Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-03-01
2001-05-15
Lee, John D. (Department: 2874)
Optical waveguides
Polarization without modulation
C385S027000, C385S039000, C385S040000
Reexamination Certificate
active
06233372
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a waveguide path type optical device used in an optical wavelength division multiplex transmission system, and more particular to a waveguide path type polarization independent optical wavelength tunable filter used in a wavelength tunable multiplexer-demultiplexer for multiplexing and demultiplexing any signal optical wavelength.
2. Description of the Related Art
There is proposed, as a conventional filter, an optical wavelength filter wherein a channel type optical waveguide path is provided on a substrate, a surface wave exciting electrode is provided on the substrate through a buffer layer in the vicinity of a middle portion of the input terminal and output terminal of the channel type optical waveguide path, and an element for separating two linearly polarized lights orthogonal to each other is provided in the vicinity of said middle portion and the output terminal (Japanese Patent Application Laid-Open No. 4-51114).
There is also proposed an optical wavelength filter wherein a channel type optical waveguide path is provided on a substrate exhibiting an acousto-optic effect, and a surface wave exciting electrode, a phase shifter electrode and a polarized light separation element are provided on the channel type optical waveguide path (Japanese Patent Application Laid-Open No. 4-159516).
Recently, however, as optical communication systems have been put to practical use, demand rises for a high-capacity, multifunctional, advanced communication system. Also, the addition of new functions of such as generating a higher speed optical signal, multiplexing wavelengths within a single light transmission path, and switching over and changing light transmission paths are required.
In these circumstances, as optical fiber amplifiers have been, in particular, rapidly put to practical use, an optical wavelength division multiplex transmission (WDM) system is being developed actively.
The WDM transmission cannot be realized without a wavelength tunable multiplexer-demultiplexer for multiplexing and demultiplexing any signal optical wavelength. A wide range of wavelength tunable widths, high speed operation and the like are required of an optical wavelength tunable filter used in the wavelength tunable multiplexer-demultiplexer.
As for optical wavelength tunable filters, TE (transverse-electric)-TM (transverse-magnetic) mode conversion type filters using the AO (acousto-optic) effect capable of easily tuning a selected wavelength by changing the frequency of a surface acoustic wave are being developed extensively.
FIG. 1
is a view for showing the structure of a conventional TE-TM mode conversion type waveguide path type polarization independent optical wavelength tunable filter.
The wavelength tunable filter shown therein is obtained by forming a titanium (Ti) diffusion optical waveguide paths
1
a
,
1
b
and anisotropic optical waveguide paths
2
a
,
2
b
,
2
c
and
2
d
are formed on the surface of a substrate
4
a
of an X-cut lithium niobate (LiNbO
3
).
Portions having slightly higher refraction indexes than the substrate
4
a
become the Ti diffusion optical waveguide paths
1
a
,
1
b
and the anisotropic optical waveguide paths
2
a
,
2
b
,
2
c
and
2
d
. The Ti diffusion optical waveguide paths
1
a
and
1
b
are formed by thermally diffusing Ti to the substrate
4
a
, whereas the anisotropic optical waveguide paths
2
a
,
2
b
,
2
c
and
2
d
are formed by thermally diffusing Ti to the substrate
4
a
and then conducting ion (proton) exchange processing.
A surface acoustic wave (SAW) excitation interdigital transducer
5
a
and surface acoustic wave absorbers
6
a
and
6
b
are provided right over the Ti diffusion optical waveguide paths
1
a
and
1
b.
The polarization beam splitter
11
a
consists of the Ti diffusion optical waveguide paths
1
a
,
1
b
and the anisotropic optical waveguide paths
2
a
,
2
b
. The polarization beam splitter
11
b
consists of the Ti diffusion optical waveguide paths
1
a
,
1
b
and the anisotropic optical waveguide paths
2
c
,
2
d
. The TE-TM mode converter
12
a
consists of the Ti diffusion optical waveguide paths
1
a
,
1
b
and the surface acoustic wave (SAW) exciting interdigital transducer
5
a.
Now, the operational principle of this wavelength tunable filter will be described.
First, description will be given to the operations of the TE-TM mode converter
12
a
of the polarization beam splitter, taking the polarization beam splitter
11
a
as an example.
FIG. 2
is a view for explaining the operation of the polarization beam splitter
11
a.
In
FIG. 2
, a TE polarization component
15
a
, which is an extraordinary ray, and a TM polarization component
15
b
, which is an ordinary ray, of a light
14
a
incident on the Ti diffusion optical waveguide path
1
a
are separately introduced to the anisotropic optical waveguide path
2
a
and the Ti diffusion optical waveguide path
1
a
, respectively at a polarized light separation basic structural part
10
a.
Further, the TE polarization component
15
a
is multiplexed to the Ti diffusion optical waveguide path
1
b
at a polarized light separation basic structural part
10
b
. Both of the polarization components
15
a
and
15
b
of the incident light
14
a
are, therefore, polarization-separation outputted to the Ti diffusion optical waveguide paths
1
b
and
1
a
, respectively.
Likewise, as regards a light
14
d
incident on the Ti diffusion optical waveguide path
1
b
, a TE polarization component
15
g
and a TM polarization component
15
h
of the incident light
14
d
are separation-polarization outputted to the Ti diffusion optical waveguide paths
1
a
and
1
b
, respectively.
It is noted that the TE mode refers to the component of the wave-guided light
14
a
which electric field is parallel to the substrate, whereas the TM mode refers to the component of the guided wave light
15
which electric field is perpendicular to the substrate.
Next, the operation of the TE-TM mode converter
12
a
will be described.
FIG. 3
is a view for explaining the operation of the TE-TM mode converter.
In
FIG. 3
, the surface acoustic wave excited by applying an RF signal
13
a
from an oscillation circuit to the interdigital transducer
5
a
acts as a periodic refractive index grating for the wave-guided light
15
a
and
15
b.
In this case, it is assumed that the guided wave
15
a
has only a TE polarization component and the guided wave
15
b
has only a TM polarization component.
By using the X cut lithium niobate substrate and setting the direction of optical wave transmission to Y direction, the Ti diffusion optical waveguide paths
1
a
and
1
b
differ in the effective refractive index of both the TE and TM modes.
If the following phase matching conditional equation (1), where the refractive index grating period formed by the surface acoustic wave is &Lgr; and the effective refractive indexes of the TE mode and TM mode are NTE and NTM, respectively, is satisfied, a wavelength &lgr; is subjected to TE-TM mode conversion by interaction with the refractive index grating.
&lgr;=|
NTE−NTM
|&Lgr; (1)
The refractive index grating period &Lgr; is inversely proportional to the frequency f of the RF signal
13
a
. Due to this, it is possible to change easily wavelength &lgr; to be subjected to TE-TM mode conversion by changing the frequency of the RF signal
13
a
. Thus, by appropriately setting the frequency of the RF signal
13
a
, any wavelength can be subjected to TE-TM mode conversion.
In case of the mode converter
12
a
, therefore, the TE polarization wave-guided light
15
a
, only if it is the wave-guided light of a wavelength satisfying the phase matching condition with respect to the refractive index grating period &Lgr;, is TE-TM mode converted to a TM polarization wave-guided light
15
c
on the Ti diffusion optical waveguide path
1
b.
On the other hand, on the Ti diffusion optical waveguide path
2
a
, the TM polarization guided wave
15
b
Lee John D.
McGuireWoods LLP
NEC Corporation
Stahl Michael J.
LandOfFree
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