Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-03-03
2001-12-04
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S012000, C385S136000, C385S137000
Reexamination Certificate
active
06327405
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to temperature stabilization of optical devices and components for optical communications systems, and more particularly to systems and methods for temperature stabilization of Bragg grating structures used in add/drop filter systems and the like.
BACKGROUND OF THE INVENTION
Wideband communication systems using optical signals now commonly employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to transfer and transmit vast amounts of data at very high data rates. In these multiplexing systems, multiple signal-bearing channels are centered on specified spaced apart wavelengths and sources (e.g. lasers) are set to those wavelengths. Whether combining signals into the multiplex format, or demultiplexing by separating or dropping out signals, the filters and couplers used must have center wavelength stability, with a high degree of precision. This in turn means that temperature variations cannot be permitted to affect operation over a substantial range, such as 0-70° C. For a number of reasons, Bragg grating devices are widely employed in wavelength division multiplex systems because a precise wavelength response can be set by selection of the periodicity of the grating written in a device. Because the periodicity of the grating is affected by temperature, however, due to the temperature coefficient of the waveguide or fiber material, the wavelength selectivity can be unacceptably changed by ambient conditions. For example, without any form of temperature compensation, the center wavelength can vary by as much as +0.01 nm/C. Channel placements and bandwidths for individual signals are such that this amount of variation of length, in a normal temperature environment, would shift the wavelength selectivity such that the coupler or filter is no longer matched to the wavelength of the corresponding laser or other source.
In a prior application, Ser. No. 09/128,476 entitled “Methods of Fabricating Grating Assisted Coupler Devices”, filed Aug. 4, 1998 by Anthony S. Kewitsch, et al., and assigned to the assignee of the present invention, this problem is confronted by a disclosed temperature compensation system in which a “prepackage” or support structure is disposed within a cylindrical housing, with the prepackage structure using a number of spaced apart hubs mounted on and/or selectively movable relative to Invar rods. The prepackage structure enables attachment of a span of a fiber-based coupler, and the arrangement provides for the use of thermally dissimilar metal elements (stainless steel) and adjustment means for varying the strain on the critical fiber length which includes a Bragg grating. While this arrangement is fully satisfactory from the operational standpoint, and provides for the needed tensioning of the fiber optic components during the initial writing phase and thereafter despite temperature variations, it is more complex than desirable for high production processes. Further, it is desirable to provide even better temperature compensation, to a level of + or −0.001 nm/° C. center wavelength variation over temperature. In addition, it is desirable to provide means for axial twisting of the fiber optic component, for purposes of achieving polarization independence and reducing polarization mode dispersion (PMD).
SUMMARY OF THE INVENTION
Devices in accordance with the invention suspend the waist region of a fused fiber coupling closely along a base support having a low thermal coefficient of expansion (i.e. Invar or ceramic). The ends of the span are held in spaced apart compensating elements of higher expansion material (e.g. stainless steel) which are attached to the ends of the base support, and have expandable lengths, in relation to their temperature coefficients of expansion, to tend to move the mounting points inwardly to compensate for the thermal expansion of the base support. The net adjustment is arranged to be precisely enough to counteract the effect of temperature on the span. Each end of the span of optical fiber that includes the Bragg grating is mounted in a holder, such as a split-sleeve ferrule, which can be rotated so as to impart a degree of twist to the waist region, compensating for polarization dependence and PMD. This arrangement permits ready assembly of a minimum number of parts and enables fine tuning of both temperature compensation and strain on the suspended span of optical fiber so as to set the desired wavelength selectivity.
Methods of assembling a temperature compensated fused fiber coupler first elongate one or more lengths of optical fibers to form a small diameter waist region. A fused elongated fiber section is mounted under tension between elements that are held on a temporary carrier, to define a span which can be exposed to an illumination pattern, which writes the Bragg grating to provide a selected nominal periodicity. The span is then transferred to the temperature compensated structure, together with the end elements, which can be rotated about the longitudinal axis of the span to reduce polarization dependence and polarization mode dispersion. Then the unit is temperature cycled and the wavelength response of the filter is monitored using a broadband light source and a wavemeter. The temperature compensating structure can be adjusted by varying the relatively expandable lengths of the compensating elements, and the grating periodicity can be fine tuned by incremental shifting of the end elements on their supports.
This arrangement also has the advantage that it facilitates incorporation of a number of temperature compensated couplers in a single housing for assembly of multiplexers and demultiplexers. For this purpose, optical fibers splice together the separate couplers in the desired circuit pattern. The couplers are physically independent, however, being held in the housing in an elastomeric mold which does not introduce any stress that might cause wavelength shifts, or effect the temperature compensation action.
A number of specific features in accordance with the invention contribute to the usefulness of the apparatus and process. In the temperature compensated structure the elongated low expansion base supports spaced apart planar compensating elements which are attached to the base at the ends opposite the intermediate tensioned fiber span. The attachment is advantageously accomplished along a joinder length by laser welding, and the distance between the opposed weld ends (as well as the lengths of the compensating elements that are free to move relative to the base) determine the temperature compensating characteristic. Fine tuning is readily feasible by further precise welding along the joinder line. The mounts for the tensioned fiber spans are in this example cylindrical ferrules with at least one slot for receiving pairs of fibers at one end of the waist region, so that the needed degree of twist can be imprinted by rotation of one or both ferrules about the longitudinal axis of the tensioned span. In addition, micro-adjustments of tension and therefore wavelength sensitivity can be made by laser beam imprints, known as laser “hammering”.
The low expansion coefficient base can alternatively be of a non-metallic material such as a ceramic. In this event a conductive coating can be used on end surfaces of the base, so that sheet elements of stainless steel or other metal can be brazed on, to receive separate compensating pads which can then be laser welded.
REFERENCES:
patent: 5243680 (1993-09-01), Soane
patent: 5694503 (1997-12-01), Fleming et al.
patent: 6101301 (2000-08-01), Engelberth et al.
patent: 6112553 (2000-09-01), Poignant et al.
patent: 6147341 (2000-11-01), Lemaire et al.
patent: 6181851 (2001-01-01), Pan et al.
Leyva Victor
Lu Huey
Yeh Xian-Li
Arroyo Optics Inc.
Bovernick Rodney
Doan Jennifer
Jones Tullar & Cooper P.C.
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