Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2002-02-12
2004-08-24
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S122000, C524S495000
Reexamination Certificate
active
06782154
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to switching optical path lines in an optical communication system and, more particularly, to an ultrafast, high-sensitivity, waveguide, all-optical switch, made from single-walled carbon nanotube (SWNT) polymer composites, having a switching speed of less than 1 picosecond (ps) for light with a wavelength of about 1.55 micrometers (&mgr;m).
BACKGROUND OF THE INVENTION
The continued development of optical communications requires fast information processing. Therefore, ultrafast, all-optical systems for basic processing at both ends of an optical transmission line are replacing electronic systems. The advantages of all-optical systems include the avoidance of repeated conversions between electrical and optical signals and the faster speed of optical devices over their electronic counterparts.
The reliability and economy of an optical communication network using optical fibers cannot be fully realized by simply connecting two distant components through the optical fibers. Therefore, to further enhance reliability and economy, attempts have been made to improve the availability of the optical fibers by providing an optical switch or switches in the optical fibers. Optical switches function to switch optical information to a standby line, detour around obstacles, or switch optical information to an unused line.
Mechanical optical switches are one type of optical switch used in optical communication systems. Mechanical optical switches function to switch the optical paths by mechanically moving optical components such as the optical fibers. This type of optical switch has a number of drawbacks. The switching speed is low, on the order of millisecond (ms), and the number of switches possible is limited by wear of the parts caused by the mechanical switching operations.
A semiconductor waveguide optical switch is another type of optical switch used in optical communication systems. Semiconductor waveguide optical switches have a switching speed on the order of nanoseconds (ns) and are free from wear. An optical switch having an X-junction optical waveguide shown in
FIG. 1
is known in the prior art and is described in U.S. Pat. No. 5,148,505 issued to Yanagawa et al.
As shown in
FIG. 1
, thin semiconductor layers of a predetermined composition are sequentially laminated as a lower clad layer, a core layer, and an upper clad layer on a semiconductor substrate
51
to form optical waveguides
52
and
53
in a ridge configuration. Optical waveguides
52
and
53
intersect each other in the shape of the letter “X,” with a branch angle of &thgr; degrees, to form a junction point or branch point
54
. The entire surface of the structure is covered with a thin insulation film. An electrode
55
is used to inject a current of a predetermined value to the optical waveguides which intersect at the branch point
54
.
Portions
52
a
and
53
a
of the optical waveguides
52
and
53
which lie on one side of the optical waveguides with respect to the branch point
54
constitute input ports, respectively, and the portions
52
b
and
53
b
on the opposite side of the branch point
54
constitute output ports, respectively. With the optical switch of the construction shown in
FIG. 1
, when a predetermined amount of current is injected via the electrode
55
, the refractive index of that portion of the core layer into which the current is injected is lowered by the action of the injected carriers. As a result, light waves incident on the input port
53
a
are subjected to total reflection at the interface between the current injection area and the non-injection area and then transmitted from the output port
52
b
to the exterior. On the other hand, when no current is injected via the electrode
55
, light waves incident on the input port
53
a
pass straight through the branch point
54
and are transmitted from the output port
53
b
to the exterior.
Thus, the light waves incident on the input port
53
a
are transmitted from the output port
52
b
or
53
b
depending on whether a current is injected via the electrode
55
or not. In this way, the optical switch of
FIG. 1
performs the switching operation.
A branching interference-type modulator shown in
FIG. 2
is known as another example of an optical switch. The modulator combines Y-junction optical waveguides of the type shown in FIG.
3
. As shown in
FIG. 3
, each of the Y-junction optical waveguides is constructed by sequentially laminating thin semiconductor layers of a predetermined composition as a lower clad layer, a core layer, and an upper clad layer on a semiconductor substrate
61
to form an optical waveguide
62
. The optical waveguide
62
includes a main optical waveguide
62
a
as an input port for light waves and two output optical waveguides
62
b
and
62
c
branching from the main optical waveguide
62
at a predetermined branch angle (&thgr;).
Assume that the cross sections of the main input optical waveguide
62
a
and the output optical waveguides
62
b
and
62
c
are the same. Then, the light waves incident on the main input optical waveguide
62
a
are transmitted outwardly from the output optical waveguides
62
b
and
62
c
as light waves of the equal light outputs. More specifically, the light waves of the light output “2” incident on the main input optical waveguide
62
a
are equally divided and then transmitted out from the output optical waveguides
62
b
and
62
c
as light waves of light output “
1
.”
As shown in
FIG. 2
, the output optical waveguides
62
b
and
62
c
of one Y-junction optical waveguide are respectively connected to the input optical waveguides
62
b′
and
62
c′
of the other Y-junction optical waveguide. Electrodes
63
a
and
63
b
are respectively formed on the connecting portions of the waveguides. A predetermined voltage can be applied to the electrodes
63
a
and
63
b
. With the modulator, light waves incident on the main input optical waveguide
62
a
are equally divided by the output optical waveguides
62
b
and
62
c
. In this case, for example, because the guided light propagating from the output optical waveguide
62
c
to the optical waveguide
62
c′
is subjected to the phase shift according to the voltage applied via the electrode
63
a
, the guided light is combined or interfered with the guided light propagating from the output optical waveguide
62
b
to the optical waveguide
62
b′
. As a result, the light output of the light wave transmitted from the main optical waveguide
62
a′
varies according to the phase difference between the guided light propagating through the optical waveguide path
62
c
-
62
c′
and the guided light propagating through the optical waveguide path
62
b
-
62
b′.
In the case of the branching interference type-modulator, the mode interference of the light waves propagating through the optical paths is used. For this reason, the light output of the light waves to be transmitted depends on the polarization and wavelength of the light waves to be propagated. Accordingly, this type of modulator can be properly operated only for the guided light of a specified polarization and a specified wavelength.
Ultrafast, all-optical switches are crucial for future, high-bit-rate, time-division-multiplexing optical communication systems or free-space, optical-digital, computing systems. So far, many types of ultrafast, all-optical switches have been studied and demonstrated using optical nonlinearities in optical fibers and semiconductor materials. Such devices must satisfy several requirements. They must be compact and, in this respect, semiconductor devices are preferable to optical fiber devices. In addition, they should be independent of polarization and, in this respect, normal-incident devices are preferable to waveguide devices. In the normal-incident devices, however, large optical nonlinearities are needed because the interaction length is small.
The nonlinear optical properties of active semiconductor waveguides have proven to be of great inter
Ajayan Pulickel M.
Chen Yuchuan
Lu Toh-Ming
Raravikar Nachiket R.
Schadler Feist Linda S.
Lee John D.
Pak Sung
RatnerPrestia
Rensselaer Polytechnic Institute
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