CA1102598A

CA1102598A - Hermetic optical fibre feedthrough

Info

Publication number: CA1102598A
Application number: CA332,455A
Authority: CA
Inventors: Tibor F. I. Kovats
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1979-07-24
Filing date: 1979-07-24
Publication date: 1981-06-09
Also published as: GB2057936B; NL8003100A; JPS5619019A; SE8004567L; GB2057936A; IT8022269A0; IT1130724B

Abstract

HERMETIC OPTICAL FIBRE FEEDTHROUGH

Abstract of the Disclosure A hermetic feedthrough for a fiberoptic device package is fabricated by locating a fiber end part along the central axis of a tube and injecting alloy through a port in the tube wall to solidify in the space between the fiber and the wall. The tube itself is then soldered into a bore through a wall of the package. The alloy used is characterized by low thermal coefficient of expansion, minimal relaxation after solidification, and appreciable expansion as it solidifies.

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Description

11f~2~8 This invention relates to a hermetic feedthrough for a fiber optic device package, to a package incorporating such a feedthrough and to a method of making the feedthrough.
A hermetic seal is frequently required in order to combat the adverse effects (fluctuating performance and reduced lifetime) of moisture and gaseous contaminants on, for example, optical lasers and photodiodes.
Rich et al (Ceramic Bulletin Vol. 47, No. 2, 1978) describe a fiber optic hermetic seal which is made by threading fiber through a hole in a closed end of a hollow copper right circular cylinder, then pouring molten 60:40 tin:lead solder into an open end of the cylinder to surround the fiber. They describe how the solder squeezes against the fiber, forming a hermetic seal, because the net thermal expansion coefficient of the copper-solder combination is much greater than that of the fiber. The resulting stresses in the glass at the glass-metal interface are compressive which is beneficial in inhibiting crack growth in the glass.
However, a consequence of the solder expansion is that stresses are variable during normal operational temperature cycling.
Moreover, 60:40 solder is subject to creep and stress relaxat~on effects.
The Rich feedthrough also has several shortcomings where high volume fabrication in practical device packages is contemplated. Firstly, if high alignment accuracy is important, the mere fact that the threading hole is large enough to enable rapid threading of the fiber means that it is too large to ensure accurate alignment at the cylinder central axis.
Secondly, if the cylinder is to be small, in keeping with the development of small scale device packages, then the open end of the cylinder must also ., -- 1 --: .
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S~8 be small, which renders solder pouring difficult.
To overcome these defects there is proposed by the present invention in place of lead-tin solder a material exhibiting:
(i) appreciable expansion on solidification.
(ii) negligible volume change after solidification.
(iii) low coefficient of thermal expansion.
One example is a tîn:bismuth (60:40) alloy, sold under the tradename Cerrocast (RTM) which shows a volume change on solidifying of +0.5%, a negligible volume change, after solidification (of the order of 0.01%), and a coefficient of thermal expansion of the order of 0.000015/C. In order to adapt this technique for automated production, it is proposed that the alloy be injected through the wall of a cylindrical tube with the fiber located along the central longitudinal axis of the tube.
Thus according to one aspect of the invention there is provided a method of making a hermetic feedthrough at which an optical fiber enters a package housing, said method comprising supporting a tube in one reference position, passing an optical fiber end portion through the tube, supporting a fiber part projecting beyond the tube in a second reference position so that the fiber end portion is located substantially aligned with a central axis of the tube, injecting a molten alloy into the tube through a bore in a wall thereof, solidifying the lnjected alloy by cooling and sealing the tube w~thin a bore in the package housing.
Accord~ng to a second aspect of the invention there is provided in a fiber optic device package, a hermetic feedthrough at which an optical fiber enters a package housing, the hermetic feedthrough comprislng a tube, a portion of the fiber being located along a ., ~:~
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-S~8 longitudinal axis of the tube, an annular region between the fiber and an in-side surface of the tube being filled by injected -alloy, said tube being sealedwithin a bore through a wall of the housing.
~n the hermetic feedthrough defined, the tube inside surface preferably tapers toward one end thereof and an alloy injection port extends through a wall of the tube adjacent its other end.
The alloy is preferably one including bismuth in such propor~
tion that the alloy expands appreciably on solidification. The alloy should furthermore exhibit low thermal coefficient of expansion and minimal relaxation effects.
Preferably the hermetic feedthrough further incorporates a barrel soldered into the bore in the housing wall, and the tube is soldered within the barrel. The housing can be a two part housing, the feedthrough ex-tending through a wall of one of said parts. The tube and said packaged housing are preferably made of brass or Kovar (TM). Typically the hermetic feedthrough has dimensions: tube length in the range 0.25 to 1 inch; tube diameter in the range .03" to .05". The tube inside diameter preferably tapers to about 0.025"
thereby to inhibit alloy from spewing from said one end during alloy injection.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: -Figure 1 is a perspective view showing an early stage in the fabrication of a hermetic feedthrough according to the invention; and Figure 2 is a perspective, part sectional view showing a later stage in the fabrication of the hermetic feedthrough.
Figure 3 is a plan view in part section of a device package incorporating the hermetic feedthrough of Figure 1.
Referring in detail to Figure 1 there is shown an optical ~, ~

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With the fiber section 16 located in groove 18, a closure member 24 can be pivoted from the open position (Figure 1 ) to a closed position ( Figure 2~ , in which it resiliently clamps the fiber 1 and closes off the end of the tube 14.
The tube 14 has an inlet port 26 for injection of molten alloy at one end. The tube internal bore tapers inwardly, 28, to a diameter slightly larger than the fiber diameter at the other end of the tube.
Reciprocally mounted above the port 26 is a thermally controlled syringe 30. The syringe incorporates a heater element 32 around a cylinder 35 filled with injectant alloy 34. A hollow needle 36 has its --end formed as a concave rim 38 in order that, when the needle end is pressed against the tube 14, the needle end and the tube at the circumference of port 26 form a seal preventing escape of injected alloy 34.
In operation, once the fiber 1 and tube 14 are supported in the positions shown, the closure member 24 is closed and alloy 34 is introduced into the cylinder 35. The alloy is characterized by a low co-efficient of thermal expansion, minimal relaxation effects after solidification and by the property of expanding on solidification.

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S~8 An alloy of tin t60%) and bismuth (40%) sold under the tradename Cerrocast (RTM) has the required characteristics, thus:
(i) coefficient of thermal expansion - 0.000015/C
(ii) linear shrinkage after solidification - 0.01%
(iii) linear growth on solidification - 0.5%.
Other alloy materials which key well to glass, which have both a low thermal coefficient of expansion, and a low relaxation coefficient, which exhibit expansion on solidification and which have desired flow and sealing characteristics will be known or can be easily determined by those familiar with the art. A further example is the alloy bismuth (58%)i tin (42%).
The alloy 34 is heated by element 32 until it is at a temperature at which it manifests the desired flow properties and is then injected down the needle by means of a reciprocal piston 40. Molten alloy passes through the port 26 and into the tube 14. The closure member 24 prevents molten alloy from escaping at the tube end. In addition, by the time alloy reaches the other end of a tube it is cooled and is consequently too viscous to escape through the comparatively narrow region between the fiber 1 and the tube beyond the taper 28. The alloy is then allowed to solidify by cooling. At this time the alloy and fiber are subjected to a small pressure owing to the alloy expanding within the tube 14.
A slight excess of pressure is desirable s~nce it enhances ; the hermetic seal at the glass-alloy and alloy-tube interfaces and discourages crack format~on in the glass. Moreover, since the alloy has negligible shrinkage and low thermal coefficient of expansion, the pressure exerted by it is substantially invariable which aids reproducibility of optical performance at the feedthrough.
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- ~ . , - ` . : .-, , Turning now to Figure 2 there is shown a laser package utilizing the hermetic feedthrough of Figure 1. The GaAlAs double heterostructure injection laser 42 is mounted in an open topped housing 44. Through a side wall of the housing extend hermetically sealed leads 46, 46a to the laser 42, 46b to a detector 54, 46c for a thermoelectric cooler (not shown) and 46d for a temperature sensor (not shown). In an end wall is mounted an optical fiber feedthrough. The feedthrough has a brass barrel 48 with a radially extending flange 56 which is soldered, 50, into a bore through the housing wall. The tube 14, with fiber 1 in place, is then soldered, 52, into an internal bore extending through the barrel 48. The barrel 48 has a serrated outer surface 46 in order to retain a length of resilient tubing (not shown) which is placed over the barrel and the fiber projecting from it in order to prevent the bending radius of the fibre falling below a lower limit. The function of the barrel 48 is threefold. Firstly, it provides mechanical strength at the fiber feedthrough. Secondly, it provides alignment of the tube 14 with the site of laser 42 within the housing. Thirdly, it functions as a heat sink so that when soldering the tube 14 within the housing wall, there is insufficient heat inadvertently to melt the Cerrocast alloy.
To complete the package once all required components have been inserted, a lid (not shown) is seam welded onto the housing upper rim 58.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. A method of making a hermetic seal between a metal tube and an optical fiber, the method comprising supporting an end portion of the optical fiber within the tube with the fiber end portion located substantially aligned with a central longitudinal axis of the tube to define an elongate chamber between the tube and the optical fiber, closing ends of the chamber, injecting molten fusible alloy through a port in a wall of the tube to fill the enclosed chamber, the fusible alloy characterized by appreciable expansion on solidification, and solidifying the injected fusible alloy by cooling whereupon resulting expansion of the alloy produces contact pressure at an interface between the alloy and the optical fiber.

2. A method as claimed in claim 1 in which the tube is supported in one reference position, the optical fiber end portion is passed through the tube, and a fiber end part projecting beyond the tube is supported in a second reference position so that the fiber end portion is located substantially aligned with the central longitudinal axis of the tube.

3. A method as claimed in claim 1, in which the tube has a necked down portion closing at least one end of the chamber.

4. A method of making a hermetic feedthrough at which an optical fiber enters a fiber optic device package, said method comprising making a hermetic seal as claimed in claim 1, the method further comprising sealing said tube within a bore within the housing of the package.

5. A method as claimed in claim 2, in which said first and second reference positions are provided by a jig having longitudinally contiguous, vertically offset V-grooves.

6. A method as claimed in claim 5, the jig having a closure member to close one end of the tube during alloy injection.

7. A method as claimed in claim 1, in which said alloy exhibits expansion on solidification of at least 0.2%.

8. A method as claimed in claim 1, in which said alloy exhibits a coefficient of thermal expansion of less than .00002/°C.

9. A method as claimed in claim 1, in which said alloy exhibits linear growth after solidification of less than 0.1%.

10. A method as claimed in claim 1, in which said alloy is injected from a syringe.

11. A method as claimed in claim 10, in which the syringe has an outlet nozzle shaped to the outer surface curvature of said tube.

12. A method as claimed in claim 11, in which said syringe has a barrel surrounded by a heating coil.

13. A hermetic seal between an optical fiber and a metal tube, the fiber located substantially along a central longitudinal axis of the tube, the tube having a port in a wall thereof for injection of molten fusible alloy into an elongate chamber between the tube and the optical fiber, which fusible alloy, in a solidified state, fills the chamber and is characterized by appreciable expansion on solidification thereby producing a contact pressure at an interface between the alloy and the optical fiber.

14. A hermetic seal as claimed in claim 13, in which said tube is necked down at one end thereof substantially to enclose one end of the chamber.

15. A hermetic seal as claimed in claim 13, in which said alloy exhibits expansion on solidification of at least 0.2%.

16. A hermetic seal as claimed in claim 13, in which said alloy includes bismuth.

17. A hermetic seal as claimed in claim 13, in which said alloy is 40% bismuth, 60% tin.

18. A hermetic seal as claimed in claim 13, in which the alloy has a coefficient of thermal expansion less than 0.00002/°C.

19. A hermetic seal as claimed in claim 13, in which said alloy exhibits linear shrinkage after solidification of less than 0.1%.

20. A hermetic seal as claimed in claim 13, in which the tube length is in the range of 0.25" to 1".

21. A hermetic seal as claimed in claim 13, in which the tube diamter is in the range 0.03" to 0.05".

22. A hermetic seal as claimed in claim 14, in which the tube inside diameter tapers to about 0.025", thereby to inhibit alloy from spewing from said one end during injection of alloy.

23. In a fiber optic device package a hermetic feedthrough at which an optical fiber enters a package housing, the hermetic feedthrough comprising a hermetic seal as claimed in claim 13, said tube being soldered within a bore through a wall of the package housing.

24. A hermetic feedthrough as claimed in claim 23, in which a barrel is soldered into said bore in the housing wall and the tube is soldered within the barrel.

25. A hermetic feedthrough as claimed in claim 24, in which the barrel has an outer serrated surface.

26. A fiber optic device package incorporating a hermetic feedthrough as claimed in claim 23, the package housing being a two-part housing, said feedthrough extending through a wall of one of said parts.

27. A package as claimed in claim 26, in which said tube and said package housing are made of one of the group consisting of brass and Kovar (TM).