The light and the distance

Engineers at C-Cor developed a system to verify the performance of optical transmitters and receivers at distances up to 1500 km.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 12/1/2005

PROJECT PROFILE: C-Cor Access and Transport Group

Optical components that operate at 10 Gbps and are used in metro and regional networks to carry data, voice, and video.

Verify that optical components will reliably transmit and receive data over 1500 km of optical fiber. Characterize the devices for optical signal-to-noise ratio (OSNR) and nonlinear effects to find the maximum reliable transmission distance if devices are unable to operate at 1500 km.

• Agilent Technologies: digital communications analyzer (DCA), multiwavelength meter, optical attenuator. www.tm.agilent.com.
• Ando (now part of Yokogawa): optical spectrum analyzer. www.yokogawa.com.
• Anritsu: pattern generator/bit-error-rate tester. www.us.anritsu.com.
• C-Cor: optical transmitters and receivers and erbium-doped fiber amplifiers. www.c-cor.com.
• ILX Lightwave: laser driver. www.ilxlightwave.com.
• Newport: laser driver. www.newport.com.
• Peleton: multiwavelength laser source. www.peleton.com.
• Spirent Communications: performance analysis system. www.spirentcom.com.

Project Description

When C-Cor Access and Transport Group (Wallingford, CT; www.c-cor.com) acquired a line of optical transmitters and receivers, engineers needed to verify their performance at distances up to 1500 km. "We needed to understand the limitations, if any, of our components," said chief technologist Zoran Maricevic.

To characterize the components, Maricevic's team measured how optical signal-to-noise ratio (OSNR), chromatic dispersion, and nonlinear effects caused degradation in performance. They developed a test bed that simulates a transmission line where self-phase modulation (SPM), cross-phase modulation (XPM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS) cause an increase in OSNR and a corresponding decrease in the bit-error rate (BER).

Maricevic developed a test bed (figure) that contains 1500 km of optical fiber with erbium-doped fiber amplifiers (EDFAs) every 80 km. He made BER and OSNR measurements on each of 40 of 44 possible DWDM channels while keeping the other channels "dark." The measurements let him verify that the components worked and provided a measurement baseline. The components operate on optical channels 16–59 (191.6–195.9 THz). Next, Maricevic ran a frame-error test, using the performance-analysis system to send and receive Ethernet packets.

A test bed lets engineers measure how nonlinear effects on DWDM transmissions reduce transmission distance and increase bit errors.

After verifying that single-channel transmissions met requirements, Maricevic ran tests with all channels turned on. He used C-Cor's optical transport modules to generate signals in 20 DWDM channels and used the multiwavelength laser source for the remaining channels. Nonlinear effects from adjacent channels cause BER and OSNR degradation.

Light from a digital "1" can degrade a transmission in the transmission's channel, causing SPM, or it can interfere with transmission in neighboring channels, causing XPM. XPM causes 6 dB more interference than SPM. The engineers measure XPM in a channel near the center of the frequency band, a location at which XPM interference occurs from both above and below the channel's frequency. "You also need to run live traffic on at least the two most adjacent channels on either side of the channel under test," explained Maricevic. He performed his tests on channel 39 (193.9 THz) with live traffic from the performance analyzer on channels 37, 38, 40, and 41. The remaining channels carried either live traffic or unmodulated light from the laser source.

C-Cor engineers learned the value of having clean eye diagrams at transmitter outputs (measured with the DCA). They also learned that you must make optical spectrum measurements everywhere in an optical transmission line. The company reports that the performance of its PLEXiS platform exceeded customer requirements. "I can't overemphasize availability of quality, reliable test gear for gathering test results and verifying all the assumptions made," said Maricevic.