Optical assembly with improved dimensional stability

Details Optical assembly with improved dimensional stability Optical assembly with improved dimensional stability

1998-04-14

2001-03-20

Lee, John D. (Department: 2874)

Optical waveguides

Integrated optical circuit

C427S180000, C428S614000, C438S613000

Reexamination Certificate

active

06205264

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to optical assemblies with improved dimensional stability of optical alignment and, in particular, to an optical assembly comprising creep resistant solder or adhesive connections.
BACKGROUND OF THE INVENTION
In order for optoelectronics technology to have wide applications in consumer and telecommunication products, it is desirable to reduce the assembly cost and to improve the reliability of the products. One of the challenges for optical packaging is to connect the components with precision alignment and to maintain the stability of the alignment during device operation with fluctuations in ambient temperature and stress at the interconnect.
Precision alignments are necessary for coupling optical fibers or waveguides to optically active devices such as lasers, light-emitting diodes (LEDs), or photodetectors, and to passive devices such as other fibers. At present, such coupling is usually accomplished by active alignment, i.e., by monitoring the levels of output light intensity for different coupling conditions, and then forming a permanent assembly by soldering, brazing or adhesive bonding. While more accurate alignment is possible, such active alignment, often carried out on one or a few devices at a time, is time-consuming and costly.
For low-cost optical assembly, passive alignment using solder bonding is attractive. The optical monitoring is omitted, and a reasonably accurate device assembly can be achieved by utilizing the well-known solder self-alignment process or by using a convenient alignment fixture such as a substrate with V-grooves.
Reducing creep in bonds and joints is of paramount importance in optical packaging because of the need for maintaining positional accuracy of the components over extended periods of time. Creep is a permanent dimensional change from prolonged stress or exposure to elevated temperature. In lightguide ocean cables and many other optical assemblies, optical fiber is attached in aligned position with solder. Creep can destroy the alignment and coupling efficiency to and from the fiber.
Time and temperature dependent processes such as creep and stress relaxation become significant and affect the dimensions and properties of a material at high homologous temperatures (T/T
mp
, where T and T
mp
are the operating temperature and the melting point in absolute units respectively). These processes are important for the commonly used low temperature solders, which typically have melting points less than 200° C., since even room temperature corresponds to a homologous temperature of greater than 0.6. For applications requiring creep resistant joints (e.g. for multimode optical fiber interconnections with an alignment position tolerance of about 5-10 &mgr;m, or for single-mode optical interconnections with a tolerance of 1 &mgr;m), higher melting temperature solders such as the 80 Au-20 Sn eutectic (melting point of 278° C.) are used because of their well known creep resistance. However, high soldering temperatures can degrade the properties of optical fiber and other electronic components. Thus it is highly desirable to have a solder that has a low melting point in combination with a high creep resistance. A high mechanical strength is also helpful to prevent component misalignment during handling.
From the manufacturing point of view, low bonding and assembly temperatures are desirable for the ease of processing and the low cost for optoelectronic assembly as well as to minimize the possible damages to devices upon high temperature exposure. Therefore there is a need for a low melting temperature solders or low curing temperature adhesives which have substantially improved creep resistance so that the stability of optoelectronic assembly is assured.
SUMMARY OF THE INVENTION
In accordance with the invention, an optical assembly with improved dimensional stability comprises components bonded with a solder or adhesive containing insoluble, non-coarsening dispersoid particles having a diameter of 5000 nm or less. In solder, the particles are preferably dispersed in a solder material having an average grain size of approximately 10,000 nm or less. The dispersoid particles in solder act as physical barriers substantially impeding the motion of grain boundaries and inhibiting grain growth during thermal and stress cycling. This, in turn, substantially inhibits coarsening. The result is an assembly with substantially improved dimensional stability, with the rate of creep deformation at least halved.


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patent: 5346775 (1994-09-01), Jin et al.
patent: 5548677 (1996-08-01), Kakii et al.
S. Jin, Deformation-Induced Anisotropic Cr-Co-Fe Permanent Magnet Alloys, IEEE Transactions on Magnetics, vol. MAG-15, No. 6, Nov. 1979, pp. 1748-1750.

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