160 Gbit/s optical-time-division-multiplexing (OTDM) transmitter/receiver employing electroabsorption (EA) modulators are described. In the 160 Gbit/s OTDM transmitter, the optical multiplexer, which implemented four EA modulators, is used and the generation of authentic 160 Gbit/s OTDM signal is realized. The optical multiplexer also enables to generate the phase-coded OTDM signal such as carrier-suppressed return-to-zero (CS-RZ) signal at 160 Gbit/s by changing driving temperatures of the EA modulators. In the 160 Gbit/s receiver, the EA modulator is also used in an optical demultiplexer and a phase-locked-loop (PLL) for clock extraction. As both optical demultiplexer and PLL are insensitive to polarization state of incoming signal, highly stable operation is achieved. We also show some results of transmission experiment using the developed OTDM transmitter/receiver and discuss the advantage of a switching capability of modulation format in the 160 Gbit/s signal transmission.

40 Gbit/s optical transceiver using a novel OTDM MUX module has been developed. OTDM (Optical-Time-Division-Multiplexing) MUX module, the core component of the transmitter, consisted of a optical splitter, two electro-absorption (EA) modulators and a combiner in a sealed small package. As the split optical paths run through the "air" in the module, greatly stable optical phase relation between bit-interleaved pulses could be maintained. With the OTDM MUX module, the selection between conventional Return-to-Zero (conventional-RZ) format and carrier-suppressed RZ (CS-RZ) format is performed by slightly changing the wavelength of laser-diode. In a receiver, 40 Gbit/s optical data train is optically demultiplexed to 10 Gbit/s optical train, before detected by the O/E receiver for 10 Gbit/s RZ format. Back-to-back MUX-DEMUX evaluations of the transceiver exhibited good sensitivities of under -30 dBm measured at 40 Gbit/s optical input to achieve the bit-error-rate (BER) of 10-9. Another unique feature of the transceiver system was a spectrum switch capability. The stable RZ and CS-RZ multiplexing operation was confirmed in the experiment. Once we adjust the 40 Gbit/s optical signal to CS-RZ format, the optical spectrum would maintain its CS spectrum shape for a long time to the benefit of the stable long transmission characteristics. In the recirculating loop experiment employing the OTDM MUX transceiver, the larger power margin was successfully observed with CS-RZ format than with conventional-RZ format, indicating that proper encoding of conventional-RZ and CS-RZ was realized with this prototype transceiver. In the case of CS-RZ format, the error free (BER < 10-9) transmission over 720 km was achieved with the long repeater amplifier span of 120 km.