Hiroto Kawakami’s research while affiliated with Nippon Telegraph and Telephone and other places

In this paper, we experimentally and theoretically show the improvement in noise characteristics in a distributed Raman amplifier (DRA) system for wavelength division multiplexing (WDM) transmission, utilizing our proposed pumping technique. We show that forward (Fwd) pumping is clearly superior to backward (Bwd) pumping in terms of noise figure (NF) defined by amplified spontaneous emission (ASE) noise and gain. We also show that bi-directional pumping is a more desirable configuration for NF improvement. However, it is known that Bwd pumping is preferable to Fwd pumping in suppressing signal quality degradation caused by nonlinear optical effects, especially the interaction between signal and pump light. To compare these advantages and disadvantages of Fwd pumping, we conducted WDM transmission experiments using a recirculating loop including DRA and erbium doped fiber amplifiers. We measured the Q-factors of 9-channel 131.6-GBaud polarization division multiplexed probabilistically shaped 32-quadrature amplitude modulation signals while changing the ratio of Fwd pumping to Bwd pumping in the DRA. By introducing the previously proposed low-noise Fwd pumping technique, a higher Q-factor could be achieved with a lower signal launch power, even when the total Raman gain remained constant. A Q-factor improvement of 0.4 dB and signal launch power reduction of more than 3.4 dB were simultaneously achieved. We also show that when Fwd pumping was performed with a conventional pump light source, the disadvantage of Fwd pumping was more noticeable than its advantage.

In this paper, we discuss the beat-noise induced by a distributed Raman amplifier system with forward pumping and propose a technique to suppress it. To construct a Raman amplifier, several techniques are required. First, the pump light must be depolarized due to the polarization dependence of the Raman gain. Second, the intensity noise of the pump light must be suppressed to stabilize the Raman gain. Due to the fast response of Raman amplifiers, especially forward-pumped, the instability of the pump power induces relative intensity noise (RIN) in the amplified signal light. To achieve depolarization, an orthogonally polarized pump light emitted from two different laser sources was widely used in conventional Raman pump units. These sources often have a fiber Bragg grating (FBG) to lock their wavelengths. In this paper, we show that a FBG can induce fluctuations in Raman gain. We also show that an orthogonally polarized pump laser light can induce beat noise in the amplified signal. We propose a pump unit that can suppress this beat noise. We measure the RIN of the amplified light using our proposed technique and demonstrate its advantages over a conventional Raman pump unit. Finally, we present experimental results of an optical data transmission with a forward-pumped Raman amplifier. Utilizing our proposed noise suppression technique, the signal-to-noise ratio of the probabilistically-shaped 36-QAM signal after a 1,920-km transmission is improved compared with the results using a conventional Raman pump unit.

We demonstrate 50-Tb/s WDM transmission within 6.25-THz band over 240-km SMF using a novel hybrid inline repeater consisting of PPLN-based OPAs and incoherent-forward-pumped DRA. 1-Tb/s/lambda PS-64QAM channels are transmitted within each the S/C-band or C/L-band.

We investigate the random intensity noises that occur in forward-pumped distributed Raman amplifier systems. First, we show pump-to-signal intensity noise transfer characteristics, which strongly depend on the group velocities of pump light and signal light in optical fiber used as a gain medium. When signal light is in the C-band, dispersion shifted fiber (DSF) transfers much larger noise compared with standard single mode fiber. Next, we discuss the origin of the noise induced in pump light. We define the concept of synthesized polarization and show that fluctuation in the state of synthesized polarization (SOSP) can induce a large gain instability even if the relative intensity noise (RIN) of each pump laser source is negligible experimentally. Next, we propose a novel optical depolarizer for pump light. It can simultaneously depolarize pump light generated by multiple laser sources. Moreover, it can manage optical phases precisely to suppress the gain instability induced by the fluctuation in SOSP. Finally, we present a measurement of the Q factor of a 16-QAM signal (32 Gbaud) after 35-km transmission, with and without a forward-pumped Raman amplifier. Two pump laser sources with RINs of 134 to 126 dB/Hz were depolarized by the proposed depolarizer, and the generated on-off gain was 7 dB. Though amplified signal light was in the C-band and the gain medium was DSF, 16-QAM transmission specifications were successfully improved, and acceptable noise was observed for 5 hours operation.

We demonstrate the acquisition of signal power evolution and Raman gain spectra (RGS) in multi-span Raman-amplified links with digital backpropagation. We successfully identify the individual RGS of each Raman-amplified span separately.

We have achieved the longest three mode-multiplexed full C-band transmission yet attained over 3060 km. In wideband mode-multiplexed transmission over weakly-coupled fewmode fibers (FMFs), the width of signal impulse responses is dependent on wavelength, and also exhibits a linear growth with increased transmission distance because of the presence of differential mode delay (DMD). Those properties are technically challenging issues for future deployable space division multiplexing (SDM) transport systems since they make a system design complicated, especially for terrestrial FMF transmission links. In this paper, we describe how we applied a cyclic mode permutation (CMP) technique to achieve wideband long-haul FMF transmission where spatial channels are cyclically interchanged in each span to suppress DMD-induced pulse broadening. The CMP technique enabled DMD-unmanaged long-haul transmission across 4.4-THz optical bandwidth over two-mode FMF whose DMD varied in the range from 33.7 to 44.3 ps/km, resulting in 3060-km FMF transmission with a net capacity of 40.2 Tb/s.