Coherent Modulation Overview

Coherent fiber optic transmission systems, such as those supported by NeoPhotonics products, use higher order modulation (HOM) formats to transmit more information in a given slice of the optical spectrum.  These techniques are similar to those used for years in microwave communications devices, but are now able to be applied in the much higher frequency optical regime.  A modulation methodology allows information to be carried by an optical or radio-frequency wave. A few modulation schemes have been used for 100 Gb/s coherent systems, including DP-BPSK (mainly used for undersea cable systems), DP-QPSK (mainly used for long-haul terrestrial systems), and DP-16QAM (mainly used for metro/regional terrestrial systems). Here “DP” stands for “dual-polarization”, “BPSK” stands for “binary phase shift keying”, “QPSK” stands for “quadrature phase shift keying”, and “16QAM” stands for “16 quadrature-amplitude-modulation”.

QPSK uses four phases, e.g., 0, π/2, π, and 3 π /2 to transport twice as much information as BASK or BPSK, and achieves 2 bits per symbol. As a result, QPSK in combination with dual polarizations (i.e., DP-QPSK) can achieve 2×2=4 bits per symbol.  A DP-16QAM signal contains not only dual polarizations, but also both amplitude and phase modulations from symbol to symbol, and therefore further improves the spectral efficiency of DP-QPSK by two times, i.e., it has a spectral efficiency of 8 bits/s/symbol. A simple illustration is shown in the figure below, where we can see that to change from one symbol to another in a QPSK constellation diagram requires only phase variation, while to change of one symbol to another in a DP-16QAM constellation diagram requires both phase and amplitude variation.

The process can continue to higher levels of modulation, such as 32 QAM and 64QAM.  As higher orders are used the points in the constellation diagram move ever closer together and become more vulnerable to noise.  A critical contributor is phase noise in the transmit and local oscillator laser and therefore it becomes even more critical to have a narrow laser linewidth when higher order modulation is used.

The modulation formats such QPSK, 16QAM and 64QAM indicate how many bits can be conveyed in one symbol.  The second major determiner of link capacity is the symbol rate, which is also known as the baud rate.  First generation coherent systems used a baud rate of 32 Gbaud, which with overhead and QPSK modulation transmitted 100Gbps.  Current systems operate at 64 Gbaud, which gives twice the data rate of an equivalent 32 Gbaud system.  96 Gbaud systems are being introduced and 128 Gbaud is under development.