First demonstration of a C + L band CDC-ROADM with a simple node configuration using multiband switching devices.

While ultrahigh-baud-rate optical signals are effective for extending the transmission distance of large capacity signals, they also reduce the number of wavelengths that can be arranged in a band because of their wider bandwidth. This reduces the flexibility of optical path configuration in reconfigurable optical add/drop multiplexing (ROADM) networks. Particularly in colorless, directionless and contentionless (CDC)-ROADM, the effect reduces the add/drop ratio at a node. Multiband ROADM systems are an effective countermeasure for overcoming this issue, but they make the node configuration more complicated and its operation more difficult. In this paper, we analyze the challenges of C + L band CDC-ROADM and show that optical switch devices that operate over multiple bands are effective in meeting them. For this purpose, we built a C + L band CDC-ROADM node based on C + L band wavelength-selective switches and multicast switches and confirmed its effectiveness experimentally. In particular, to simplify the node configuration, we propose a reduction in the number of optical amplifiers used for node loss compensation and experimentally verify its feasibility.

Optical frequency distribution using laser repeater stations with planar lightwave circuits.

We report a cascaded optical fiber link which connects laboratories in RIKEN, the University of Tokyo, and NTT within a 100-km region using a transfer light at 1397 nm, a subharmonic of the Sr clock frequency. The multiple cascaded link employing several laser repeater stations benefits from a wide feedback bandwidth for fiber noise compensation, which allows constructing optical lattice clock networks based on the master-slave configuration. We developed the laser repeater stations based on planar lightwave circuits to significantly reduce the interferometer noise for improved link stability. We implemented a 240-km-long cascaded link in a UTokyo-NTT-UTokyo loop using light sent from RIKEN via a 30-km-long link. In environments with large fiber noise, the link instability is 3 × 10-16 at an averaging time of 1 s and reaches 1 × 10-18 at 2,600 s.

Dual-carrier IQ modulator with a complementary frequency shifter.

We devised a dual-carrier IQ modulator consisting of a novel complementary frequency shifter (CFS) and two IQ modulators. The CFS generates two optical subcarriers from a single light source and outputs them from separate ports without using any optical demultiplexing filters. We fabricated the modulator in a hybrid configuration of silica planar lightwave circuits and a LiNbO(3) phase-modulator array. Frequency-spacing-tunable operation of the CFS was demonstrated. With the modulator, we successfully generated a 100-Gb/s/pol dual-carrier OFDM-QPSK signal.