Digital coherent optical communication systems: fundamentals and future prospects

Abstract

Abstract:The recently-developed digital coherent receiver enables us to employ a variety of spectrally-efficient modulation formats such as M -ary phase-shift keying (PSK) and quadrature-amplitude modulation (QAM). Moreover, in the digital domain, we can equalize linear transmission impairments, which may stem from group-velocity dispersion (GVD) and polarization-mode dispersion (PMD) of fibers for transmission, because the phase information is preserved after coherent detection.This paper reviews the history of coherent optical communications, the principle of coherent detection, and the concept of the digital coherent receiver. After that, we discuss digital signal processing (DSP) for mitigating transmission impairments, coherent transmission characteristics of multi-level optical signals, and future prospects of coherent optical communications. Keywords: coherent optical communications, multi-level modulation, homodyne detection. Classification: Fiber-optic communication Lett., vol. 16, no. 5, pp. 179-181, Feb. 1980. [4] Y. Yamamoto, "Receiver performance evaluation of various digital optical modulation-demodulation systems in the 0.5-10 μm wavelength region," IEEE J. Quantum Electron., vol. QE-16, no. 11, pp. 1251-1259, Nov. 1980 Express, vol. 16, no. 2, pp. 804-817, Jan. 2008. [15] H. Sun, K.-T. Wu, and K. Roberts, "Real-time measurements of a 40-Gb/s coherent system," Opt. Express, vol. 16, no. 2, pp. 873-879, Jan. 2008. Lightwave Technol., vol. 29, no. 4, pp. 417-425, Feb. 2011. [18] References

Optical carrier extraction from carrier-less phase modulated optical signals

“…2. The power is normalized so that its peak value at LO ¼ 0 is the same as that evaluated by (2). The numerically evaluated carrier power agrees well with that obtained analytically.…”

“…Although post-detection carrier phase estimation in the digital domain using a free-running local oscillator (LO) is widely favored in current coherent transmission systems [2], the original approach to homodyne detection using a carrier-phase synchronized LO still has advantages such as potentials of lower-power consumption and compact receiver structures. The optical carrier extraction is also needed in coherent signal processing such as phase-sensitive amplification [3].…”

Pre-Equalizing Electro-Optic Modulator Utilizing Polarization-Reversed Ferro-Electric Crystal Substrate

“…In order to compensate for the signal distortion resulting from the chromatic dispersion effect, the digital coherent method utilizing high-speed analog-to-digital (A/D) converters and digital signal processors has been developed and implemented [5]- [8]. This technique is robust and applicable for any modulation format including vector modulation schemes.…”

High-performance compound-semiconductor integrated circuits for advanced digital coherent optical communications systems

“…Then, 40-Gb/s  40-wavelength (or -channel) systems were deployed, and the total transmission capacity reached 1.6 Tb/s. To cope with the increasingly rapid growth of communications traffic, innovative 100-Gb/s digital coherent technology [1,2], which combines coherent detection and digital signal processing (DSP), has recently been introduced. Digital coherent technology enables us to employ a variety of spectrally efficient modulation formats such as phase shift keying (PSK) and quadrature amplitude modulation (QAM).…”