Optical waveguides – Accessories – Attenuator
Reexamination Certificate
2000-11-15
2001-11-20
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Accessories
Attenuator
C385S031000, C385S037000, C385S043000, C385S048000
Reexamination Certificate
active
06321022
ABSTRACT:
TECHNICAL FIELD
Mechanical actuation, preferably under servo-control, adjusts optical signal attenuation over a continuum. Consistent performance requires attention to friction reduction and thermal compensation.
BACKGROUND OF THE INVENTION
Variable optical attenuators (VOAs) are key components of optical networks, including local and long distance telephone networks. High performance telecommunication systems of the future are expected to rely on VOAs to perform a variety of functions such as filtering, switching, splitting, coupling, and combining as well as to participate in other functions such as multiplexing and demultiplexing.
Optical signal attenuation can be accomplished in a variety of ways through diverting all or a portion of an optical signal from an original pathway. The diverted optical signal can be discarded for performing such operations as in filtering or can be transferred to one or more additional pathways for performing such operations as switching, splitting, and coupling. In addition, optical signals traveling in different original pathways can be diverted in whole or in part into a common pathway, such as in combining operations.
The “variable” capability of VOAs distinguishes them from simpler discrete state devices, such as on-off switches, that direct all or none of the optical signal to selected pathways. A range of operating positions within VOAs provides a further capability to distribute or collect optical signals in different proportions among pathways.
U.S. Pat. No. 5,146,519 to Miller et al. and U.S. Pat. No. 5,353,363 to Keck et al. disclose VOAs using fiber couplers to provide the variable attenuating function. The fiber couplers include one or more fibers within an overcladding that are drawn down to a reduced diameter at a midsection. Bending or twisting about the midsection of the coupler changes the optical characteristics (e.g., propagation constant) of the coupler to progressively couple or uncouple light between adjacent fibers or change modes of transmission within a single fiber.
Correlating a desired amount of attenuation with a command signal presents difficult mechanical challenges because of a high sensitivity of the fiber couplers to bending. In addition, friction and backlash among moving elements of a bending actuator and thermal size variations of the actuator and fused coupler mountings undermine repeatability.
SUMMARY OF INVENTION
A variable optical attenuator (VOA) in accordance with one or more embodiments of this invention includes an electro-mechanical actuator that provides precise repeatable control over an optical attenuator to closely correlate a desired amount of optical attenuation with a command signal. The actuator maintains precise alignments, eliminates backlash, reduces friction, and compensates for thermal expansion and contraction characteristics among its components.
One such embodiment includes a frame and a fiber optical coupler having first and second fiber sections joined by an intermediate fiber section. A flexure has a fixed portion mounted on the frame and a free portion suspended from a resilient portion out of frictional engagement with the frame. The first fiber section of the optical coupler is attached to the frame. The second fiber section of the optical coupler is supported for limited angular movement by the free portion of the flexure.
A kinematic chain of an actuator connects a source of motion to the free portion of the flexure for displacing the free portion of the flexure with respect to the fixed portion of the flexure. The free portion of the flexure is displaceable with respect to the fixed portion of the flexure without substantially changing in angular orientation. A bearing carried by the free portion of the flexure permits angular movement of the second fiber section with respect to the free portion of the flexure. The intermediate section of the optical coupler bends in response to the displacement of the free portion of the flexure to adjust transmission characteristics of the optical coupler.
The coupler is preferably a fused optical coupler in which the intermediate portion tapers to a reduced diameter. The flexure preferably takes the form of a parallelogram with the resilient portion formed by two resilient arms extending between the fixed and free portions of the flexure. The resilient arms, which extend nearly parallel, perform several important functions. For example, the resilient arms exhibit flexibility in a first orthogonal direction and rigidity in a second orthogonal direction for supporting the free portion of the flexure from the frame. The resilient arms permit the free portion of the flexure to be displaced by the kinematic chain while remaining angularly aligned with the kinematic chain. In addition, the resilient arms preload the kinematic chain to avoid backlash.
A sleeve preferably surrounds the second fiber section of the optical coupler and supports the bearing that allows angular movement of the second fiber section with respect to the free portion of the flexure. The bearing, which is preferably spherical, is preferably received in a cylindrical surface of the free portion of the flexure to permit both angular and axial movements of the second fiber section. The permitted angular movement allows smooth bending of the intermediate section of the fiber coupler accompanying displacement of the second section of the coupler. The permitted axial movement of the second fiber section accommodates changes in position between the relatively displaced fixed and free sections of the flexure.
Another embodiment of the variable optical attenuator features improvements in thermal compensation. A frame supports an optical attenuator having first and second relatively displaceable sections for attenuating the optical signals as a function of their relative displacement. The first section of the attenuator is attached to the frame. The second section of the attenuator is supported for movement with respect to the frame. A source of motion is also attached to the frame. A kinematic chain connects the source of motion to the second section of the attenuator for displacing the second section with respect to the first section of the attenuator through a range of relative displacement positions.
The first section of the attenuator is spaced from the source of motion through a first distance that varies as a function of temperature. The second section of the attenuator is spaced from the source of motion through a second distance that varies as a function of both temperature and the relative displacement. Portions of the second distance contributed by the relative displacement, however, do not significantly vary as a function of temperature. Throughout the range of displacement positions, the distances between the first and second sections of the attenuator and the source of motion vary by substantially the same amounts as a function of temperature so that the relative displacement positions remain substantially independent of temperature.
The kinematic chain preferably includes among a series of components a cam having a dimension that varies as a function of position. A first portion of the cam including the variation in dimension is made of a material (e.g., Invar) having a lower coefficient of thermal expansion than the materials (e.g., aluminum) of the other components of the kinematic chain. A remaining portion of the cam can also made of the same material.
Any change in the thermal expansion characteristics of the remaining portion of the cam can affect the relative thermal expansion characteristics between the first and second distances. A reduced thermal expansion of the remaining cam portion can be offset by a corresponding increase in the thermal expansion characteristics of one or more components along the first distance or by a corresponding decrease in the thermal expansion characteristics of one or more components along the second distance.
REFERENCES:
patent: 5146519 (1992-09-01), Miller et al.
patent: 5333217 (1994-07-01), Kossat
patent: 5353363 (1994-10
Corning Incorporated
Eugene Stephens & Associates
Ryan Thomas B.
Sanghavi Hemang
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