Postioning VCSELs for better testing

NASA Tech Briefs, Dec 2000

Using National Instruments' LabVIEW, a MODE/Emcore team achieved a sixfold gain in testing time.

MODE/Emcore of Albuquerque, NM, manufactures vertical-cavity surface-emitting lasers (VCSELs) and the sunglass-type lenses, called attenuated caps, for these VCSELs. The goal of the manufacturing team was to develop a percentage of transmittance system for the caps that would exceed the current system's capabilities for quality testing during production. This included higher throughput by testing more fixtures, as well as increased accuracy by introducing z-axis motion, facilitating cap testing from the inside rather than outside.

Using National Instruments LabVIEW and motion control on a Windows 98-- based PC, the team moves the fixture to be tested on an x-y stage and then lifts the tester VCSEL into the cap with z-axis control. The earlier system tested one fixture of 120 caps in approximately 30 minutes. The new system was designed to test six fixtures - 720 caps - in the same period of time.

The VCSEL Cap Test Station, built by Alliance Program member B&B Technologies for MODE/Emcore in Albuquerque, is designed to test the percentage of light transmission of VCSEL caps during production. These caps, or lenses, are manufactured by applying a thin metallic surface to a glass lens and housing. The thickness of the metal determines the percentage of transmission of the laser light at a specific wavelength.

During production, this thickness must be constantly monitored for quality control purposes. To test each cap, a laser must be positioned at the cap's opening and the transmission measured by a photodetector. High-precision automated positioning tables speed up this process so users can test many caps in a short period of time. The new test plate holding six cap fixtures was designed by the team.

The test station uses LabVIEW and motion control to move an x-y stage and a z-axis controller. An NI universal motion interface sends motor signals to "amplifiers," and an NI 4-axis motion controller controls the stage movement. The machine moves through 120 positions, using a z-axis to raise a VCSEL into position. This z-axis gives the team the ability to test caps from the inside, which is how they are used in the field. Repeatability and correlation to the final product were both greatly improved. A digital multimeter from NI reads the signal from the photodetector. Using LabVIEW, in addition to light intensity measurements the team also captures information that includes lot number, fixture number, cap in test, actual transmission value, file information, and calibration factors. This information can also be dumped into a database. System advantages include the repeatability of the computer-based system - the operator can walk away, and the system takes care of the rest. Automated reporting, complete with a color map of the results, is also a plus.

The team has also added the capability to print the binning map. The operator uses this map to separate the parts. The previous tester required manual cutting and pasting of data into a macro to create the binning map.

Finally, the sorting capability of the cap fixtures helps scientists more quickly complete their tests. It is also easy to modify the application according to specific needs.

System Design

Automation Solutions of Albuquerque, a distributor for Industrial Devices Corp. (IDC), supplied positioning tables, step motors, and microstepping drives for the x, y, and z axes. The test plate mounts on a 7-in.-travel IDC CP8 precision positioning table, or y axis. The y axis is mounted to a 12-in.-travel IDC CP8 precision positioning table, or x axis. Together they are set on a precision-machined table of three-quarter-inch aluminum. The laser is mounted to a 1-in.-travel IDC CP3 precision positioning table mounted separately from the x and y axes. With the test plate extending from the x and y axes, the laser is free to move up and down underneath each of the 720 caps. A photodetector is mounted above the test plate to measure the transmission through the caps. To eliminate any ambient light that may affect the measurements, the test fixtures and positioning tables are put in a light-- proof enclosure with a sealed access door for loading and unloading the fixtures. The enclosure attaches directly to the aluminum plate, which is then mounted atop a small Hoffman enclosure that houses the computer and microstepping drives.

For more information on this application, contact Don Lifke, coauthor of this article with Chris German of B&B Technologies, at MODE/Emcore, 5741 Midway Park NE, Albuquerque, NM; e-mail: don_lifke@ emcore. com.