Optical waveguides – Accessories – Attenuator
Reexamination Certificate
1999-09-10
2001-01-09
Nguyen, Thong (Department: 2874)
Optical waveguides
Accessories
Attenuator
C385S037000, C385S043000
Reexamination Certificate
active
06173106
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical attenuating device and more particularly to a thermally compensated, fiber coupler, variable optical attenuator (VOA).
BACKGROUND OF THE INVENTION
VOAs of various designs are widely used in the local and long distance telephone networks. Applications of these devices include, e.g., light filters, switches, splitters, combiners/couplers of light signals, light multiplexers, etc. It is expected that thermally compensated, fiber coupled, VOA's will become vital for the development of high performance telecommunication systems. The ability to design viable and reliable packages, which might be subjected to various thermal and mechanical loads during 20-25 years of their projected service life, is of practical importance.
Optical systems utilizing fiber coupled variable taps are already known (see, e.g., Miller et al. U.S. Pat. No. 5,146,519 entitled “Rotary Variable Optical Tap”; Keck et al. U.S. Pat. No. 5,353,363 entitled “Optical Fiber Bendable Coupler/Switch Device”). The variable attenuation of the light signal is achieved by bending or rotational (torsional) loading of a bi-conical coupler comprising a pair of optical fibers fused together at a narrowed region. Typically, the coupler is bent in the narrowed region, whereby a coupling ratio can be selected. Structurally, fiber coupled variable optical attenuating (FC-VOA) packages are multi-material assemblies in which the material interactions, their sizes and configurations, and the loads, whether thermally induced or mechanical, are as important as the performance characteristics of the employed materials. The thermal contraction mismatch of the fused silica glass used in the biconical coupler and fibers relative to the enclosure materials (metal alloys or packaging plastics), including adhesives, results in thermally induced stresses that affect the light attenuation and control thereof. In order to minimize the adverse effect of thermally induced stresses in the fused coupler caused by the thermal contraction mismatch between the fused glass coupler, the loading mechanism, and the enclosure, the fiber may be mechanically tuned by alignment techniques. The tuning, however, results in a new temperature dependent change in the light attenuation caused by the combined action of the thermal mismatch and the tuning induced misalignments. The loading devices used in FC-VOA packages tend to not be fully thermally compensated and therefore produce additional mismatch stresses when assembled in the protective enclosure. These packages are also sensitive to mechanical shock and vibrations and, therefore, reproducibility of the coupling ratio varies over time. In addition, FC-VOA packages containing loading devices providing variable bending or rotation are very expensive to manufacture.
It is desirable to obtain a high accuracy, totally thermally self-compensated fiber coupled variable optical attenuating system that can be easy-tunable over large deflection and temperature ranges, is relatively inexpensive, which is made from commercially available and easily machinable materials, and which will be reliable in operation. In addition to this, the enclosure design should be adequate not only to mechanically protect the brittle glass components but also to compensate for and minimize the possible thermally induced shift in the optical attenuation.
SUMMARY OF THE INVENTION
The present invention is directed to a variable optical attenuator. In an embodiment of the invention, a fiber coupled variable optical attenuator includes a fused bi-conical silica glass coupler comprising a pair of optical fiber waveguides that are fused together at a narrowed, waist region. The coupler is mounted into a protective enclosure with one end of the coupler fixed to the enclosure and one end free. The enclosure is made from metal alloys or packaging plastics.
In order to reduce the thermal mismatch stresses localized in the clamped portion of the coupler, the coupler is mounted through a low modulus material bushing which acts as a stress-reducing buffer. The free end of the coupler is deflected using a motorized mechanism. The mechanism includes an electric step motor, an eccentric motion producing shaft and a push rod that is perpendicular to the motor and shaft, connected in series. The end of the push rod engages the coupler through a V-groove in contact with a spherically shaped sleeve around the free end of the coupler. The kinematic chain formed by the motor, shaft, push rod and coupler mechanically transfers the rotational motion of the motor through the axial motion of the rod to the lateral deflection of the coupler's free end. The dimensions and thermo-mechanical parameters of the chain members and the enclosure are established to satisfy conditions of nearly total thermal compensation within the enclosure enveloping this chain. Total thermal compensation can be defined here as sufficient dimensional stability of the optical device such that changes in the VOA output due to temperature change are negligible in use. The motorized mechanism is also equipped with a rigid frame that limits axial deflection of the coupler and protects it from excessive deflections caused by mechanical shocks associated with accidental drops when mishandled or during shipping. The free end of the coupler is equipped with a sleeve having a spherical polished surface positioned in contact with the end of the push rod. The contact sleeve is designed to thermally compensate the axial expansion of the enclosure due to the temperature change and to transfer the vertical (axial) motion of the push rod into lateral elastic deflection of the coupler. The moving portion of the coupler is balanced with respect to the geometrical center of the spherical surface of the contact sleeve to protect the coupler when mechanically shock-excited.
In order to reduce the thermal stresses in the fiber waveguides, the enclosure is also equipped with thermal compensating devices positioned in series between the enclosure and optical fibers exiting the enclosure. The thermal compensating device is a bi-material laminate cylinder that includes several thin material layers (e.g., washers) made from a low elastic modulus, high thermal expansion coefficient material (e.g., RTV silicone), and separated by metal disks, and is provided with a central aperture coinciding with the axis of the fiber waveguide. With the proper selection of the thermo-elastic properties of the materials as well as dimensions of the layers, the thermal compensating device added to the chain dramatically reduces the thermal strain in the fiber that is inevitably present. The function of the metal disk is to limit the deflection of the elastomer under conditions of loads externally applied to the fiber. A thick single elastomer washer will deflect under load more than two washers of half thickness if the washers are bonded to high elastic modulus materials that limit lateral deformation of the elastomer. The fiber is fixedly bonded to the innermost member of the thermal compensator that is in turn carried through the laminated layers of elastomer and metal. The outer member is fixedly bonded to the enclosure. This arrangement allows relative motion between inner and outer members, which is caused by thermal expansion/contraction of the materials in such manner as to maintain a fixed dimensional relationship between the inner member and the optical fused coupler.
In another embodiment of the invention, a device for variably attenuating an optical signal transmitted through the device includes an enclosure for the device; an optical fiber coupler having a first end section, a second end section, and a waist section between the first and second end sections; a sleeve encircling at least a portion of the first end section; a clamping structure for fixedly securing the first end section of the coupler with respect to a mounting surface of the enclosure; a sleeve encircling a portion of the second end section of the coupler; a motor connected to the m
DeBoynton William L.
Uschitsky Michael
Cherry Euncha
Corning Incorporated
Greener William
Malley Daniel P.
Nguyen Thong
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