Optics get temperature calibration

A 10-Gbits/s optical networking module that converts a 16-bit parallel bus into an optical signal. The optical transmitters on the module employ a directly modulated, uncooled laser that transmits signals over distances of 12 km.

Optimize the laser bias to meet performance requirements by calibrating the transmitter's modulation current and DC bias current from 0°C to 70°C. The system must generate data streams without a bit-error-rate tester.

• Agilent Technologies 86140B optical spectrum analyzer (OSA) and 34970A data-acquisition/switch unit. www.tm.agilent.com.
• Network Elements Lithium multiprotocol module. www.nei.com.
• Newport 8800 photonics test system mainframe with one 1x2 and two 1x16 optical-switch modules. www.newport.com.
• Elgar/Sorensen DLM8-75 power supplies. www.elgar.com.
• Tektronix CSA8000B communications signal analyzer with 80C08-CR1 optical sampling module. www.tektronix.com/optical.
• TestEquity 1020S temperature chamber. www.testequity.com.

Network Elements (Beaverton, OR; www.nei.com) manufactures optical networking modules used in 10-Gbits/s SONET and Ethernet networks. The company must calibrate the modulation current and DC bias current transmitted by the modules to meet Telcordia performance requirements for extinction ratio, launch power, and other parameters.

An automated system calibrates 10-Gbits/s optical transmitters–part of an optical transceiver–with respect to temperature.

At each temperature, the system sends a data stream to drive a device under test's (DUT's) transmitter. After measuring the DUT's output with a signal analyzer and a spectrum analyzer, the system adjusts the transmitter's modulation current and DC bias current until it achieves the optimal eye performance over the temperature range. It then stores those settings in the transceiver's nonvolatile memory. The transceiver's onboard processor uses those settings to set the transmitter's output.

Testing the networking modules requires engineers to generate pseudorandom bit sequences (PRBSs) with lengths up to 2³¹–1 bits, which simulates worst-case traffic content. Engineers programmed the company's multiprotocol module (see figure) to generate the PRBS test signal for bit rates of either 9.95 Gbits/s for SONET or 10.3 Gbits/s for 10 Gigabit Ethernet. A computer sets the PRBS data stream's length through a LAN.

A Newport 8800 optical test system switches the data stream to any of 16 DUTs in the temperature chamber. Each DUT uses its optical receiver to convert the optical PRBS data stream into a 16-line electrical bus, which a test fixture then loops back to the DUT's optical transmitter.

Another optical switch in the 8800 selects one DUT's optical output signal and routes it to another switch that sends the transmitter's output signal to either the signal analyzer or the OSA. The signal analyzer displays an eye diagram from which it measures extinction ratio, launch power, and mask margin. Upon receiving the measurements, the PC decides if it needs to adjust the DUT's modulation and bias currents. To make the adjustments, the PC uses its parallel port to generate the I²C protocol, which lets it communicate new settings to the DUT. The process repeats until the system finds the optimal modulation and bias currents for all 16 DUTs at each of the three temperatures.

Test engineering manager Fred Barbee says that Network Elements saved more than $150,000 when test engineers incorporated the company's multiprotocol module in the test system. The savings come from not buying a 10-Gbits/s bit-error-rate tester to generate the PRBS test signals. In addition, parallel tests on the modules reduce the average temperature ramp and settling time per DUT. Barbee estimates an overall throughput increase of approximately 8X over single-unit testing.