Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-03-13
2003-04-08
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S245000, C385S008000
Reexamination Certificate
active
06545792
ABSTRACT:
(1) FIELD OF THE INVENTION
The present invention relates to switching devices. This invention more specifically relates to optical switching devices for routing optical signals between multiple inputs and multiple outputs.
(2) BACKGROUND OF THE INVENTION
The use of optical fibers, particularly as a telecommunication transmission medium, has numerous advantages over existing telecommunication media (e.g. copper cable). For example, optical fibers may sustain a broader bandwidth signal and may therefore convey substantially larger quantities of information over a given period of time. Further, optical fibers emit little or no electromagnetic or radio frequency radiation and therefore have negligible environmental impact. Conversely, optical fibers are relatively insensitive to electromagnetic and radio frequency interference from the surrounding environment. As a result, optical based communication promises to play a major role in the development of national and global information infrastructure, as applications such as super-computing, telecommunications, and military C
3
I rely on the ability to route data at increasingly high bit rates.
To be viable, the above-mentioned applications must include some means for controllably redirecting a signal, or at least a portion thereof Many telecommunications applications require the capability to switch a signal from one wire in an input array of M wires to an output signal in one wire of an array of N output wires. Telecommunications switching, transport and routing systems make widespread use of networks called multistage interconnection networks (MIN), to accomplish this function.
In current optical network development, building intelligent optical networks is becoming highly important. In these networks, all data may be easily and quickly transported through optimized paths according to network management commands. For example if one path is impeded or otherwise not working properly, an intelligent optical network may route optical signals via another path. A critical component in these intelligent optical networks is an all-optical switching device, also referred to as an optical router. The all-optical switching/routing device cross-interconnects different input and output fiber ports together, or even different wavelength channels to redirect the data through the network system according to management commands. Optimized routing paths may be selected for the best data transporting performance according to one or more requirements.
Optical switching devices currently under development and/or in production may generally be classified into one of three types. A first is micro-electro-mechanical-system (MEMS) based micro-mirror technology (see for example U.S. Pat. No. 6,097,859, in which advanced photolithographic technology is utilized to make micro-mirrors that may be moved by the application of a voltage. Input fibers are configured such that light signals are incident on the micro-mirrors. By controlling the voltages applied to the mirrors, the incident light may be redirected into different output fibers in order to realize a crossbar connection between input and output fibers. However, since the fundamental operation of a MEMS type of switching device is based on mechanical rotation or shifting, the switching speeds tend to be limited. Current switching speeds for MEMS type devices are typically about 10 milliseconds. Further, highly accurate alignment is required such that even a minor misalignment tends to degrade switching performance. Therefore, the stability and reliability of MEMS type devices may be limited in environments where mechanical vibrations are prevalent.
A second type of optical switching device is air-bubble based optical switching (see for example U.S. Pat. No. 4,988,157). A distribution of air-bubbles may be created in light paths to realize total internal reflection and to redirect the light signals to different switching elements and finally to different output fibers, The air-bubbles may be created or eliminated by an electric voltage signal (e.g. a pulse). However, since a bubble-type switching device generally utilizes a phase change in ink materials, the switching speeds tend to be limited. Also, bubble generation and elimination generally requires a complex temperature controlling system, resulting in an expensive switching device.
A third type of all-optical switching device is a polarization-based device (see for example U.S. Pat. No. 4,516,837 to Soref et al., U.S. Pat. No. 4,852,962 to Nicia, U.S. Pat. No. 5,276,747 to Pan, and U.S. patent application Ser. No. 09/342,422 to Faris et al.,). The U.S. Patents and Patent Application cited in this paragraph are fully incorporated herein by reference. These include liquid crystal (LC) based optical switching devices. This type of all-optical switch tends to have advantages over the other two types of optical switching technologies. First, polarization based switches typically have good stability since they have no moving parts used in the switching function. Moreover, a polarization-based switch may have a high switching speed, since it depends only on the switching speed of the active materials used in the switch. For example, a switch using a nematic LC material may have a switching speed on the order of one millisecond. A switch using a ferroelectric LC material may have a switching speed on the order of one microsecond (or faster). Improvement of the switching speed in polarization based optical switches tends to be limited only by the ability to develop new active materials. One further advantage is that a polarization-based type of switching device generally has a compact profile and fabrication costs that are relatively low. This type of switch is therefore, becoming more important in optical communication research and development.
However, a generally significant disadvantage to polarization based switching devices is that they typically require incident light to be polarized. If a conventional absorptive polarizer is used either in or prior to the switch, then at least 50% of the incident light (i.e. about −3 dB) is lost. This is not acceptable for most practical applications, especially since it is common for light to be routed through numerous switching devices. Since conventional optical fibers do not preserve the polarization state of light, a polarizer is required either in or prior to each switch, resulting in rapidly accumulating losses. While some advancements have been made in polarization preserving fiber optics (see for example U.S. Pat. No. 4,904,052), these fibers generally do not preserve the polarization state with the purity required by polarization based optical switches. Further, polarization preserving optical fiber is generally significantly more expensive than conventional optical fiber. One possible solution is to utilize an optical amplifier either before or after each switching device, although this generally results in a bulkier device and also significantly increases costs.
Therefore, there exists a need for a polarization independent all-optical switching device that is also a high-speed, compact, and high-capacity device having a constant signal pathlength for all I/O permutations.
SUMMARY OF THE INVENTION
In one embodiment, the present invention is a polarization independent optical interconnect device (i.e. an optical switch) for selectively interconnecting a plurality of optical signals between a plurality of inputs and a plurality of outputs. This embodiment includes a plurality of beam splitting elements, at least two phase shifting elements disposed on at least two adjacent sides of at least one of the polarizing beam splitting elements, and a plurality of mirrors. In one variation, this embodiment may be an M×N optical interconnect device wherein one of the plurality of polarizing beam splitting elements divides unpolarized light from one of the plurality of inputs into two mutually orthogonal components, with each of the orthogonal components being routed separately through the device. Another
Brill, Esq. Gefow D.
Crispino Ralph J.
Epps Georgia
O'Neill Gary
Reveo Inc.
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