Photonics gears up for automation

Nov. 8, 2001
Despite their high-tech image, fiber-optic components get assembled largely by hand. New automation promises to bring their production into the 21st century.

A computer simulation of a laser welding system from Adept Technology Inc., San Jose, shows dual high-precision four-axis laser-pointing mechanisms that put two weld beams on packages to within ±5µm. Fiber pigtailing is one of the primary application areas.

It's not a pretty picture. Estimates are that some 50 million fiber-optic components will be in production within two years. They will go into mass-produced telecom and computer gear that is expected to be an engine of economic growth. But today, the vast majority of fiber-optic parts are put together by hand through methods appropriate for one-of-akind research gear, but which are uneconomical for volume production.

No question that manual assembly methods hike manufacturing costs and slow throughput. One side effect, say experts, is that production yields for photonic components are typically 80% at best. A typical assembler scraps as many as three units daily that cost an average of $1,000 to $2,000 each. Savings could amount to a half-million dollars yearly for each such assembler that automated methods replace.

Moreover, economies from reduced scrap and more-efficient assembly dwarf those available from taking labor out of the production process, say automation suppliers. Estimates are that assembly represents 65 to 85% of the cost of most photonics products, compared to less than 10% of costs for products in mature industries.

It is for such reasons that manufacturers of automation equipment have taken aim at photonics. One indicator of progress so far came this past September: Robotic system supplier Adept Technology Inc. revealed some of the results it has gained from five years of research into photonics assembly.

Adept says it concentrated on operations that combine motion control, vision, and high-precision alignment stage technology, and which are common to a wide variety of photonics assembly equipment. These operations include pigtailing (attaching fibers to components), device assembly, and vision-guided placement of optical components. The company has developed standard assembly products for pigtailing both active and passive components using epoxy and laser-welding bonding methods.

"We have designed piezo-actuated nanopositioners able to work in an industrial environment but with resolutions and repeatability equivalent to the highest quality lab-grade equipment," says Ray Karam, general manager of Adept's photonics operation in Santa Barbara, Calif. "You can find other actuators with the same resolution as ours, but they are too fragile for typical industrial uses where high impact loads from operators and machinery are commonplace. You can hit ours directly and they will still resolve to better than 5 nm."

The firm's technology of choice for hitting sub-five-nanometer resolution is piezoelectric actuation. Piezoactuators produce a relatively small displacement in response to an electrical signal. Versions of the devices created for use in automation can move relatively heavy loads, typically on the order of a kilogram or more, with settling times below 5 msec and no degradation in performance qualities such as repeatability.

Piezo positioning also has no problem with static friction. Stiction can greatly hamper conventional bearing and ballscrew technology working at supersmall displacements. "In applications involving the alignment of a fiber with a laser source, moving just 20 nm would change the light level noticeably and would be enough to cause significant insertion loss," says Adrian Goding, an Adept Technology technical marketing manager. "You need at least 10 nm of reliable noncross-coupled resolution to handle this chore. Conventional bearing stages would have a hard time moving just 10 nm without significant elastic stiction and parasitic off-axis motion." All in all, researchers at Adept figure stiction problems and parasitic movement coupled between axes make it tough to get repeatable movement below about 50 nm with precision bearing ways.

Nevertheless, precision ways are excellent at handling longer displacements. So some photonic assembly equipment takes the approach of putting a piezoactuator atop a motorized bearing stage. For example, the Adept NanoStage uses this scheme to provide 28 mm of travel in the Z direction while its nanopositioner travels up to 0.20 mm in X and Y.

Indications are that such an approach can handle optical positioning needs for the foreseeable future. "Piezo technology can align down to 0.1-nm resolution with the right kind of control," says Adept's Karam. "No photonics application requires better than 5-nm alignment resolution, so it is clear that photonics is not yet pushing the limits of piezo nanopositioning technology."

The feedback control that lets piezobased nanopositioners provide such performance typically incorporates proximity sensors as the feedback element. Closedloop systems employing these sensors can deliver 20-microns travel with an rms repeatability of ?8 angstroms, says Adept, and do so at room temperature without Herculean efforts at vibration isolation or thermal stabilizing.

A test case
It is interesting to examine one automated assembly task done as a technology demonstration. Researchers at Adept devised an automated station that positioned and epoxied together components comprising a fiber collimator. The collimator, used in relatively high volumes, basically converts light coming from the point source at the end of the fiber into parallel rays that interact with filters, prisms, switching mirrors, and other elements in a device.

Assembly steps include temporarily splicing-in a light emitter and detector, repeatedly tweaking optical alignment to peak the light transmission, and making adjustments as the epoxy cures. When manually assembled, as is typical today, total assembly and test time is about 30 min.

In contrast, semiautomated tools reduced assembly time to 15 min with epoxy cure time comprising the bulk of this period. (Because this effort was just for demonstration purposes, researchers made no attempt to minimize cure time.) Alignment tasks that typically soaked up 6 min of manual work finished in just 20 sec.

The Adept assembly cell uses heated chucks to hold parts, the better to control epoxy curing profiles. An active alignment process peaks light reflected from a lens to exactly align a dual-fiber capillary with a fixed component. Once light output has peaked, the two parts of the device are held fast while an applicator epoxies them together.

The collimator demonstration is semiautomatic, performing the critical autoalignment and bonding processes but requiring manual loading and unloading of parts. Equipment suppliers say semiautomatic tools of this sort can also serve as a vehicle for refining product design rules. These rules, in turn, can make possible future versions of products that lend themselves to assembly by fully automated methods.

Unfortunately, there are obstacles to be hurdled before full automation becomes practical. Rapid changes in optical technology have kept manufacturing processes from stabilizing. And there are no formal packaging or handling standards for components such as laser diodes or connections to optical fibers. Ditto for dimensional controls on these items. Finally, experts say cleanliness specs and raw material quality both need upgrades to handle full automation.

Basic tasks in laser welding

A computer model (top) and a real example of a typical laserwelding task: Mating a laser diode to a ferruled fiber and clip. The procedure is often complicated by packaging obstructions that force the necessary butt and shoulder welds to take place at an angle.

A typical laser-welding task in photonics is that of bonding in place a ferruled fiber and clip that has been aligned to a laser diode. One complicating factor is that the package walls somewhat obstruct the approach to the weld points, forcing welds to be made at an angle. Conventional laser-welding systems tend to have trouble with such applications. It is difficult for them to hold alignment during the weld process due to a lack of stiffness. The ferrule must then be laser hammered back into peak alignment. The results may be inconsistent.

The NanoBonder LWS laser-welding system has no such problems, claims developer Adept Technology Inc. Its two laser beams are vision guided and deliver ±5- m accuracy. The system employs an XYZ auto-alignment stage consisting of two piezo-based fine-alignment stages (X and Y axes) mounted atop a longtravel XYZ motorized ball-screw stage. An Adept motion and vision controller synchronizes the alignment stages, weld heads, and vision. It also lets third-party instruments communicate with the system.


See you later collimator
In studying automation methods for epoxy bonding, automation supplier Adept Technology Inc. devised an assembly station for a collimator as a test case. The collimator consists of a dual-fiber capillary, a gradient-index (Grin) lens, and two glass tubes. The station holds the Grin lens fast while moving the capillary via blind search and peak-finder algorithms that minimize insertion loss in the collimator while optimizing the output of the collimated beam. Chucks holding the piece-parts are heated to promote uniform epoxy curing.

The firm recently made its semiautomatic epoxy bonding system commercially available. It includes a closed-loop XY piezo stage sitting atop a motorized crossed roller-bearing stage, the patter providing long travel in the Z direction. The system also includes the laser source, detector, epoxy dispensing and curing system, and all accessories needed to prep, view, orient, gap, clock and bond the piece parts.




Adept Technology designed a moving chuck on its XY piezo flexure nanopositioner stage to follow a search pattern which homes in on the point of peak transmission from a fiber to a gradient index lens.


About the Author

Leland Teschler

Lee Teschler served as Editor-in-Chief of Machine Design until 2014. He holds a B.S. Engineering from the University of Michigan; a B.S. Electrical Engineering from the University of Michigan; and an MBA from Cleveland State University. Prior to joining Penton, Lee worked as a Communications design engineer for the U.S. Government.

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