Temperature-Compliant Tube for Fiber Optic Components

The present invention relates to temperature compensation, and more particularly to a temperature-compliant tube for fiber optic components that compensates for the difference in thermal expansion coefficients between a package and an optical fiber connecting an internal component to an external point.

Fiber optic components, such as laser diode sources and photodiode optical receivers, are limited in their operating and storage temperature range, at least in part, by thermal expansion mismatch between the optical fiber and package body material. Expansion mismatch typically results in fiber-to-component misalignment and/or fiber breakage when components are repeatably heated from room temperature to above 100 degrees centigrade. Both effects render the component useless. As shown in FIG. 1 an optical component, such as a laser diode or photodiode, is mounted on a substrate within a package. An optical fiber is aligned with the optical component and is attached at "A", such as by soldering, welding, adhesive bonding or the like. The fiber exits the package through an exit port and is soldered into the tube at "B". The fiber is metallized and the solder at "B" provides a hermetic seal for the package. The thermal coefficient of expansion for the fiber is approximately 0.5E-6 in/in/degC, and the package body is typically fabricated of a metal having a thermal coefficient of expansion of approximately 6.0E-6 in/in/degC. When the temperature is raised the change in length of the package between points "A" and "B" exceeds that of the fiber. As a result the fiber is put in tension and the solder joint at "A" may yield, causing misalignment of the fiber with the optical component. If the elastic limit of the fiber is exceeded, the fiber may even break between "A" and "B".

Therefore it is desired to extend the temperature range within which the fiber/package combination may be exposed without misalignment or breakage of the fiber.

Accordingly the present invention provides a temperature-compliant tube for fiber optic components that adds an insert tube within an exit port of a package body. The insert tube extends into the package body to reduce the length of constrained fiber, and has a thermal coefficient of expansion greater than that of the package body. The expansion of the insert tube compensates for the expansion of the package body relative to the fiber to produce a "zero stress" condition at elevated temperatures.

The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.

FIG. 1 is a cross-sectional view of a prior art mechanical configuration for coupling an optical fiber to an optical component within a package body.

FIG. 2 is a cross-sectional view of a mechanical configuration using a temperature-compliant tube for coupling an optical fiber to an optical component within a package body according to the present invention.

Referring now to FIG. 2 a package body 10 is shown having a pedestal 12 upon which is mounted a substrate 14. On the substrate 14 is mounted an optical component 16, such as a laser diode or a photodiode. An optical fiber 18 enters the package body 10 through an exit port 20 and is attached to the substrate 14 at point "A" by any suitable means, such as soldering. An insert tube 22 also is inserted through the exit port 20 and extends into the interior of the package body. The exterior end 24 of the insert tube 22 is in the form of a flange that is hermetically, fixedly attached to the outer end 26 of the exit port 20 at point "B". The fiber 18 is contained within the insert tube and hermetically affixed at point "C" to the interior end 28 of the insert tube, such as by soldering, welding, adhesive bonding or the like.

The insert tube 22 is fabricated from a metal having a thermal coefficient of expansion greater than that of the package body to offset the expansion of the package relative to the fiber. Zero differential expansion, and thus zero stress on the fiber and the attachment point "A", is achieved by tailoring the actual length of the mechanical elements and their thermal expansions. The zero stress condition is given by the expression:

where a.sub.n are the thermal coefficients of expansion for the respective elements, and 1.sub.n are the respective lengths of the elements. As shown in FIG. 2 l.sub.1 is the restrained length of the fiber 18 between points A and C, with a corresponding coefficient a.sub.1 ; l.sub.2 is the length of the insert tube 22 between points B and C, with a corresponding coefficient a.sub.2 ; and l.sub.3 is the overall length of the package body 10 between points A and B, with a corresponding coefficient a.sub.3. The result is to enable the optical package with attached fiber to be cycled over a wide temperature range without causing strain-induced fiber-to-optical component misalignment or mechanical failure of the fiber. This is achieved while still maintaining a hermetic exit port construction.

Thus the present invention provides a temperature-compliant tube for coupling an optical fiber to an optical component within a hermetically sealed package, the tube having a temperature coefficient of expansion greater than that of the package body to compensate for the differential in temperature coefficients of expansion between the optical fiber and the package body, and thus to produce zero stress on the fiber at the coupling point.