Connecting Photonic Devices: Two Routes to Achieve Accuracy
Julie A. Harrell and Mark Rodighiero, Miyachi Unitek Corp.; and EXFO Life Sciences & Industrial Div.
Because photonic components such as multiplexers, carrier and pump lasers and attenuators must fit within a network and survive up to 20 years, how they are connected is a critical issue.
Photonic devices that contain lenses, crystals, laser diodes and photodetectors demand exact alignment. The fiber optics that conduct signals in and out of these devices are precisely attached and aligned to the internal optical components. Consequently, placement of these miniature parts requires such a high degree of accuracy that manufacturers must rely on high-precision automated equipment.
The technical challenge that faces manufacturers is one of placing and attaching miniature optical components within devices that require positioning tolerances as small as 50 nm in several axes. The other challenge is producing the product quickly enough to satisfy customer demands.
Two methods, adhesive bonding and laser welding, are widely employed to join individual components used in photonic devices. Adhesive bonding creates a seal by curing an acrylic or epoxy adhesive medium to join two parts. Such adhesives can be roughly divided into two chemistries, acrylics and epoxies. These materials use energy, such as UV light for acrylics or epoxies and heat for epoxies, to join (cure) the adhesive monomers into long-chain, branched or network polymers.
Applications for adhesive bonding include active components such as laser-diode die attachment and passive components such as fiber-to-ferrule bonding.
The equipment used for adhesive bonding can be relatively inexpensive and readily available, as opposed to that for laser welding. Laser welders are more costly and are sometimes difficult to obtain in a timely manner for specific applications, while adhesive bonding apparatus is easily manufactured, including the jigs to hold parts steady during the curing process.
The trade-off for lower cost, faster manufacturing procedures is that, under extreme conditions, adhesive seals and bonds can shift during curing, causing devices such as pump lasers to fail premarket testing. UV adhesives (both epoxies and acrylics) are often inferior to thermally cured materials, particularly with regard to damp heat resistance.
However, UV materials allow manufacturers to align parts actively with a wet bond, then lock the bond with UV illumination. Newer UV-cured materials offer substantially better performance than previous materials. An advantage of UV adhesion over thermal bonding is the relative simplicity of UV lamps compared with over-curing processes. Thermally activated materials, while viewed as high-performance materials, require expensive fixtures and thermally compensating jigs to maintain the desired alignment during the curing cycle, which is usually done in batch fashion in an oven. For device manufacturers, the two most critical adhesive choices are the glue and the curing profile.
Photonics for photonics
Laser welding is a proven alternative to adhesive joining, which offers many advantages in critical applications (Figure 1). Compared with solder and epoxy adhesive attachment methods, laser welding offers significantly higher production rates; easier automation; excellent long-term stability of mechanical and optical performance; low contamination; strong, clean joints; and low ecological impact.
Figure 1. In a laser welding system, flexible optical fibers can deliver high-energy laser pulses through a focusing assembly to the part surface accurately and repeatably under microprocessor control.
A critical concern for laser welding applications is postweld shift, which is the contraction of the materials as the weld spots solidify and cool. Fillet, butt and lap welds incur shifts. Correct preweld engineering can compensate for these shifts with appropriate process parameters, beam delivery components, materials and weld joint design (Figure 2).
Laser welding is widely applied in the production of active photonic components. Better part design, material selection and laser welding techniques are becoming available for managing postweld shift.
Even if the preweld engineering process doesn’t eliminate postweld shift, laser welding systems can compensate. They use the welding laser to ease the part back into the proper location with a process known as laser hammering. Production-equipment manufacturers offer automated systems, so device manufacturers can use relatively unskilled operators to assemble and align devices in a high-volume manufacturing environment.
Figure 2. Commercial laser welding systems reduce postweld shift through branch-to-branch energy balance, beam profile and proper beam distribution.
The ultimate expense of laser welding lies in the positioning systems — but adhesive joining requires similar apparatus.
The typical installed cost for a laser welder with three beam-delivery systems ranges from $62,000 to $76,000. The motion control system adds $50,000 to $250,000, depending on the number of axes and the required precision of the alignment. A Class I enclosure, vibration isolation, computerized alignment algorithm and system integration can add another $60,000 to $200,000 to the installed price.
One critical consideration in choosing a bonding method is the long-term mechanical stability of the bond. Bellcore tests require accelerated environmental aging trials lasting several months to confirm that performance does not change appreciably over the expected lifetime of the product. Laser-welded parts exhibit no long-term mechanical drift, but adhesively bonded and soldered joints have had trouble in this area in the past.
Figure 3. Adhesive bonding systems can provide automated curing to improve postbond shifts and long-term stability. Courtesy of EXFO.
A test can be performed that can predict how much the microscopic adhesive volumes might shrink. The test enables preadhesion engineering similar to preweld engineering. Programming the cure cycles based on accurate predictions of the shrinkage and shift of polymers, acrylics and epoxies can significantly affect the outcome of a product, ensuring that it passes Bellcore long-term stability standards (Figure 3).
Another issue in the choice of bond is manufacturing time, once all the optical components and fibers are in position. Whereas adhesive curing times range from several minutes to an hour, a laser weld takes 5 ms, and the part can be moved within 100 ms, greatly reducing production time.
The author, Julie Harrell, can be reached at (518) 265-2808; or at firstname.lastname@example.org.