Introduction

Intent

At the optical interfaces within wavelength-division multiplexed (WDM) networks, it is desirable to measure parameters that provide information about the integrity of the physical plant. Such parameters are necessary to monitor network performance as an integral part of network management. They are also necessary to assure proper system operation for installation and maintenance of the network.

Ideally, such parameters would directly correspond to the bit error ratio (BER) of each channel of a multichannel carrier at the particular optical interface. Related parameters such as Q-factor or those calculated from optical eye patterns would provide similar information, that is, they would correlate to the channel BER. However, it is difficult to obtain access to these parameters at a multichannel interface point. It is necessary to demultiplex the potentially large number of channels and make BER, Q-factor, or eye-diagram measurements on a per-channel basis.

In contrast, useful information about the optical properties of the multichannel carrier is readily obtained by measuring the optical spectrum. Wavelength-resolved signal and noise levels provide information on signal level, signal wavelength, and amplified spontaneous emission (ASE) for each channel. Spectral information, however, does not show signal degradation due to waveshape impairments resulting from polarization-mode dispersion (PMD), and chromatic dispersion. Also, intersymbol interface and time jitter are not revealed from an OSNR measurement. In spite of these limitations, OSNR is listed as an interface parameter in ITU-T draft Recommendation G.692; "Optical interfaces for multichannel systems with optical amplifiers." It is also proposed that OSNR be listed in ITU-T draft recommendation G.959.1, "Optical networking physical layer parameters."

This OFSTP provides a parameter definition and a test method for obtaining optical signal-to-noise ratio (OSNR) using apparatus that measures the optical spectrum at a multichannel interface. Three implementations for an optical spectrum analyzer (OSA) are discussed: a diffraction-grating-based OSA, a Michelson interferometerbased OSA, and a Fabry-Perot-based OSA. Performance characteristics of the OSA that affect OSNR measurement accuracy are provided.