Divide and conquer optical measurements
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2003
A wavelength monitor unit (WMU) that measures optical wavelength and power in dense wavelength division multiplexing (DWDM) optical-network transmitters. A WMU contains two photodiodes that detect a laser's optical output and a thermistor that a transmitter uses to monitor the laser's temperature.
THE CHALLENGEDevelop a test system that measures the thermistor and photodiode outputs over a range of temperatures and wavelengths. Temperatures range from 0°C to 70°C. Wavelengths range from a DWDM channel's center wavelength ±1 nm to the center wavelength.
THE TOOLS- Agilent Technologies 8164 tunable laser source, 81560A optical attenuator, 8153A optical multimeter, 34970A data-acquisition/switch unit, and E3631A DC power supply. www.tm.agilent.com .
- Burleigh WA1000 optical wavelength meter. www.exfo.com.
- National Instruments PCI-6032E and PCI-6033E multifunction data-acquisition cards and LabView graphical programming language. www.ni.com.
- Thermotron S-8C temperature chamber. www.thermotron.com.
Laser diodes in DWDM systems require precise wavelength control because temperature and age affect wavelength. To keep the wavelength within limits, transmitters contain wavelength monitor units (WMUs). A major WMU component manufacturer contracted Bloomy Controls (www.bloomy.com) of Windsor, CT, to develop a test system that could characterize new WMUs in both development and production.
To test a WMU, the system (see the figure) produces an optical signal of a known wavelength and power and applies that signal to the DUT in a temperature-controlled environment. The tester measures the thermistor's resistance to verify that it accurately measures the known temperature. It also measures the current outputs of two photodiodes. The system simultaneously tests 32 WMUs under numerous temperatures, wavelengths, and power levels.
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An automated system tests wavelength monitor units from DWDM transmitters in both engineering and production. |
"When coupled to a laser, each photodiode sits behind a thin-film bandpass filter that provides two spectrally sensitive current outputs," explains Peter Blume, president of Bloomy Controls. "The photodiode currents are proportional to the laser's power transmitted through each filter's pass band. The laser channel's power is related to the sum of the two photodiode currents and the laser's wavelength is related to the ratio of the two currents."
To generate the known inputs, the system uses a tunable laser source that sweeps the signal's wavelength across each WMU's center frequency. An optical tap diverts 5% of the signal power to an optical wavelength meter, which measures the tunable laser's output wavelength. Erbium-doped fiber amplifiers (EDFAs) amplify the laser in the C band (1530–1570 nm) and L band (1570–1610 nm), respectively.
An optical attenuator adjusts the amplified laser signal and diverts 5% of the signal power to an optical multimeter, which measures the power. An optical splitter divides the test signal into 32 separate fibers for input to 32 WMUs. A temperature chamber controls the WMUs' temperature.
The system measures the thermistors' resistances and photodiode's output currents with two data-acquisition cards. A 32-differential-channel card measures the thermistor resistances and an eight-channel card uses two of its channels to measure the photodiode outputs. After the switching unit selects a pair of photodiodes, transimpedance amplifiers convert the photodiodes' output currents into voltages that the data-acquisition card can measure.
RESULTSThe system helped the manufacturer bring a new product from prototype to high-volume production in just nine months. Bloomy engineers accomplished the goal by automating tests performed in the engineering lab. They reused much of the code written for engineering evaluations when the product was ready for production. Today, the system tests more than 15,000 WMUs per year.
Martin Rowe, Senior Technical Editor, m.rowe@tmworld.com
