Automating Laser to Fiber Alignment

Author(s):
Scot Blackford - Emergent Information Technologies Inc.

Industry:
Telecommunications

Products:
Motion Control, LabVIEW,

The Challenge:
Automating the process of aligning lasers to fibers and reducing alignment time by a factor of ten.

The Solution:
Developing an automated precision alignment solution which features an intuitive pull-down menu driven front panel that integrates the customers existing motorized stages and instrumentation.

Searching for Light
The recent growth of the fiber optics industry has been phenomenal. Glass fiber cables are being laid on seabeds across the oceans to residential doorsteps. This has ignited an exponential growth in demand for components to support this expanding glass network.

One of the key components of this infrastructure is the laser to fiber assembly, or pigtail as it is referred to in the industry. When producing these assemblies it is essential that the fiber be aligned exactly at the lasers focal point for transmission of maximum optical power and to minimize signal loss. The fibers range in size, from four microns (four millionths of a meter) to a few hundred microns (smaller in diameter than a human hair). Because of the incredibly small size of these fibers, aligning them to lasers has been compared to trying to find a few specific blades of grass on a football field. Engineers and scientists have been aligning pigtails in the laboratory by manually using micrometers for several years. However, it is a time consuming, laborious process, which does not lend itself to high-volume production.

Improving Production Throughput Capabilities
Emergent Information Technologies, Inc. was contracted to develop an automated alignment solution for our customer, a leading manufacturer of electro-optical components. They had to improve its production through put capabilities to meet customer demands. Using their existing technique technicians, they were producing about five assemblies per day, provided that they could achieve alignment.

Developing an automated alignment solution required two steps. The first was to correctly position the fiber in a retaining ferrule as part of a pre-alignment process. The next step was to position the pre-aligned fiber assembly at the laser’s focal point. This required movement in the vertical, lateral, and along the optical axes. This second process, an alignment that required movement in three axes, was taking technicians up to 90 minutes to complete, and they were not always successful aligning the devices.

Pre-aligning the Assembly
For the pre-alignment phase of the assembly, we used the NI PCI 7344 motion control board to control a commercial, off-the-shelf amplifier and servo motor stage. With the LabVIEW pop-up control panel, they had intuitive submicron control of the positioning process. This automated approach dramatically improved pre-alignment accuracy and repeatability.

Automating the Laser Fiber Alignment
To automate the laser fiber alignment phase of the assembly process, we developed a system to control three microstepper motor driven stages and two piezoelectric stages. The National Instruments hardware consisted of a PCI CAN bus board, a PCI GPIB board and a PCI DIO board. Our LabVIEW-based software featured a multitiered menu driven user interface. When started, the software presents the user with a single menu pull down with three menu options - auto align, supervisor, and exit. The auto align option automatically sequences through the steps to align pigtail assemblies. The software prompts the operator to mount the laser and fiber on the stages, controls the laser, monitors the laser power meter, aligns the fiber in front of the laser, separates the stages for adhesive application, repositions the fiber at the peak power position, and prompts the operator to remove the assembly after the adhesive has cured.

Using the manual system, the technician had to manually record the laser identification, serial number, laser power, coupling loss, and alignment position. Then, another person entered it into a database. Our automated system stores all the information directly into the database saving up to ten minutes per assembly and eliminating the possibility of errors in the data. When the supervisor mode is selected, a dialog box pops up and prompts the user for a password. After successfully logging into the supervisor mode, it offers the user three menu bar pull downs featuring, utilities for homing and moving the stages, laser and power meter control functions, several alignment options and, the auto align function.

Searching for Laser Power
All laser alignment systems must define a region of interest to search for the laser power. These regions are scanned using a variety of search methods. Several of the popular methods use raster or spiral searches or variations of the two. These methods spend much scan time searching areas where there is no light. Other algorithms find power and search for a peak - this search method is prone to find false and lower power peaks rather than the peak focal point power. For this project, we developed a modified raster algorithm that greatly reduces search times, ignores false laser low power peaks, and finds the true peak laser power focal point without scanning areas devoid of any laser light.

Reduced Alignment Time and Improved Accuracy
Emergent’s alignment solution yielded not only a 30-fold reduction in alignment time, but it also produced an improvement in pre-alignment accuracy and thereby optical coupling by a factor of 28. The automated data storage routin eliminated an additional ten minutes of overhead time and removed the possibility of transcription errors. The manual pre-alignment of the fiber in the ferrule was producing an error of up to seven microns off the desired position. Our automated solution consistently produced assemblies with less than 0.5 microns (500 nanometers) from the peak power position.

Finally, the automated pigtail alignment system reduced eliminated the occurrences of the pigtails that our customer could not manually align. We achieved these benefits by making expert use of the inherent connectivity and scalability of NI software and hardware. The software reduced to the alignment time to three minutes - far exceeding design goals. The customer has since proceeded with automating several other alignments and test processes and continues to build on the LabVIEW platform using other National Instruments components.

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