PDM-QPSK Modulator With a Hybrid Configuration of Silica PLCs and LiNbO$_{3}$ Phase Modulators

We discuss detailed characteristics of a polarization-division-multiplexed quadrature-phase-shift-keying (PDM-QPSK) modulator with a hybrid configuration of silica planar lightwave circuits (PLCs) and LiNbO₃ (LN) phase modulators. With a polarization multiplexing circuit consisting of a half-wavelength plate (HWP) and a Mach-Zehnder-type polarization beam combiner (PBC) in a PLC, the modulator outputs a PDM-QPSK signal with a inter-polarization-channel crosstalk smaller than -40 dB. The modulator also has a small insertion loss of 5.0 dB (7.7 and 8.4 dB for each polarization), and successfully operates at 100 Gb/s.

I. Introduction

The rapid increase in IP data traffic has accelerated the research and development of optical transmission technologies to further increase capacities in wavelength-division-multiplexed (WDM) networks [1]–[7]. Polarization-division multiplexing (PDM), in which two orthogonal polarization are used as two independent optical carriers, is one of the essential technologies for achieving such large-capacity transmissions because PDM can double the spectral efficiency (SE) compared to the conventional single-polarization modulations. Another key technology for achieving large SE and large capacity is multilevel modulation with advanced formats, such as N-level phase-shift keying (N-PSK), N-level quadrature amplitude modulation (N-QAM), and orthogonal frequency-division multiplexing (OFDM). DP-16 QAM was used to achieve the record capacity of 69.1 Tb/s with a SE of 6.4 b/s/Hz [6]. On the other hand, PDM-quadrature-PSK (PDM-QPSK) is the leading candidate for the upcoming commercial deployment of 100-Gb/s/ WDM systems [1], [7].