Kohichi Tamura

Fundamentals of stable continuum generation at high repetition rates

A continuum generated from highly nonlinear seed pulses (N/spl Gt/1) propagating in a medium with only self-phase modulation (SPM) or with SPM and anomalous dispersion is highly sensitive to the noise of the input pump pulse. The combination of SPM and normal dispersion improves the stability. However, more efficient spectral broadening schemes are desirable for generating a broad-band continuum at gigahertz rates. The adiabatic compression of weakly nonlinear pulses (N/spl sime/1) via the soliton effect efficiently generates a broad-band continuum that is robust against noise. Detailed characterization of continuum generation in several different fibers is reported.

640-Gbit/s optical TDM transmission over 92 km through a dispersion-managed fiber consisting of single-mode fiber and "reverse dispersion fiber"

A 640-Gbit/s optical time division multiplexed signal was successfully transmitted over a 92-km zero-dispersion-flattened transmission line. The transmission line consisted of single-mode fiber, dispersion-shifted fiber, and reverse dispersion fiber. By using reverse dispersion fiber instead of dispersion slope compensation fiber, we were able to increase the transmission distance from 63 to 92 km because reverse dispersion fiber has less polarization mode dispersion and a flatter dispersion profile.

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.

Time-domain ABCD matrix formalism for laser mode-locking and optical pulse transmission

ABCD matrix formalism in the time domain has been newly developed on the basis of laser beam and resonator analyses which were developed under a Gaussian paraxial approximation. We derive matrix elements for amplitude and frequency modulations, group velocity dispersion, optical bandpass filter dispersion, and lumped self-phase modulation under a parabolic approximation. Applications to AM, FM, and stretched-pulse laser mode-locking are described by using these time-domain matrices. An application to pulse transmission in a dispersion-allocated system is also described.

50-GHz repetition-rate, 280-fs pulse generation at 100-mW average power from a mode-locked laser diode externally compressed in a pedestal-free pulse compressor

280-fs pedestal-free pulses are generated at average output powers exceeding 100 mW at a repetition rate of 50 GHz by compression of the output of a mode-locked laser diode (MLLD) by use of a pedestal-free pulse compressor (PFPC). The MLLD consists of a monolithically integrated chirped distributed Bragg reflector, a gain section, and an electroabsorption modulator. The PFPC is composed of a dispersion-flattened dispersion-decreasing fiber and a dispersion-flattened dispersion-imbalanced nonlinear optical loop mirror. Frequency modulation for linewidth broadening is used to overcome the power limitation imposed by stimulated Brillouin scattering.

40 Gbit/s soliton transmission field experiment over 1,020 km and its extension to 1,360 km using in-line synchronous modulation

We report the details of a 40-Gbit/s soliton transmission field experiment over 1,020 km, which used a dispersion-compensation technique. The transmission distance was extended to 1,360 km by installing in-line synchronous modulation.

Femtosecond fiber laser at 10 GHz and its application as a multi‐wavelength optical pulse source

In order to realize ultrafast optical communication in the future, it is necessary to develop a stable short pulse optical source that can be operated in the GHz frequency band. This paper reports the oscillation characteristic of a stable, harmonically and regeneratively mode-locked, fiber laser. This laser can produce stable pulses with a repetition rate of 10 GHz, a pulse width of 1.0 to 3.0 ps and a wavelength of 1.55 μm. Using this laser and a dispersion decreasing erbidium (Er)-doped fiber amplifier, femtosecond pulses with a wavelength of 1.55 μm, a pulse width of 170 to 250 fs and a repetition rate of 10 GHz were generated. Using an optical fiber amplifier with a much higher output power, the average optical output of 0.53 W was obtained, where the peak power of the femtosecond optical pulse was as high as 190 W. Using the 10 GHz femtosecond pulses with high power, pulses with eight wavelengths were generated where wavelengths were between 1.552 and 1.559 μm, and the pulse width and repetition rate were 16 to 17 ps and 80 GHz, respectively. This light source is expected to play an important role in multi-wavelength optical communication.