Dual-Carrier Dual-Polarization IQ Modulator Using a Complementary Frequency Shifter

We discuss the detailed characteristics of a dual-carrier dual-polarization (DP) in-phase-and-quadrature (IQ) modulator. As the dual-carrier generator, we used a complementary frequency shifter (CFS), which generates two frequency-spacing-locked optical subcarriers and outputs each of them from different output ports without any optical demultiplexers. We fabricated the modulator, which integrates a CFS, a quad-parallel IQ modulator, and a polarization-multiplexing circuit, with a hybrid configuration of silica planar lightwave circuits and a LiNbO₃ chip. With the fabricated modulator, we demonstrated that the CFS enables us to flexibility change the relative and absolute optical frequencies of the subcarriers with a high isolation ratio of about 40 dB. We generated a 400-Gb/s dual-carrier DP 16-level quadrature amplitude modulation (16QAM) signal by driving the modulator with high-speed digital-to-analog converters.

I. Introduction

To cope with the ever-increasing demands for data transmission capacity, technologies to achieve high per-channel data rates and high spectral efficiencies (SEs) in optical wavelength-division multiplexing (WDM) transmission systems are being investigated very actively [1]–[8]. While current systems with a per-channel rate of 100 Gb/s employ single-carrier dual-polarization quadrature phase-shift keying (DP-QPSK), future systems with per-channel rates of 400 Gb/s or higher will probably use multicarrier (or superchannel) approaches because achieving such high rates only by increasing baud rates and/or modulation levels with a single-carrier signal seems to be difficult. Actually, most recent large-capacity transmission experiments have been performed by using high-order modulation formats, such as quadrature amplitude modulation (QAM), and multicarrier transmission technologies, such as orthogonal frequency-division multiplexing (OFDM) or Nyquist wavelength-division multiplexing (Nyquist WDM), in combination [1]–[6].