For major telecommunications carriers and service providers, it is time for a move to 100G coherent optical for long-haul and ultra-long-haul dense wavelength division multiplexing (DWDM). The rapid growth of high-bandwidth applications like on-demand video and cloud-based services conspire to make carrying capacity an absolute top priority. For now, the solution is to squeeze more spectral efficiency out of existing fiber. To accomplish this, carriers are looking beyond conventional on-off keying (OOK) to coherent dual-polarization quadrature phase-shift keying (DP-QPSK), quadrature amplitude modulation (QAM), and many variations of orthogonal frequency-division multiplexing (OFDM).
With coherent lightwave techniques moving from R&D to manufacturing and production deployment, many development laboratories still content themselves with in-house coherent receivers and analysis software, often coupled to adaptive equalizers, to maximize eye openings under all conditions. While this approach is important for receiver development, it may frequently miss critical sources of signal distortion in the transmission system, and the slow response of these systems may prohibit rapid identification of root causes of failure.
It is therefore crucial to have characterized, calibrated, and repeatable analysis methods and equipment. However, many of the test standards needed to ensure interoperability, such as the eye masks commonly used for serial data communications systems, remain undefined for the current leader, the DP-QPSK format, which is sanctioned by the Optical Internetworking Forum (OIF). Nevertheless, testing to a specific Q factor can at least allow cross-checking of component performance. It is vital to devise a test strategy that can be adapted to accommodate different coherent modulation schemes as the various standards and technologies evolve over the next several years.
When designing and deploying coherent long-haul fiber transmission systems, it is important to ensure that the coherent optical transceiver achieves predictable bit-error-rate performance and repeatable Q factors. In this article, we look at test tools, in particular coherent lightwave signal analyzer technologies, which make it possible to discover and mediate impairments of a physical design whether at R&D, manufacturing, or during deployment.
Understanding What Went Wrong
In any transmission system, the ability to determine “what went wrong” when a transceiver fails in production or in the field is critical to success. Unfortunately, the conventional method of direct detection is insufficient for the measurement of phase-modulated signals. For instance, a photodiode used in the traditional way, as a receiver set up to detect OOK or amplitude modulation, will respond with all ones when presented with a phase-modulated signal in which the optical carrier is modulated in phase but not in amplitude. As a result, conventional eye-diagram analyzers are not recommended in their current configuration because they cannot be used to plot conventional quantities.