Automation cuts calibration time

A 43-channel power monitor for dense wavelength division multiplexing (DWDM) optical networks. The device uses an optical waveguide to steer the channels into 43 photodiodes, where each optical signal is converted into an electrical current proportional to power.

Develop a calibration station for production that measures the spectrum of each channel across the C-band. The station must calibrate each device based on an optical signal of known power and must monitor and control the DUT's temperature. The system must notify an engineer after each calibration.

• Agilent Technologies:
• lightwave measurement system with tunable laser source; polarization controller. www.tm.agilent.com.
• dBm Optics:
• component spectrum analyzer (CSA) with power/wavelength reference option, optical reference channel, and photodiode measurement card. www.dbmoptics.com.
• Keithley Instruments: source-measurement meter; switch system; thermo-electric cooler (TEC) temperature controller. www.keithley.com
• National Instruments: graphical programming software. .

Optical power-monitoring devices must be calibrated to accurately convert light into current. Test and measurement specialist Eric Desfonds at MetroPhotonics (Ottawa, ON, www.metrophotonics.com) developed a station that calibrates the DUT's output based on known inputs.

An automated system calibrates a 43-channel DWDM optical power monitor by supplying it with a known power and wavelength.

Two low-capacitance, low-inductance cable assemblies carry the 43 electrical-current signals and a common return to the switch system. The switch system scans the DUT's 43 outputs and routes them to a photodiode measurement card in the component spectrum analyzer (CSA), which provides the appropriate bias voltage and measures the photodiode current.

The CSA measures current at the four polarization states from which it extracts an optical spectrum and calculates the DUT's measurement errors. In addition, the CSA measures the DUT's leakage current under operating voltage bias. Because it knows how much power is reaching the DUT, the CSA can calculate the DUT's response for each wavelength.

Prior to the addition of the CSA, the calibration station included an analyzer that couldn't measure electrical signals. So, the source-measure meter measured the 43 photodiode currents. Desfonds now uses the meter to monitor the DUT's temperature only.

After measuring the photodiode currents when the photodiodes are illuminated and when dark, the CSA creates a file with the DUT's calibration data and saves it to an internal hard drive. It then sends the file to a network disk drive using FTP. Upon completing a calibration, the CSA sends an e-mail to the operator or engineer, indicating that it is ready to calibrate another DUT.

The station reduced calibration time by 50%—from 60 min to 30 min—over its previous configuration while improving accuracy and repeatability. Many of these improvements come from the station's automated correction for laser-power fluctuations, its internal biasing mechanism, and the elimination of operator interventions during the calibration procedure.

"Part of the speed improvement also came from connecting the CSA directly to the network," reports Desfonds. "By sending the data directly to the network using FTP rather than by downloading through the IEEE 488 port to the PC, we bypass the slower IEEE 488 link." The multichannel capabilities of the CSA may let him further reduce test time by testing more than one DUT channel at a time.