Impact of Phase Noise and Compensation Techniques in Coherent Optical Systems

Abstract

Abstract-One of the most severe impairments that affect coherent optical systems employing high-order modulation formats is phase noise due to transmit and receive lasers. This is especially detrimental in uncompensated links, where an ideal compensator for channel distortions and laser phase noise should first eliminate receive phase noise, then equalize channel distortions, and only later compensate for transmit phase noise. Unfortunately, the simultaneous presence of transmit and receive phase noise makes very difficult to discriminate between them, even in the presence of a pilot tone. Moreover, the picture is different for optical systems using single-carrier or orthogonal frequency division multiplexing, where transmit and receive phase noise components may have a different impact. All these aspects are analyzed and discussed in this paper. A novel digital coherence enhancement (DCE) technique, able to significantly reduce the phase noise of transmit or receive lasers by using an interferometric device plus a very simple electronic processing, is also described. The performance of this technique and the statistical properties of the residual phase noise are analytically derived and verified by simulations, showing a high increase of the maximum bit-rate-distance product. The practical implementation of DCE is finally discussed and some alternative implementation schemes are presented.Index Terms-Coherent detection, group velocity dispersion (GVD), optical communication, orthogonal frequency division multiplexing (OFDM), phase noise.

Analytical BER performance in differential n-PSK coherent transmission system influenced by equalization enhanced phase noise

“…[ [26][27][28][33][34][35][36]. Among these methods, the digital coherence enhancement shows the best performance [26,36], while it requires a complicated hardware implementation to measure the LO laser phase fluctuation.…”

“…Among these methods, the digital coherence enhancement shows the best performance [26,36], while it requires a complicated hardware implementation to measure the LO laser phase fluctuation. By contrast, EEPN does not exist in the coherent transmission systems using optical dispersion compensation, such as dispersion compensating fibers (DCFs) and dispersion compensating modules (DCMs) [18,19,22,25,37,38].…”

“…I. Fatadin et al have also studied the influence of the EEPN in quadrature phase shift keying (QPSK), 16-level quadrature amplitude modulation (16-QAM) and 64-QAM coherent transmission systems [24]. Meanwhile, the influence of EEPN on different CPE algorithms has also been investigated in detail by using numerical simulations [25][26][27][28][29]. Due to the EEPN, the requirement of laser 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 linewidth can not be generally relaxed for the transmission system with a higher symbol rate.…”

Digital Coherence Enhancement Enabling 6-GBd DP-64QAM Using a 1.4-MHz Linewidth Laser

“…The signal from the photodiodes was digitized and resampled to 12 GSa/s. The performance of the linewidth measurement is principally limited by the interferometer bandwidth, the receiver bandwidth and the photodetector noise [10]. Fig.…”

“…A method for independently measuring and digitally compensating the phase noise of the LO laser was first proposed in [9] and was also where the term 'Digital Coherence Enhancement' was first introduced. This technique was subsequently verified for long-haul transmission in [10]. In these works, the process was demonstrated for a standard Distributed Feedback (DFB) laser.…”