Characterization, Measurement and Specification of Device Imperfections in Optical Coherent Transceivers

Abstract

Transceiver imperfections become the primary source of impairment as baud rate and modulation order grow in advanced optical coherent communications. Thus, transceiver imperfections, both linear and nonlinear, need to be appropriately characterized, measured, and specified. Treatments for linear imperfections are relatively mature. This study reviews the transceiver's linear imperfection modeling, characterization, and measurement technologies. In practical applications, in-field measurement using the transceiver and a few low speed additional devices is preferred. In the case of nonlinear imperfections, the situation is complex. One important task is to estimate the nonlinear system performance from the nonlinear characteristics of the devices. In this study, we attempt to establish a connection between them by examining different technologies. Although the orthogonal component has a good correlation with nonlinear system performance, its measurement is prohibitively complex. In the nonlinear noise to power ratio measurement, a certain frequency component of the input signal is notched, and the re-growth component at the notch frequency is measured at the nonlinear device output. The ratio between the re-growth component power and the output signal power is the noise to power ratio. While this method is easy to carry out, it does not correctly estimate nonlinear impairment in general. The reason for this is that the signal incurs different nonlinear responses in two conditions, i.e., with or without a notch. This method accurately estimates the nonlinear impairment in some special but useful cases, such as Gaussian input signals and nonlinear systems whose dominant nonlinear term is the even order term.

Characterization of Nonlinear Distortion in Intensity Modulation and Direct Detection Systems

“…Fig. 7 shows the equivalent additive noise model, which was established to estimate the system performance [12]. In this study, we established two models.…”

“…The second is the same orthogonal model, where the equivalent linear model is described by the tap coefficient 𝑔(𝑘) and an orthogonal component with a timecycle shift of 1000 symbols is used as the equivalent additive noise. This implies that the additive noise and nonlinear system input are independent [12]. To focus on the nonlinear effect, we assumed that the receiver includes an ideal linear equalizer.…”

“…This method requires only a spectrum analyzer; hence, it is a costeffective candidate compared to orthogonal component analysis. However, NPR measurement by notch is not a general method and only works correctly for Gaussian inputs or some special nonlinear systems with non-Gaussian inputs [12]. In some cases, the measurement error is as large as 8 dB [13].…”

Digital Four-Dimensional Transceiver IQ Characterization

“…Various techniques for IQ-impairment mitigation have been investigated and are summarized in a previous study [7]. When considering IQ impairments on only transmitter (Tx)-or receiver (Rx)-side IQ impairments, using a well-known widely linear (4×2) AEQ [8] can be a solution, but impairments of the opposite side remain.…”

Nonlinear noise spectrum measurement using a probability-maintained noise power ratio method

“…"non-specific" means that the system nonlinearity contains more than even-order components 32,33 . Thus, the NPR method is severely restricted to cases with Gaussian stimuli or the systems with special nonlinearity 32,33 . It cannot be used for non-specific communication systems based on pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) signals 45,47 , which is the main reason why the NPR method has faded out over the years.…”