Optical waveguides – Polarization without modulation
Reexamination Certificate
2002-01-17
2004-11-02
Font, Frank G. (Department: 2877)
Optical waveguides
Polarization without modulation
Reexamination Certificate
active
06813398
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a tunable electrooptic add-drop filter apparatus and method. In particular, the invention relates to a tunable electrooptic add-drop filter apparatus and method in filters fabricated on a birefringent electrooptic substrate whereby a narrow range of optical frequencies are added to an optical fiber and a narrow range of optical frequencies are dropped from a fiber while leaving other frequencies unaffected.
BACKGROUND OF THE INVENTION
Wavelength division multiplexing is widely used in fiber optic communication to increase the data capacity of an optical fiber. Currently, 16, 32, or more data channels are transmitted in parallel on a single mode fiber using different optical carrier frequencies for each channel. To combine and separate these channels, a variety of frequency-selective components have been developed, including multilayer dielectric coatings, fiber Bragg gratings, arrayed waveguide gratings, and Mach-Zehnder chains. None of these techniques satisfies industry requirements for high-speed tunability and wide frequency tuning range.
An example of a prior-art electrooptic tunable filter (EOTF) for performing the add-drop function is illustrated in FIG.
1
. Such a filter has been demonstrated previously by applicants at Texas A&M University in the substrate material lithium tantalate (LiTaO
3
) [1].
The substrate for the prior art tunable filter (
FIG. 1
) is a single-crystal of a birefringent electrooptic material such as lithium niobate (LiNbO
3
) or lithium tantalate (LiTaO
3
). The waveguides are single mode for both TE and TM polarizations. Between the two directional coupler polarizing beam splitters (PBS's) is a spatially periodic dielectric film to produce polarization conversion, and electrodes for applying a tuning voltage.
Wavelength selectivity is determined by the phase-matching condition which governs coupling between the polarization states induced by the periodic film, as described by
Δ
=
2
π
v
(
n
1
-
n
3
)
c
±
2
π
Λ
,
(
1
)
where &Dgr; is the phase mismatch constant, &ngr; is the optical frequency, n
1
and n
3
are the principal refractive indices of the birefringent substrate material, and &Lgr; is the spatial period of the film. The frequency &ngr;
j
for which maximum polarization conversion occurs corresponds to the phase-matching condition &Dgr;=0. For frequencies far from &ngr;
j
, &Dgr; is large and very little polarization conversion will take place.
Tuning is accomplished in this prior art device by applying a voltage to electrodes on the surface of the substrate. The resulting electric field in the waveguide region causes a change in the birefringence (n
1
−n
3
) via the linear electrooptic effect (Pockels effect). It follows from eq. 1 that a change in the birefringence causes a change in the frequency for which phase-matched polarization coupling occurs.
Waveguides can be fabricated by a process which involves (1) deposition of a thin (90 nm) layer of titanium on the surface of the substrate, (2) patterning the titanium by a process of photolithography and etching, and (3) diffusing the titanium into the substrate at 1050° C. To produce the electrode pattern, a uniform aluminum film is deposited on the substrate, patterned photolithographically, and etched. Finally, in the case of the tunable filter, a silicon dioxide film is deposited at 350° C., masked with photoresist at room temperature, and etched to produce a periodic bar pattern in the film. Strain resulting from the mismatch in thermal expansion coefficients between film and substrate causes polarization conversion in the waveguides.
Another prior art design which is the subject of the co-pending patent application Ser. No. 09/737,206 is illustrated in FIG.
2
. This design is promotes ease of manufacturing for the EOTF by eliminating the need for PBS's, which are difficult to produce with the required tolerances. The design of
FIG. 2
differs from that of
FIG. 1
at least in that the strain-inducing strips are offset by &Lgr;/2 in the top waveguide relative to the bottom one, and the optical path difference in the top waveguide differs from that in the bottom one by &lgr;/2.
An expanded view of a section of waveguide containing the strain pads and electrodes for electrooptic tuning is shown in FIG.
3
. The diagram in
FIG. 3
could represent either the upper or lower waveguide in the EOTH of either
FIG. 1
or FIG.
2
. The performance of the tunable prior art filters of
FIG. 1
or
FIG. 2
is determined by the dependence of polarization conversion on optical frequency in this waveguide section.
From eq. (1) it is determined that the spatial period &Lgr; of the strain pads in the EOTF is determined by the center frequency &ngr;* of the spectral region in which the filter is intended to operate and the refractive indices n
1
and n
3
of the birefringent substrate. The optical wavelength region of most interest for optical fiber communication is 1530-1560 nm. The frequency of the center of this wavelength regime is c/&lgr;*, with c the free-space speed of light and &lgr;*=1545 nm, is &ngr;*=2.998×10
8
/1545×10
−9
=1.929×10
14
Hz. For a lithium niobate substrate, with n
1
=2.2118 and n
3
=2.1384, it is calculated from eq. (1) that &Lgr;=21.05 &mgr;m.
A theoretical plot of the efficiency for polarization conversion X on optical frequency, measured relative to the frequency for maximum polarization conversion, is given in FIG.
4
. The plot assumes that the substrate material is lithium niobate (LiNbO
3
), a total length for the polarization conversion region of 3.6 cm, and a uniform coupling constant induced by the strain pads of 0.139 cm
−1
. The same conditions apply to the plot of
FIG. 5
, in which the frequency scale is expanded.
The length of the polarization conversion region was chosen to give the first nulls in the conversion spectrum at ±100 GHz relative to the central peak where the conversion efficiency is a maximum. This would correspond to application in a wavelength-division-multiplexed (WDM) communication system in which the channel spacing corresponds to the International Telecommunication Union (ITU) specification of 100 GHz channel spacing. Thus, when a particular channel is selected, the adjacent channel would correspond to a null, to reduce crosstalk between channels.
The present invention addresses several deficiencies with the prior-art EOTF designs described by the diagrams in
FIGS. 1-3
and the calculated response curves of
FIGS. 4 and 5
. Among these deficiencies are:
(1) The required tuning voltage is too high. To tune the center frequency of one of these prior art filters by 100 GHz is estimated to require about 7 V with an electrode spacing of 10 &mgr;m. Thus, tuning over 32 channels would require a voltage swing of 32×7=224 V, and to tune over 64 channels. requires a voltage swing of 448 V. Such large voltages applied over such a short distance causes degradation or even destruction of the EOTF.
(2) The nulls in the conversion spectra of
FIGS. 4 and 5
are not equally spaced at the desired 100 GHz separation. This adversely affects the need to minimize crosstalk in a WDM system.
(3) The time delay experienced by the light in traversing the EOTF is different for the TE and TM polarizations due to the birefringence of the substrate. For example, for a substrate length L
sub
of 7 cm, with n
1
−n
3
=0.0734, the delay difference is L
sub
(n
1
−n
3
)/c, with c the free-space speed of light, which is calculated to be 7×0.0734/2.998×10
10
=17 ps. Such a delay leads to degradation of the “eye diagram” in a high-data-rate system, particularly at data rates of 10 Gb/s or higher.
(4) The length of the polarization conversion region required to achieve a 50 GHz channel spacing for WDM is not compatible with the size of available electrooptic substrate
Eknoyan Ohannes
Taylor Henry F.
Font Frank G.
Mooney Michael P.
Shaffer Jr. J. Nevin
Texas A & M University System
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