Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2000-12-18
2003-05-27
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S005000, C385S023000, C385S039000, C359S244000
Reexamination Certificate
active
06571028
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to optical communications. More particularly, the present invention is directed to an optical switch for an optical network.
BACKGROUND INFORMATION
The enormous increase in data traffic, largely due to the growth in Internet traffic, has spurred rapid growth in broadband communication technologies. Fiber optics, which offers the largest bandwidth of any communication system, is the medium of choice for carrying the multitude of data now being sent through networks. While fiber can theoretically carry over 50 terabits per second, current optical communication systems are limited to 10 gigabits per second due to the limitations of the switching nodes.
Switching nodes consist of systems dedicated to switching the optical signals between lines as well as providing other signal processing functions, such as amplification and signal regeneration. Switching nodes include components such as optical switches, add/drop multiplexers, channel converters, routers, etc.
Prior art “optical” switches used in switching nodes are typically not entirely optical and therefore operate relatively slowly and have limited bandwidth. One type of known prior art switch is an opto-mechanical switch. Opto-mechanical switches use moving (e.g., rotating or alternating) mirrors, prisms, holographic gratings, or other devices to deflect light beams. The mechanical action may involve motors, or piezoelectric elements may be used for fast mechanical action. For example, Lucent Corporation and other companies have introduced a type of opto-mechanical switch referred to as a micro-electro-mechanical switch (“MEMS”). MEMS consist of arrays of actuated micro-mirrors etched onto a silicon chip in a similar manner to that of electrical integrated circuits. The mirrors change angle based upon an electrical signal and route an incident optical signal to one of many output fibers.
Another example of an opto-mechanical switch is a device from Agilent Technologies that steers optical signals through the controlled formation of gas bubbles within a liquid waveguide. A bubble is formed at the junction of one input and several output waveguides. The bubble will reflect an optical signal down one output while a lack of bubble will allow the signal to propagate through another waveguide.
A major limitation of opto-mechanical switches is low switching speeds. Typical switching times are in the millisecond range. The advantages of opto-mechanical switches are low insertion loss and low cross-talk.
Other prior art devices use electro-optic materials which alter their refractive indices in the presence of an electric field. They may be used as electrically controlled phase modulators or phase retarders. When placed in one arm of an interferometer, such as a Mach-Zender interferometer, or between two crossed polarizers, the electro-optic cell serves as an electrically controlled light modulator or a 1×1 (on-off) switch. The most prevalent technology for electro-optic switching is integrated optics since it is difficult to make large arrays of switches using bulk crystals. Integrated-optic waveguides are fabricated using electro-optic dielectric substrates, such as Lithium Niobate (“LiNBO
3
”), with strips of slightly higher refractive index at the locations of the waveguides, created by diffusing titanium into the substrate. The major drawbacks of Lithium Niobate technology is the high expense of the material and difficulty in creating low loss waveguides within it.
Liquid crystals provide another technology that can be used to make electrically controlled optical switches. A large array of electrodes placed on a single liquid-crystal panel serves as a spatial light modulator or a set of 1×1 switches. The main limitation is the relatively low switching speed.
Other prior art optical devices include acousto-optic switches which use the property of Bragg deflection of light by sound. An acoustic wave propagating along a dielectric surface alternatively puts the material in compression and tension. Thus, the acoustic pressure wave periodically alters the refractive index. The change in the refractive index is determined by the power of the acoustic wave, while the period of the refractive index change is a function of the frequency of the acoustic wave. Light coupled with the periodically alternating refractive index is deflected. A switching device can be constructed where the acoustic wave controls whether or not the light beam is deflected into an output waveguide.
Some prior art optical devices use magneto-optic materials that alter their optical properties under the influence of a magnetic field. Materials exhibiting the Faraday effect, for example, act as polarization rotators in the presence of a magnetic flux density B. The rotary power &rgr; (angle per unit length) is proportional to the component B in the direction of propagation. When the material is placed between two crossed polarizers, the optical power transmission T=sin
2
&THgr; is dependent on the polarization rotation angle &THgr;=&rgr;d where d is the thickness of the cell. The device is used as a 1×1 switch controlled by the magnetic field.
Finally, prior art optical devices do exist that can be considered “all-optical” or “optic-optic” switches. In an all-optical switch, light controls light with the help of a non-linear optical material. They operate using non-linear optical properties of certain materials when exposed to high intensity light beams (i.e., a slight change in index under high intensities).
FIG. 1
illustrates a prior art all-optical switch
20
that uses an interferometer. Switch
20
includes material
14
that exhibits the optical Kerr effect (the variation of the refractive index with the applied light intensity) which is placed in one leg of a Mach-Zender interferometer. An input signal
10
is controlled by a control light
16
. As control light
16
is turned on and off, transmittance switch
20
at output
12
is switched between “1” and “0” because the optical phase modulation in Kerr medium
14
is converted into intensity modulation.
FIG. 2
illustrates a prior art all-optical switch
30
that uses an optical loop. Switch
30
is a non-linear optical loop mirror (“NOLM”) that includes a fused fiber coupler (splitter)
34
with two of its arms connected to an unbroken loop of fiber
32
. A signal arriving at the input
36
to coupler
34
is split and sent both ways around fiber loop
32
. One of the lengths of loop
32
contains a Kerr medium. The Kerr medium is pumped via another high intensity control beam that alters the refractive index of the material and thus slightly changes the speed at which the signal beam propagates through. When the two signal beams recombine at the other end of loop
32
interference effects determine the amplitude of the output
38
. Although a NOLM operates at high speeds (tens of picoseconds), it requires long lengths of fibers and is not readily integratable.
The retardation between two polarizations in an anisotropic non-linear medium has also been used for switching by placing the material between two crossed polarizers.
FIG. 3
illustrates a prior art all-optical switch
30
using an anisotropic optical fiber
42
that exhibits the optical Kerr effect. In the presence of a control light
43
, fiber
42
introduces a phase retardation &pgr;, so that the polarization of the linearly polarized input light
45
rotates 90° and is transmitted at output
46
by an output polarizer
48
. In the presence of control light
43
, fiber
42
introduces no retardation and polarizer
48
blocks input light
45
. A filter
44
is used to transmit input light
45
and block control light
43
, which has a different wavelength.
FIG. 4
illustrates another prior art all-optical device
50
that uses liquid-crystal. Device
50
includes an array of switches as part of an optically addressed liquid-crystal spatial light modulator
52
. A control light
54
alters the electric field applied to the liquid-crystal layer and therefore
Ballinger Clinton T.
Landry Daniel P.
LoCascio Michael
Raynolds James E.
Evident Technologies
Kenyon & Kenyon
Rojas Omar
Sanghavi Hemang
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