High-Speed Precision Alignment of Fiber Optic Components

Author(s):
Kerry Quinn - SES Technology Integration Group

Industry:
Telecommunications

Products:
LabVIEW, Vision, Motion Control

The Challenge:
Reducing the cycle time for a precision optical device assembly system from 115 seconds to less than 60 seconds.

The Solution:
Developing a new system that entirely eliminates the need to perform the most time consuming portion of the alignment process.

LabVIEW Simplifies Component Alignment
Many of the most time-consuming fiber optic device assembly tasks involve the precise alignment of components such as lasers, fibers, fiber arrays, waveguides and detectors. Often alignment resolution in the sub-micron and arc-second range is sought. Historically, many devices were aligned by skilled operators, who manually tweaked precision alignment stages. When the industry began switching to automated alignment systems, it was natural to create an automated system, which mimicked the manual systems, which had been in use previously. Generally the manual alignment process consisted of these steps:

1.  Insert the devices to be aligned into grippers
2.  Perform a visual coarse alignment
3.  Perform a two dimensional search for initial light
4.  Perform an optimization process to maximize the amount of coupled light
5.  Bond or weld the aligned pieces together
6.  Remove the completed device from the grippers

In adhesively bonded systems, steps three and five typically consume the majority of the total time. The bonding step, step five, is set by the adhesive and adhesive curing method and often cannot be improved substantially. The duration of the initial light search, step three, in both manual and automated systems is largely a function of how close to optimally aligned the two parts are at the end of step two.

The size of most optical components and the accuracy with which they must be positioned requires that any vision alignment system be conducted under significant magnification. As the magnification increases, the depth of the field in focus shrinks. In addition, many optical components are cylindrical or spherical. It is often the case that the diameter of components is greater than the depth of field, and as a result it, can become very difficult to obtain accurate measurements of these devices using vision-based systems. It is also often difficult to obtain a good line-of-sight to components in complex assemblies and even more difficult to provide adequate illumination to the area of interest. Finally, it is often the case that there are significant part-to-part variations in the relationship between the physical aspects of the device and the position and/or direction of the device optical path.

The flexibility afforded by LabVIEW opens up an entirely new way of approaching the problem, offering, in many cases, a very significant performance gain.

Old Approach
1. Measure the physical location of the components
2. Align the physical locations of the components with each other under the assumption that this will place the device optical paths in approximate alignment
3. Perform a raster or spiral search to bring the optical paths into alignment

New Approach
1. Measure the position and direction of the actual optical path for each device
2. Move immediately to the position where the actual optical paths are aligned

Significant Time Savings
In the system described in the introduction, the approximate average breakdown of cycle time was:

• Initial search for light 55 seconds
• Final optimization of alignment 9 seconds
• Adhesive application and cure 30 seconds
• Operator time to remove and install next component 21 seconds

SES' approach was to develop a low-cost (relative to the cost of the primary alignment systems) system to measure the position and direction of the actual optical paths for each component. In this device, a single mode fiber was aligned to a focused laser source. The laser and fiber ferrules were pre-assembled into small nickel blocks. The new measurement system worked by directing the light emitted from the laser or from the fiber onto a CCD sensor connected to a National Instruments IMAQ board. Typical CCD sensors have pixel sizes on the order of a small number of microns. Within LabVIEW, it is quite simple to determine where the optical path is pointed by interrogating the pixel array to find the pixel of maximum intensity.

At the same time, the position of the laser or fiber block is accurately measured using a non-contact laser LVDT probe (fig 1). Our probe, from Keyence Corporation has a measurement resolution of 0.1 micron, a measurement "spot size" of 2 microns, and a depth of field of several hundred microns, but a wide range of resolutions, as low as 10 nm is available from Keyence. These devices can provide either RS-232 or analog output to a standard DAQ card and can be easily controlled using a set of dry-contact closures, such as on the NI SC-2064 board. The probe is scanned over the laser or fiber block using a 1micron resolution XYZ stage controlled with an NI FlexMotion 7344 card. Using data acquisition and motion cards from National Instruments provides a speed advantage because the LVDT readings can be synchronized with position measurements using the PXI or RTSI triggering bus.

Comparing a pair of measurements at different distances between the CCD and device and the change in the location of the maximum intensity pixel, it is possible to calculate the location and direction of the optical path relative to the physical location of the blocks.

In this application, the measurement data was stored into an Access database, along with the serial number of the laser or fiber block. When parts are to be assembled in the primary alignment system, the measurement data is retrieved from the database. A laser probe, similar to the one described above is used to find the physical location of the block within the precision alignment system. Finally, a simple transformation calculation is quickly performed to determine the exact translations and rotations required to bring the optical paths of two devices into alignment. This new process requires 2-3 seconds and eliminates the original 55-second search for light step.

The open architecture and wide variety of measurement and control technologies accessible through LabVIEW provides an opportunity for optical device manufacturers to significantly increase production rates. This is particularly true in cases where a novel combination of tools creates an entirely new manufacturing approach, rather than simply making incremental changes to an existing approach.

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