Glass-solder process fortifies fiber-to-silica cohesion in couplers

Attaching optical fibers to a silica substrate with glass solder results in greater bonding strength and humidity resistance than epoxy bonding

H.S. Daniel, D.R. Moore

and V.J. Tekippe

gould electronics inc.

A novel glass-solder process has been developed for making a glass-to-glass bond between optical fibers and a silica substrate. Test results demonstrate that glass-solder bonds are stronger than epoxy bonds and are not susceptible to degradation from humidity. The relatively inexpensive and rapid glass-solder technique can be incorporated into the manufacturing process of fused biconical taper couplers to achieve increased component reliability.

Properly fastening optical fibers to a silica substrate enables the control of the differential thermal expansion and contraction between the fibers and the substrate. It also allows solid thermal stability during temperature cycling.

Although the bonding technique focuses on couplers, the glass-solder process is applicable throughout the fiber optics industry.

During the last decade, the fused biconical taper process has produced various couplers. The process, which consists of fusing two fibers by heating and stretching until the appropriate amount of coupling is achieved, is inherently stable with low insertion loss. Long-term reliability is determined not by the basic process itself, but by the packaging. In addition to low cost, these couplers exhibit improved mechanical and environmental stability.

Although different epoxies and materials for enclosing and booting are used, the packaging approach used by most fused-coupler manufacturers consists of:

Affixing the fused fibers to a silica substrate using an adhesive.

Placing the substrate into an enclosure (generally a tube).

Sealing the ends with a material that provides strain relief for the fibers.

The new glass-bonding process replaces epoxy because true hermetic sealing is almost impossible to obtain with a typical acrylate-coated fiber. The method augments the epoxy with a glass-solder material that forms a true chemical bond with the optical fiber and the substrate. This method, which is accomplished quickly, delivers a stronger bond than an epoxy method and is impervious to moisture. Tests indicate that glass-solder bonds enhance the environmental stability of the couplers.

Glass solders are inorganic compositions that are used for making strong insulating and hermetic joints or seals between different materials such as glass, ceramics and metals. Usually containing mixtures of silica and other metal oxides, the solders form strong ionic bonds that resist moisture.

However, for packaging couplers, a glass solder must be chemically and physically compatible with silica fibers and substrates. The glass solder must also have a surface energy that is less than that of silica. Upon the application of heat, this energy softens and wets the surfaces at temperatures below that of the softening point of the fibers.

This attribute is essential for obtaining good adhesion and bond strength without damaging the fibers. Furthermore, the glass solder should exhibit a coefficient of thermal expansion similar to silica to prevent the formation and propagation of cracks.

Glass solders are generally available in powder form. The preferred application method involves a slurry composed of the glass powder, a binder and a carrier or vehicle (a liquid used to make the slurry). The binder, most of which is burned away when the slurry is heated, provides dimensional stability to the powder after the vehicle has evaporated. Using proven industry epoxy practices, the glass-solder slurry can be applied in small amounts.

To avoid damaging the fiber leads, the heat required to soften and fuse the glass solder should be applied locally where the glass solder has been deposited. Heat can be applied using a carbon-dioxide laser operating at 10.6 microns. The silica glass and glass solder used in this work have a large absorptivity value at this wavelength.

In practice, the laser beam is directed onto a coupler mounted on a moving platform. Beam intensity and platform motion are computer-controlled. The optical throughput of the coupler is monitored during the packaging process. However, experience shows that monitoring is not needed because the glass solder has a lower softening temperature.

Design evaluation tests

During the development of the glass- solder packaging technique, several environmental and mechanical tests were performed to evaluate the process capabilities. Mechanical tests, including vibration, impact and fiber retention, were performed to evaluate the strength of the bonds between the glass solder and the optical fibers and substrate.

For the fiber retention tests, standard singlemode fibers were secured to silica substrates with a small (approximately 2-millimeter diameter) bead of glass solder. Tensile forces were applied to the fiber leads by attaching known weights.

All five test samples supported tensile loads in excess of 5 kilograms of force (49 Newtons) without failure of the glass-solder bonds. Similar samples that were prepared using epoxies typically failed at lower pull-out forces.

The five packaged couplers using the glass-solder technique were also subjected to mechanical vibration and impact tests. The vibration tests were conducted from 10 to 55 hert¥in accordance with the test conditions specified in Bellcore TR-NWT-001209. The average change in insertion loss following the vibration tests was approximately -0.1 decibel.

The couplers were then subjected to impact tests conducted from a height of 1.8 meters. The average change in insertion loss following the impact tests was approximately -0.1 dB.

To test the thermal compatibility of the glass solder with silica fibers and substrates, 17 packaged couplers were subjected to temperature cycle tests between -40 and +125C. The couplers were tested for five temperature cycles and were actively monitored.

The couplers exhibited minimal sensitivity to changes in temperature. The typical change in insertion loss during the temperature cycle test was less than 0.1 dB. Furthermore, only two couplers exhibited changes in insertion loss of more than 0.2 dB.

To evaluate the ability of the glass solder to withstand prolonged exposure to hot, humid environments, 10 partially packaged couplers were subjected to 2500-hour damp-heat tests conducted at 85C and 85% relative humidity.

To accelerate the detrimental effects of humidity, the couplers were secured to silica substrates using the glass solder and then inserted into an environmental chamber. The couplers were not housed inside a protective tube or sealed from the humid environment. A second group of five couplers, packaged in a similar manner using only epoxy, was tested for comparison purposes. Both sets of couplers were monitored periodically.

After approximately 600 hours, the humidity began to weaken and break down the epoxy, causing the insertion loss of the epoxy-packaged couplers to change substantially. In comparison, no significant degradation in the performance of the glass-solder couplers was observed. Furthermore, although the behavior of the epoxy-packaged couplers was unpredictable, all the glass-soldered couplers behaved identically.

The insertion loss of the coupled leg of the glass-solder couplers slowly decreased during the test, indicating a slight increase in coupling ratio. This action was probably due to the diffusion of moisture into the glass fibers of the coupling region, which was unprotected from the hot, humid environment. To evaluate this hypothesis, the above test was continued beyond 2500 hours without the high humidity. As expected, the insertion loss returned to its original value within a short time in the dry heat environment.

Qualification tests

Following the development of the glass-solder packaging process, several couplers were built for qualification testing in accordance with the test criteria specified in Bellcore TR-NWT-001209. This test consisted of a prescribed sequence of successive tests, including thermal aging, humidity resistance, temperature cycling, water immersion, vibration, cable retention and impact resistance.

Three different groups of couplers were assembled: dual window (1310/1550-nanometer) 50/50 couplers in the glass-solder package (where light is split equally between two fibers), 10% tap couplers (for example, couplers with a 10/90 splitting ratio) in the glass-solder packages and 10% tap couplers in epoxy packages. The tap couplers were measured to compare the relative performance of the two packaging processes.

Although these qualification tests were short term, the glass-solder-packaged tap couplers showed superior performance in all test categories. The results obtained for the temperature cycle test for both the epoxy package and the glass-solder package indicated smaller changes for the glass-solder package. Also, the results for the entire lot were more consistent. Although both package types performed well, the glass-solder packages outperformed the epoxy packages.

Applications

The design and qualification test results indicate that the glass-solder process is superior to the epoxy package alone for fused couplers. Other fiber-optic applications prevail where similar improvements in performance could be expected. For example, fiber-optic sensors used in hostile environments could benefit from this assembly technique. Similarly, enhanced performance of pigtailed sources and detectors could be expected when the glass-solder process is used to attach optical fiber to these devices.

Preliminary tests demonstrate this process can also be used to attach optical fibers to ceramic ferrules in fiber-optic connectors. In general, this glass-solder process should be applicable whenever a strong, permanent bond is desired between an optical fiber and silica substrate.

Using epoxy bonding as a packaging material offers advantages: easy application, low cost and proven technology. Also, despite its simplicity, this packaging technique furnishes adequate stability and reliability.u

Dr. H.S. Daniel is a research scientist, Dr. D.R. Moore is vice president for engineering, and Dr. V.J. Tekippe is president and general manager at the fiber optics division of Gould Electronics Inc., Millersville, MD.