1 . 6 bit / s / Hz , 6 . 4 Tbit / s OCDM / WDM ( 4 OCDM x 40 WDM x 40 Gbit / s ) transmission experiment

We demonstrate the transmission of 6.4 Tbit/s OCDM/WDM (4 OCDM x 40 WDM x 40 Gbit/s) using only C-band wavelength region. A record of 1.6 bit/s/Hz spectrum efficiency is achieved by QPSK optical encoding/ decoding along with optical time-gating and optical thresholding in addition to polarization multiplexing. Introduction To meet the rapid increase of the demand for multi-terabit/s transmission capacity, the spectrum efficiency will become a key issue for the full utilization of limited wavelength resources. Some approaches have demonstrated such as spectrum efficiency of 1.28 bit/s/Hz using vestigial sideband filtering with polarization multiplexing /1/, and that of 0.8 bit/s/Hz polarization interleave multiplexing /2/. Optical code division multiplexing (OCDM) is another candidate for the ultimate spectrum efficiency. OCDM provides unique attributes such as asynchronous transmission, a potential of communication security, soft capacity on demand, and high degree of scalability /3/. We have demonstrated a 1.52 Tbit/s OCDM/WDM link with 0.4 bit/s/Hz spectrum efficiency /4/. Here, we report a record of 1.6 bit/s/Hz spectrum efficiency OCDM/WDM transmission experiment by applying the quaternary phase shift keying (QPSK) optical encoding/decoding /5/ accompanied by ultrafast optical time-gating and optical thesholding for interference noise suppression. As a result, 6.4 Tbit/s OCDM/WDM (4 OCDM x 40 WDM x 40 Gbit/s) transmission using only Cband wavelength region is experimentally demonstrated by simultaneous multi-wavelength optical encoding of a single supercontinuum (SC) source /4/. To the authors’ knowledge, this transmission capacity is a record of OCDM/WDM transmission experiment. Operation principle As shown in Fig. 1(a), time spread QPSK pulse codes are used as the optical codes and optical transversal filters are used as optical encoders and decoders. The encoded signal is time-despread by the decoder, which is used as the counterpart of the optical encoder. The output shows the correlation waveform, that is, the matched filtering response in the time domain. When the receiver’s optical code is matched with the transmitter’s optical code, the auto-correlation waveform having a sharp peak at the center is observed. In the case of unmatched codes, on the other hand, the cross-correlation waveform is formed. Furthermore, in order to achieve the strict sense of high process gain, optical time-gating must be introduced to extract only the mainlobe of auto-correlation. As a result, the interference noise arising from interference codes existing outside the time-frame of mainlobe greatly reduces /3,4/. In addition, by the introduction of thresholding in the optical domain, the interference noise existing inside the time-frame of mainlobe also greatly reduces. The transversal filter consists of tunable taps, 5 ps delay lines, programmable quaternary optical phase shifters, and a combiner. An optical code of 3-chip QPSK pulse code sequence with a chip interval of 5 ps is generated. The impulse response of the optical encoding has a periodicity of 200 GHz, because the chip interval of optical encoder is 5 ps, as shown in Fig. 1(b) /4/. As a result, simultaneous multi-wavelength encoding having the WDM channel spacing of 200 GHz can be achieved using a single optical encoder when a broadband optical pulse such as a SC pulse are used as a chip pulse /4/. Experiments and discussions Figure 2 shows the experimental setup of 6.4 Tbit/s OCDM/WDM (4 OCDM x 40 WDM x 40 Gbit/s) transmission. A 10 GHz, 1.5 ps pulse trains of the modeFigure 1: (a) Principle operation of QPSK codeOCDM, and (b) 200 GHz periodicity in impulse response of optical encoding in the frequency domain. Optical encoder Tunable tap 5 ps delay Phase shifter (QPSK) Combiner Optical transversal filter Optical decoder Optical time-gating QPSK Optical code [0], [1] ,[2], [3] [0],[π/2],[π],[3π/2] Optical Carrier Phase Optical Thresholding