Hirokazu Kubota

Supercontinuum generation at 1.55 µm in a dispersion-flattened polarization-maintaining photonic crystal fiber

We demonstrate the generation of symmetrical supercontinuum of over 40 nm in the 1.55 m region (1540 - 1580 nm) by injecting 1562 nm, 2.2 ps, 40 GHz optical pulses into a 200 m-long, dispersion-flattened polarization-maintaining photonic crystal fiber. The chromatic dispersion and dispersion slope of the fiber at 1.55 m are -0.23 ps/km/nm and 0.01 ps/km/nm2, respectively. This is the first report of 1.55 m band supercontinuum generation in a dispersion-flattened and polarization-maintaining photonic crystal fiber.

Ultrahigh-speed long-distance TDM and WDM soliton transmission technologies

Recent progress on time-division multiplexed (TDM) and wavelength-division multiplexed (WDM) soliton transmission is described, in which dispersion management (DM) plays an important role in increasing the power margin and the dispersion tolerance. The characteristics of the DM soliton are compared with those of return-to-zero (RZ) and nonreturn-to-zero (NRZ) pulses. With a small dispersion swing, the system can still be described as an average soliton with a nonlinear Schrodinger equation (NLSE), whereas with a large dispersion swing, the soliton-like steady-state pulse becomes a chirped Gaussian pulse, in which the master equation is closer to a linear Schrodinger equation (LSE) with a parabolic potential well. An in-line modulation scheme up to 80 Gb/s per channel and its two-channel WDM transmission over 10000 km are described. A 640-Gb/s (40 Gb/s/spl times/16 channels) WDM soliton transmission over 1000 km is also reported with a DM single-mode fiber, without the use of in-line modulation. Finally, dark soliton transmission at 10 Gb/s over 1000 km is described as a different nonlinear pulse application.

Recent progress in soliton transmission technology

Recent progress on time-division multiplexed (TDM) and wavelength-division multiplexed (WDM) soliton transmission is described, in which dispersion management plays an important role in increasing the power margin and the dispersion tolerance. The characteristics of the dispersion-managed soliton are compared with those of return to zero and nonreturn to zero pulses. With a small dispersion swing, the system can still be described as an average soliton using the nonlinear Schrodinger equation, while with a large dispersion swing, the solitonlike steady-state pulse becomes a chirped Gaussian pulse, in which the governing equation is closer to a linear Schrodinger equation with a parabolic potential well. We describe an in-line modulation scheme for up to 80 Gbit/s per channel and its two channel WDM transmission over 10 000 km. Finally, we describe 640 Gbit/s (40 Gbit/sx16 channels) WDM soliton transmission over 1000 km with a dispersion-managed single-mode fiber. (c) 2000 American Institute of Physics.

Soliton transmission control in time and frequency domains

Soliton transmission control techniques in both the time and frequency domains designed to enable ultra-long-distance soliton transmission are described in detail. Soliton transmission control in the time domain, which can be realized by synchronous modulation, is a retiming technique which removes jitter and nonlinear interaction between adjacent solitons. In addition, a transfer function reduces noise and the noise power eventually converges to a low level for any transmission distance. Soliton transmission control in the frequency domain, which can be realized with a bandpass optical filter, stabilizes the soliton pulse. A million-kilometer transmission experiment confirms the usefulness of these techniques. >

Random evolution and coherence degradation of a high-order optical soliton train in the presence of noise

The intrinsic evolution of a high-order soliton described by the nonlinear Schrödinger equation is initiated by a self-four-wave mixing effect (or modulational instability) and recurs neatly every soliton period. We show that when there is noise such as amplified spontaneous emission, however, a high-order soliton evolves randomly and independently and is distorted because the evolution is initiated by noise. Thus the time and the frequency coherence of a soliton pulse train are both greatly degraded.

YBa2Cu3O7−δ trilayer junction with nm thick PrGaO3 barrier

We have established a deposition process of high quality a axis oriented YBa2Cu3O7−δ (a-YBCO) and insulating epitaxial PrGaO3 (PGO) films to fabricate a-YBCO/PGO(2.0–3.2 nm)/a-YBCO trilayer junction. The precipitate formation on the bottom a-YBCO was greatly suppressed by the atomic layer modification of the substrate surface with a wet etching and successive atomic layer epitaxy of SrO and BaO atomic layers prior to the YBCO deposition. Crack formation and residual stress in the film due to the thermal expansion mismatch along c axis of YBCO could be eliminated by inserting a buffer layer of a-YBCO deposited with changing the substrate temperature from 580 to 735 °C. The junctions showed a clear hysteresis with its current jump as large as 30%, together with the Fraunhofer diffraction.

Long-distance optical soliton transmission with lumped amplifiers

A soliton transmission method using lumped amplifiers is presented which is named the preemphasis method. An N=1 soliton with relatively high amplitudes (1.2 >

Femtosecond optical soliton transmission over long distances using adiabatic trapping and soliton standardization

A new technique for achieving subpicosecond–femtosecond optical soliton communication over long distances is proposed. The technique uses adiabatic soliton trapping and soliton standardization in an active transmission line with a finite optical-gain bandwidth. Forces destructive to femtosecond pulse propagation, such as soliton self-frequency shift and third-order dispersion, can be completely compensated for by the bandwidth-limited optical gain. Then soliton amplitude and width (i.e., area) are fixed to a certain value that indicates an N = 1 soliton, and the soliton is standardized by trapping. The technique will be a key technology for achieving ultrahigh-bit-rate (>100 Gbit/s) optical transmission systems.

Partial soliton communication system

Abstract Erbium-doped fiber amplifiers make it possible to construct simple soliton transmission systems with lumped amplifiers. We propose a combined dispersion-compensation dynamic soliton communication method which extends the amplifier spacing to more than 100 km. Pulse broadening which is caused by the decrease in nonlinearity due to optical fiber loss can be completely compensated for by employing positive group velocity dispersion at every optical repeater.

Femtosecond erbium‐doped optical fiber amplifier

A gain of 9 dB has been obtained in an erbium‐doped optical fiber amplifier for input pulses with a peak power of 30 W, an average power of 30 μW (−15.2 dBm), and a pulse width of 260 fs. When the input peak power is increased in the soliton power regime, an adiabatic soliton narrowing occurs due to the optical gain. By increasing the pump power further, an optical gain enhanced soliton self‐frequency shift dominates, which is due to the excitation of high‐order soliton. Thus, soliton narrowing from 250 to 60 fs is observed.