Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2001-07-03
2003-09-23
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Switch
C359S225100, C310S308000, C307S143000
Reexamination Certificate
active
06625342
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of micro-electrical-mechanical systems (MEMS), and in particular, to improved MEMS devices and methods for their use with fiber-optic communications systems.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronics—typically an electronic SONET/SDH system. However SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems would require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called “wavelength routing networks” or “optical transport networks” (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC).
In order to perform wavelength routing functions optically today, the light stream must first be de-multiplexed or filtered into its many individual wavelengths, each on an individual optical fiber. Then each individual wavelength must be directed toward its target fiber using a large array of optical switches commonly called an optical cross-connect (OXC). Finally, all of the wavelengths must be re-multiplexed before continuing on through the destination fiber. This compound process is complex, very expensive, decreases system reliability and complicates system management. The OXC in particular is a technical challenge. A typical 40-80 channel DWDM system will require thousands of switches to fully cross-connect all the wavelengths. Conventional opto-mechanical switches providing acceptable optical specifications are too big, expensive and unreliable for widespread deployment.
In recent years, micro-electrical-mechanical systems (MEMS) have been considered for performing functions associated with the OXC. Such MEMS devices are desirable because they may be constructed with considerable versatility despite their very small size. In a variety of applications, MEMS component structures may be fabricated to move in such a fashion that there is a risk of stiction between that component structure and some other aspect of the system. One such example of a MEMS component structure is a micromirror, which is generally configured to reflect light from two positions. Such micromirrors find numerous applications, including as parts of optical switches, display devices, and signal modulators, among others.
In many applications, such as may be used in fiber-optics applications, such MEMS-based devices may include hundreds or even thousands of micromirrors arranged as an array. Within such an array, each of the micromirrors should be accurately aligned with both a target and a source. Such alignment is generally complex and typically involves fixing the location of the MEMS device relative to a number of sources and targets. If any of the micromirrors is not positioned correctly in the alignment process and/or the MEMS device is moved from the aligned position, the MEMS device will not function properly.
In part to reduce the complexity of alignment, some MEMS devices provide for individual movement of each of the micromirrors. An example is provided in
FIGS. 1A-1C
illustrating a particular MEMS micromirror structure that may take one of three positions. Each micromirror
116
is mounted on a base
112
that is connected by a pivot
108
to an underlying base layer
104
. Movement of an individual micromirror
116
is controlled by energizing actuators
124
a
and/or
124
b
disposed underneath base
112
on opposite sides of pivot
108
. Hard stops
120
a
and
120
b
are provided to limit movement of base
112
. Energizing left actuator
124
a
causes micromirror
116
to tilt on pivot
108
towards the left side until one edge of base
112
contacts left hard stop
120
a
, as shown in FIG.
1
A. In such a tilted position, a restoring force
150
, illustrated as a direction arrow, is created in opposition to forces created when left actuator
124
a
is energized.
Alternatively, right actuator
124
b
may be energized to cause the micromirror
116
to tilt in the opposite direction, as shown in FIG.
1
B. In such a tilted position, a restoring force
160
, illustrated as a direction arrow, is created in opposition to forces created when right actuator
124
b
is energized. When both actuators
124
are de-energized, as shown in
FIG. 1C
, restoring forces
150
,
160
cause micromirror
116
to assume a horizontal static position. Thus, micromirror
116
may be moved to any of three positions. This ability to move micromirror
116
provides a degree of flexibility useful in aligning (including aligning, pointing, and/or steering) the MEMS device, however, alignment complexity remains significant.
In certain applications, once the micromirror is moved to the proper position, it may remain in that position for ten years or more. Thus, for example, one side of an individual micromirror may remain in contact with the hard stop for extended periods. Maintaining such contact increases the incidence of dormancy related stiction. Such stiction results in the micromirror remaining in a tilted position after the actuators are de-energized. Some theorize that stiction is a result of molecule and/or charge buildup at the junction between the micromirror and the hard stop. For example, it has been demonstrated that capillary forces due to an accumulation of H
2
O molecules at the junction increases the incidence of stiction.
Thus, one solution to overcome stiction is to package the MEMS device in a hermetic or inert environment. Such an environment reduces the possibility of molecule accumulation at the junction. However, such packaging is costly and prone to failure where seals break or are not properly formed. Further, such packaging is incompatible with many types of MEMS devices. In addition, such packaging does not reduce stiction related to charge build-up at the junction.
In “Ultrasonic Actuation for MEMS Dormancy-Related Stiction Reduction”, Proceedings of SPIE Vol. 4180 (2000), Ville Kaajakari et al. describe a system for overcoming both molecule and charge related stiction. The system operates by periodically vibrating an entire MEMS device to release stiction forces. In this way, the stiction forces are overcome. While there is evidence that vibrating the entire MEMS device can overcome stiction, such vibration causes temporary or even permanent misalignment of the device. Thus, freeing an individual micromirror often requires a costly re-alignment procedure. Even where the device i
Anderson David Paul
Muller Lilac
Staple Bevan
Connelly-Cushwa Michelle R.
Network Photonics, Inc.
Townsend and Townsend / and Crew LLP
Ullah Akm Enayet
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