by Kerry Quinn
The use of automation in optical-device assembly, inspection, and test processes continues to lag even recent, scaled-back projections of demand. Automation is often touted as the key to increased production yields and device quality, and is seen as a necessary step to drive down manufacturing costs. Yet, semi- and fully automated systems have not gained more than limited market penetration.
Two major obstacles are delaying the widespread adoption of automation. First, many automation schemes are still too slow and costly to be attractive from a return-on-investment standpoint. Second, the optical-device assembly industry has not yet produced industry-standard device form factors and manufacturing cell commonalities such as those in the printed circuit and semiconductor fields. The lack of industry standards results in manufacturers delaying automation projects to avoid large financial commitments to an approach that could end up being incompatible with the eventual industry standard. In fact, these two obstacles are coupled: the cost, performance, and return on investment (ROI) would likely improve if more standards existed, and more standards would likely be developed if the ROI were sufficient to promote broader use of the technology.
Eventually, high-performance industry-standard equipment will be widely available. Meanwhile, choosing flexible, reconfigurable, and extensible software and hardware tools can mitigate the risks of large capital hardware commitments. Flexible, open-architecture tools offer the benefits of lower initial acquisition costs, the ability to fine-tune and continually optimize the system performance through software and/or hardware modifications, and the ability to adapt the system to future industry standards if necessary. Specific advantages can be realized in three areas of software and computing hardware for optoelectronic device assembly and test.
High-performance motion control
Many inspection and manufacturing processes require precise, high-performance motion control. Unfortunately, many precision-stage manufacturers have sacrificed stage performance in favor of ease of use. It is common to find motion controllers that operate using RS-232 or GPIB protocols. While it is relatively easy to develop software to move these stages, the system ends up limited in both determinism and speed. Under this approach, the software typically sends a short text-string command across the communication bus. The stage decodes the command and then starts the move. The exact time at which the stage starts or reaches a target position is not known in advance, nor is it precisely repeatable. This can present several problems, including difficulties in precisely coordinating the motion between stage stacks, accurately mapping the optical power of a device, and returning to a precise position.
Coordinating multiple-stage stacks is common in high-performance alignment of fiber arrays and waveguides. Optical-power mapping is important in device research and quality control applications. A common procedure in optical-device alignment is to perform a high-speed scan while monitoring the level of coupled optical power. Key to this approach is the ability to know precisely the stage coordinates at the time that the peak coupled power level is reached. The nondeterministic nature of the communications across the data bus severely limits the potential performance of protocol-based motion-control systems in these types of applications.
Alternatively, precise, high-performance motion control is possible through the use of PCI-based PC-card motion controllers, such as those from DeltaTau (Northridge, CA), Acroloop (Chanhassen, MN), National Instruments (Austin, TX), and others. Each of these solutions provides state-of-the-art control over a variety of motion technologies including stepper, servo, and piezo motors with deterministic control and feedback capabilities.
Often the performance gains resulting from simply discarding a protocol-based motion controller in favor of a PC-card motion controller are sufficient to turn a system that is unattractive from an ROI standpoint into an attractive one. This approach has the added benefit of moving the overall system away from proprietary hardware toward a more flexible and upgradeable software-based solution.
Flexible software platform
One example of the advantages of a flexible system for optical-device assembly is highlighted by the vision and motion precision integrated robot (VAMPIRe) alignment system. We recently developed a custom VAMPIRe system to align two fiber arrays and an intermediate waveguide for a customer in Asia. This system is built around the National Instruments LabVIEW software, the PCI/PXI motion controller, and data-acquisition hardware. The motion controllers and data-acquisition boards can be connected to each other to share synchronization-timing signals. In this mode, it is possible to simultaneously measure coupled optical power and the precise stage coordinates while the positioning stages are in high-speed motion. While this is useful for quickly and accurately aligning optical components, the same hardware can be used, with only slightly different software, to create an optical-power map showing the distribution of optical power over the entire beam path.
We used the same hardware-with the addition of a laser-height gauge-and further modified LabVIEW programming to perform noncontact prealignment of fiber arrays and waveguides without the expense, complexity, and limitations found in traditional vision-system-based prealignment systems. The significance of this approach is that a single set of hardware can be re-used, with appropriate software, to perform a variety of tasks common to the development, manufacture, and quality control of optoelectronic devices.
Hexapod robot
In addition to flexible software, it is also important to develop systems that use flexible hardware solutions. We were recently asked to assist a client in developing the kinematics and control software for one such system.
A hexapod robot offers a combination of attractive features for many optical-engineering applications including high precision, high speed, high stiffness, and the option of a programmatically relocatable center of rotation (see figure). It is very helpful to be able to position the system center of rotation at optically meaningful locations-for example, at the focal point of a lens or at a specified position along the optical axis of a device.
Here again, LabVIEW software offered a good combination of a powerful mathematics engine to drive the kinematics equations, a graphical front-end useful for system operators as well as for use in software debugging and powerful synchronization tools necessary to guide and coordinate the motion of all six legs of the system. The end result was a maneuverable and customizable precision motion platform that could be used for a wide variety of optical-device assembly and inspection tasks.
KERRY QUINN is an engineer at the Rocky Mountain office of SES Technology Integration, 6560 Odell Place, Suite G, Boulder, CO 80301; e-mail: [email protected].
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