Guide the light

Optical triplexers used to transmit data and receive data, voice, and video in fiber-to-the-home applications. The devices transmit light on 1310 nm and receive on 1490 nm and 1550 nm. Wavelengths carry data at 1.25 Gbps.

Test the filter optics while the device is still in unpackaged form to verify that the fabrication process is within tolerance. Filter optics are inaccessible on complete devices, requiring special on-wafer devices just for testing the fabrication process.

• Agilent Technologies: polarization controller, tunable laser, broad-area optical detector, digital communications analyzer, and bit-error-rate tester. www.tm.agilent.com.
• Cascade Microtech: microprobes. www.cmicro.com
• JDS Uniphase: optical modulator. www.jdsu.com.
• Keithley Instruments: nanoammeter. www.keithley.com.
• Melles Griot: fiber alignment stage. www.mellesgriot.com.
• Suss MicroTec: microprobes. www.suss.com.
• Polytec PI: fiber-alignment stage. www.polytecpi.com.

Engineers at Enablence (Ottawa, ON, www.enablence.com) need to test optical triplexers for design verification and in production. While they test fully packaged devices, they also test unpackaged devices to characterize optical filters. Production tests on optical filters let engineers verify that the manufacturing process is within tolerance.

To run the optical-filter tests, engineers build devices onto wafers that contain only the optical components. "Electronic components such as detectors can't be on the devices for these tests because they limit our access to the optical filters," said Enablence director of R&D Ashok Balakrishnan. "For testing purposes, we replace the optical detector with a multimode waveguide to steer the filtered light to the edge of the chip."

THE figure illustrates an unpackaged device used for test. The multimode waveguide must be wide enough to capture all of the filtered light, and its optical-index contrast must be high enough to steer the light to the edge of the chip while preserving it. Upon reaching the chip's edge, the light must pass to the multimode fiber with minimal loss, before it then travels to an external detector. The acceptance angle of the output fiber must be greater than that of the output waveguide, and the aperture of the broad-area detector must be larger than the core diameter of the multimode fiber. A characterized and calibrated detector converts the filtered light to an electrical signal for bit-error rate (BER) measurements and eye-diagram tests with a digital communications analyzer (DCA). A tunable laser supplies incoming light, and the modulator digitally modulates the light.

Engineers use a multimode waveguide and multimode fiber because attaching a single-mode fiber to the DUT's output requires tight alignment—the light's diameter is just 5 ìm. Single-mode fibers have core diameters of about 10 ìm, but multimode fibers have core diameters ranging from 50 ìm to 200 ìm. Thus, multimode fibers are easier to align to the DUT. Enablence uses six-axis stages to align the input and output fibers to the chip.

In addition to testing optical filters on unpackaged ICs, Enablence engineers test fully packaged ICs and unpackaged devices that have optical detectors in place of the waveguides. For testing packaged devices, they use a modulator, a BER tester, and a DCA. Because the devices are complete, engineers can use a test board and fixture. When a device has a detector but is unpackaged, engineers use microprobes to gain access to the device, and they use a nanoammeter to measure its photocurrent at low frequencies.

Enablence engineers found that active elements such as lasers and detectors contain passive optical filters that compound their test problems. Customers rely on tests of fully packaged devices, but Enablence must obtain other details of device performance. Engineers must design-in methods of testing at the more detailed levels. Multimode waveguides and fibers work well to bypass the electrical output of an integrated device.

Multimode fibers let engineers test optical filters in unpackaged devices.