Abstract
We discuss the implications of using monolithically integrated semiconductor lasers in high capacity optical coherent links suitable for metro applications, where the integration capabilities of semiconductor lasers make them an attractive candidate to reduce transceiver cost. By investigating semiconductor laser frequency noise profiles we show that carrier induced frequency noise plays an important role in system performance. We point out that, when such lasers are employed, the commonly used laser linewidth fails to estimate system performance, and we propose an alternative figure of merit that we name “Effective Linewidth”. We derive this figure of merit analytically, explore it by numerical simulations and experimentally validate our results by transmitting a 28 Gbaud DP-16QAM over an optical link. Our investigations cover the use of semiconductor lasers both in the transmitter side and as a local oscillator at the receiver. The obtained results show that our proposed “effective linewidth” is easy to measure and accounts for frequency noise more accurately, and hence the penalties associated to phase noise in the received signal.
Highlights
Laser frequency and phase noise play a major role in the performance of current optical coherent transceivers
In order ensure that only phase noise from the transmitter laser is present in the system, we model all of the components ideally, and set the standard single mode fiber (SSMF) length to 0
We have proposed an alternative figure of merit for the transmitter laser based on a phenomenological model of carrier induced frequency noise and system experiments of the induced penalty as a function of the laser resonance frequency and damping
Summary
Laser frequency and phase noise play a major role in the performance of current optical coherent transceivers. It is common to evaluate CPR algorithms in terms of optical signal-to-noise ratio (OSNR) penalty versus the laser linewidth times the symbol period of the transmitted signal (∆υ τ) [2,3,4] It is only under the assumption of spectrally flat white frequency noise that the laser linewidth is directly proportional to the power spectral density (PSD) of the frequency noise [5]. When considering monolithically integrated semiconductor lasers, the PSD of the frequency noise is no longer flat due to carrier induced frequency noise, and the relationship between linewidth and frequency noise is no longer trivial [7,8] In this case, laser linewidth poorly indicates system performance degradation due to phase noise, disqualifying it as a suitable figure of merit to benchmark CPR algorithms.
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