Gratings/OTDR combo monitors networks


Wavelength-division multiplexing requires monitoring the optical power and wavelength in each channel and the ability to measure the linked fiber status. Methods of monitoring single-wavelength optical networks measure the fiber distributed loss, and if a fault occurs, they describe the type and location of a fault, but are not sufficient on their own for monitoring WDM networks.

Researchers Chien Chung Lee, Ta Chun Kao, and Shien Chi at the Institute of Electro-Optical Engineering, National Chiao-Tung University (Taipei, Taiwan) devised a method that builds on an optical time-domain reflectometer (OTDR), integrating with it a series of fiber Bragg gratings and an acousto-optic switch (AOS).1

An optical probe pulse at 1310-nm is generated by the OTDR, which has an output power of 23 dBm, pulsewidth of 100 ns, and spatial resolution of 2 m. The probe pulse is split by the 1:2 coupler and injected into two paths (see figure). One path travels through a 450-m-long delay fiber and then into the fiber link under test. Reflections back to the OTDR provide information about the fiber distributed loss, fault type, and fault location.

The second path is sent to the system that monitors the optical power and wavelength in each channel. The pulse in the second path passes through an optical circulator and variable optical attenuator (VOA) before being detected by a receiver and converted into an electrical pulse. This triggers a pulse generator, which has a tunable delay time and pulsewidth. The optical pulse produces an AOS control signal.

The optical signal from a WDM network (simulated by a modulator and pulses from four distributed-feedback lasers, operating at wavelengths from 1549.32 to 1555.75) is sampled by the AOS with 300-ns duration. An optical circulator directs this signal into a series of fiber Bragg gratings, separated by 70 m of delay fiber.

One fiber Bragg grating is centered on each channel wavelength. The gratings have 99.95% reflectivity and a 0.5-dB bandwidth of about 0.36 nm. Because the channels are reflected back to the circulator at different gratings, this introduces a staggered time delay for the different channels. The signal is directed into the OTDR.

The OTDR detects both signals from both paths, and can clearly differentiate them. The experiment demonstrated optical power monitoring with a dynamic range of 26 dB and an accuracy of at least ±0.1 dB for each WDM channel.

For more information contact Chien Chung Lee at

Yvonne Carts-Powell


  1. C. C. Lee, T. C. Kao, and S. Chi, IEEE Photon. Tech. Lett. 13 (9), 1026 (September 2001).
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