optical signal can be received
at the output end of the channel 22. While if an appropriate
modulating field is applied to the device, the unidirectional coupling
process can be formed and the entire optical beam is coupled from the
channel 22 to the channel 24 and no optical signal is received at the
output end. Thus, a high switching contrast is achieved.
In a waveguide coupler, unidirectional coupling is used to achieve a
higher extinction ratio. To achieve this goa, the primary concern is
to electro-optically tune the coupling from the guiding channel 22 to
the coupling-out channel 24. First we only consider the coupling
between one mode (j, m) of the guiding channel 22 and one mode (j',m')
of the coupling-out channel 24. If no modulation effect is applied
onto the device, the modulated grating-coupler does not exist, so both
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the coupling process from channel 22 to channel 24 and the coupling
process from channel 24 to channel 22 exist and the coupling efficiency
can be expressed as
~l~m~,j~~ ~kJ,m~,jm $II1Z(~jm.l~m~L) 1
Y' jm, j'm'
While if an appropriate modulation effect is applied to the device, the
modulated grating-coupler is induced, and only the coupling from
channel 22 to channel 24 exists. The unidirectional coupling can be
expressed as
~4,m .lm _ ~ k 2m~,jm TJ m, $In? ~~jm,J~m~ (n -+- 1)~~~1 - ~k~~Zm~,Jm rj'm~
$In 2 (~jm, j'm~n~),
n~0 ~jm,j'm' 1'' jm,j'm'
where z is the loss coefficient of the modulated grating-coupler, which
is related to the index modulation one and the groove depth od of the
modulated grating-coupler (i.e., r~On~'Od), kj.m,,jm is the coupling
constant, L is the interaction length and 01, is a selected length
within which the coupled-out energy from the guiding channel 22 to the
coupling-out channel 24 is uniform, and N~ is defined by
NL = int(~) ( 3 )
The function int (~) is the integer closest to ~ . ~rjm,j.m~ is
defined by
z
~J",,J'",' '- (kJm j,m, ~ kl,m',jm ~ Qjm.j~m~) ~ , and
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Qjm,l'm' - kI N''" - N'~"'~~ l 2 (4b)
where k is Boltzmann constant, and N Jm.l~m~ and N ~m~~~m~ are the effective
refractive indices of the (j,m)th mode of the guiding channel 22 and
the (j',m')th mode of the coupling-out channel 24, respectively. For
the single-mode devices, the coupling efficiency and the unidirectional
coupling efficiency can be directly calculated by using Eqs. (1) and
(2), respectively. For the multi-mode devices, the total coupling
efficiency and the total unidirectional coupling efficiency can be
calculated by using the sum of normalized the mode-to-mode coupling
efficiencies defined by Eqs. (1) and (2), respectively. As an EO
modulator, the extinction ratio is completely determined by the
difference of the coupling efficiencies between the unmodulated state
(i.e., One =0) defined by Eq. (1) and the modulated state
(i.e., One = ~ r33 ~ nWE ) def i ned by Eq. ( 2 ) . Where r33 i s the
corresponding EO coefficient nW is the refractive index of waveguide
material and E is the electric field across modulating electrodes.
Note from Eq. (1) that the coupling efficiency can be zero in theory by
appropriately choosing the values of '~;m.i~m' and L. Note from Eq. (2)
that the unidirectional coupling efficiency can be up to 100% in theory
by appropriately choosing ~im.um~~ T and L, so the difference between
these two efficiencies can be achieved to be approximately 100% or a
much higher value in theory. Therefore, a much higher switching
contrast can be achieved with the EO waveguide intensity modulator.
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(For more detailed information about the theoretical study of the
unidirectional coupling between two multi-mode waveguides, see Applied
Physics Letters 72(24), 3139-3141 (1998) and SPIE PROC. Vol. 2994, 319-
329 (1997).)
5 With the appropriate selections of index modulation On~, grating depth
4d and interaction length L, we can obtain a maximum unidirectional
coupling efficiency ~' of the modulated state and a minimum coupling
efficiency ~' of the unmodulated state, we thus obtain a maximum
modulation depth (i.e., the switching contrast) ~m = ~° - ~' . So,
10 the switching contrast of the intensity modulator can be achieved to a
high value (>20dB).
The waveguide modulators based on the based on the present invention
are electro-optically modulated by applying the modulating voltage
between the upper electrode and the lower electrode and only the
waveguide material may be an EO polymer. As an EO modulator, the
poling process for the polymer material needs to be done for the
polymer to create the EO nonlinearity. The poling electrodes can be
the same as the modulating electrodes.
The EO waveguide modulators based on the present invention can also be
implemented on the EO crystals such as LiNb03 . In this situation, the
two modulating electrodes 30 and 32 need to be specially placed in
order to produce a modulated grating-coupler along the coupling-out
channel 24.
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The guiding channel 22 and the coupling-out channel 24 can be either
identical or nonidentical. They can be either single-mode or multi-
mode. The guiding channel 22 should be longer and the coupling-out
channel 24 shorter. This causes the input, the detection and the
modulation of the optical signals to be performed with ease.
The optical signal is coupled into the guiding channel 22 and is
coupled out from the coupling-out channel 24 when the modulating
voltage is applied to the coupling-out channel 24 through the
modulating electrodes.
The polymeric waveguide intensity modulators may be thermo-optically
modulated by applying the modulating voltage from two ends of the upper
grating electrode where the upper modulating electrode is taken as an
electrical heater. In this case, the lower electrode is unnecessary
and can be removed. In the thermo-optic waveguide intensity modulators
according to the present invention, the waveguide material can be a
polymer.
