16 for the ROADM
A manufacturer of reconfigurable optical add/drop multiplexers integrated a system that tests and calibrates up to 16 modules at a time.
Martin Rowe, Senior Technical Editor. -- Test & Measurement World, 10/1/2007
PROJECT DESCRIPTION
Nistica (Bridgewater, NJ; www.nistica.com) designs and manufactures ROADMs used in telecom switches produced by several manufacturers. Telecom carriers use ROADMs in the switches to divert DWDM channels to network nodes. A ROADM mounts on a network-interface board that controls the module through an RS-232 or I2C port.
A ROADM is essentially a tunable notch filter that removes a channel from a DWDM stream and diverts it to a network node. It can also add channels back into the stream. Each channel is approximately 800 pm in wavelength; a ROADM must remove a channel from the stream without affecting remaining channels. Nistica tests and calibrates each ROADM to ensure that its output—the diverted as well as the pass-though channels—operates at wavelengths and power levels that are compatible with network equipment.
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A production test system for reconfigurable optical add-drop multiplexer (ROADM) modules tests and calibrates each module for wavelength and output power. |
Nistica’s production test stations consist of optical switches and a swept-wavelength test system, which includes a swept-signal source and a detector (figure). The optical source sweeps through the C band, while the detector measures power, wavelength, insertion loss, and group delay in 3-pm increments. The calibration process produces calibration factors for the ROADM that adjust its optical output for wavelength and power. Optical switches let the system test and calibrate up to 16 ROADMs at a time.
At the start of each test, a technician inserts a piece of optical fiber into the circuit, creating an optical “short circuit.” The optical source sweeps through the C band, producing light at a known optical power. The detector measures the power every 3 pm, producing a set of reference values as a function of wavelength. Then, the technician removes the fibers and inserts the ROADMs into place. A repeat measurement reveals the difference: insertion loss, which can be as high as 90 dB before calibration. The system also measures group delay at each wavelength.
During a calibration, the test system controls the device under test’s (DUT’s) input-signal power as it sweeps across the C band and measures the output power of the removed and adjacent channels at 3-pm increments. Calibration occurs with the DUT in an oven at a known temperature ranging from –5°C to +65°C. Having acquired data at each wavelength, the system calculates calibration constants that correct the ROADM’s output power and wavelength. Calibration aligns the ROADM’s channel wavelengths to those expected by other network equipment. The system then stores each ROADM’s calibration constants in the appropriate ROADM.
LESSONS LEARNED“Tracking of the swept input signal is critical,” said Nistica CTO Tom Strasser. “3 pm is the practical wavelength limit of the test equipment in terms of resolution.” The system spends 10 s at each measurement point, and with measurements performed at 3-pm increments, there are about 25,000 sets of measurements across the C band. Strasser also pointed out that calibration is important to ensure that a ROADM sufficiently removes a DWDM channel with minimal effect on adjacent channels.
| DEVICE UNDER TEST
Reconfigurable optical add-drop multiplexer (ROADM) modules used in dense-wavelength division multiplexing (DWDM) communications networks. The modules mount on network-interface boards, and they can remove a channel destined for a specific network node. The modules operate in the C band (1520–1570 nm) on 45 channels. A ROADM can remove up to eight channels from a DWDM stream and add them back to the network. THE CHALLENGETest and calibrate ROADMs following manufacture to ensure proper optical output power, wavelength, and bandwidth. Test for insertion loss, polarization-dependent loss, and chromatic dispersion under controlled temperature conditions. Perform tests on up to 16 ROADMs at once. THE TOOLS
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