Customer Solutions
LabVIEW and PXI Automate a Filter Test System
Author(s):
Peter A. Blume, Bloomy Controls Inc.
Industry:
Telecommunications
Product:
LabVIEW, PXI/CompactPCI
The Challenge:
Automating a labor-intensive dense wavelength division multiplexed (DWDM) filter test to increase test throughput and remove human error.
The Solution:
Using National Instruments LabVIEW, PXI modules, and motion control hardware to create an automated filter test system that increases test throughput by five to 10 times.
Defining Thin-film Coatings
Precision Optics Corp. manufactures high-quality optical thin-film coatings, including narrow bandpass filters used extensively in DWDM optical networks. Thin film coatings are microscopically thin layers of dielectric materials such as metal oxides applied to the surface of a material, typically glass, to alter its optical properties. Thin films alter the transmission and reflection of light at varying wavelengths. For DWDM use, the coated glass is a high-reflectance surface at all wavelengths except at a single wavelength band less than 1 nm wide. In this band, called the pass band, the reflection typically is 10 percent for a transmission of 90 percent.
Filter Characteristics
DWDM filters are fully characterized by their wavelength-dependent insertion loss. Insertion loss is the total optical power loss due to the insertion of the filter in a light wave. Insertion loss is computed in decibels as 10*log10 (P1/P2), where P1 is the input optical power and P2 is the output optical power. Wavelength-dependent insertion loss is commonly evaluated as a wavelength (nm) versus loss (dB) curve.
In past manual testing procedures, the original test fixture consisted of an optical spectrum analyzer (OSA) with two four-axis stages containing a total of eight positioning micrometers. The operator had to configure the OSA by directly interacting with the instrument’s panel. Then, the operator carefully placed a single filter in the fixture, aligned a collimated broadband light source with the filter, and aligned a collimated detector with the transmitted light wave. This process was tedious, time consuming, and subject to operator care and skill level.
To automate the filter test procedure, Precision Optics contracted our company, Bloomy Controls, an NI Select Integrator. Precision Optics provided its experience with thin film coatings production and manual test processes, and we contributed extensive experience in designing automated test equipment for the communications industry. The two companies collaborated to develop an automated test system consisting of a light wave measurement instrument, an innovative mechanical fixture, a PXI control system, and LabVIEW software.
The light wave measurement instrument contains a wavelength tunable laser source and two optical power meters. We chose the laser source for its excellent wavelength accuracy, stability, and fast settling time. The two optical power meters measure the power of light reflected from and transmitted through the filter under test, respectively.
The test fixture holds a tray containing a grid of up to 238 DWDM filters. Each filter sets on a nest containing an aperture for passing transmitted light. Three axes of motion control position the tray under the light source to an incremental filter nest position. Two actuators position a collimator that precisely aligns the light wave perpendicular to the filter. A wide area detector measures the transmission power.
The control system comprises PXI hardware and LabVIEW software, including a PXI-1002 chassis containing a PXI-8176 embedded controller and two PXI-7334 motion controllers. The PXI-1002 is a four-slot PXI chassis that provides industrial-grade cooling in a small package that is rack-mounted in a 19-in. rack. The PXI-8176 controller contains a Pentium III processor that runs the filter test software on a Microsoft Windows 2000 operating system. The PXI-7334 is a low-cost, four-axis motion control module that controls the stepper motors through the MID-7604 and MID-7602 amplifiers. The control system communicates to the light wave measurement system through GPIB interface.
To protect the motion wiring, we developed a special chassis for housing the wiring accessories and each MID drive. Specifically, each motor contains power, encoder, and limit switch signals integrated in a single molded cable with a 25-pin sub-D connector. The MID drive is only available with screw terminals, which expose delicate wire connections. The MID drive chassis provides strain relief, wiring terminals, pull-up resistors, grounding and shielding, and D-style connectors in a robust enclosure. Hence, the motor cables are fastened directly to the chassis via enhanced DB25 connectors, and the chassis distributes the signals to the proper MID drive screw terminals.
We developed the filter test software using LabVIEW 6.1. LabVIEW provides a user-friendly graphical interface and controls all instrumentation. The user interface contains separate screens for operator and engineering access. The operator-level screen is a very simple interface that allows the user to enter the center wavelength and number of filters to test and initiate automatic testing by clicking the “Measure” button. The password-protected, engineering-level screens configure the test equipment. Specifically, the “Define Tray” screen provides the flexibility to specify the layout of the tray, including number of rows, number of columns, and row and column spacing. The “Define Scan” screen allows the user to configure the laser sweep parameters such as the output power, wavelength span, and wavelength step size.
Automatic Test Sequence
The operator first loads a tray of filters into the fixture and closes the fixture door. Then, the operator enters the channel wavelength and clicks the “Measure” button.
The software first verifies proper XY placement and alignment of the tray to ensure measurements are performed exactly in the center of each filter. Then, the software performs a calibration. This consists of a wavelength scan with nothing interrupting the path between the source and the detector. The calibration scan represents the loss due to the test equipment and is subtracted from each filter’s transmission scan to obtain the insertion loss due to the filter under test.
Next, the software positions the fixture to align the source aperture with the center of a filter nest. A vacuum is switched on to pull the filter to the source aperture. The software aligns the source to the filter by tipping and tilting the collimator until the emitter is perpendicular to the filter surface within approximately one minute.
Once aligned, the software performs the transmission scan and insertion loss is calculated and displayed on a wavelength (nm) vs. loss (dB) graph. A text file saves the data and the measurement sequence is repeated for each filter in the tray.
Increasing Throughput
The automatic DWDM filter test system reduces the operator’s involvement to loading the tray of filters and initiating the automatic test sequence. As a result, one technician can simultaneously monitor two automated testers. Because the throughput of the automated test system is 150 filters per hour, the net throughput of the automated test system has increased by a factor of five times per station and 10 times per operator while improving wavelength accuracy and eliminating human error.
For more information, contact:
Peter Blume
Bloomy Controls Inc.
839 Marshall Phelps Rd.
Windsor, CT 06095
Tel: (860) 298-9925
E-Mail: peter.blume@bloomy.com
