Toshio Goto

Compact system of wavelength-tunable femtosecond soliton pulse generation using optical fibers

Using passively mode-locked femtosecond (fs) fiber laser and polarization maintaining fibers, the compact system of wavelength-tunable femtosecond (fs) fundamental soliton pulse generation is realized. The monocolored soliton pulse, not multicolored ones, with the ideal sech/sup 2/ shape is generated, and its wavelength can be linearly shifted by varying merely the fiber-input power in the wide wavelength region of 1.56-1.78 /spl mu/m for a 75-m fiber. The soliton pulses of less than 200 fs are generated with the high conversion efficiency of 75%-85%. This system can be widely used as a portable and practical wavelength-tunable fs optical pulse sources.

Flatly broadened, wideband and low noise supercontinuum generation in highly nonlinear hybrid fiber.

We present wideband of 1180-2100 nm, flatly broadened supercontinuum (SC) generation using highly nonlinear hybrid fibers and femtosecond fiber laser. Stable and smooth spectra without fine structure are demonstrated. The hybrid fibers are constructed by fusion splicing fibers with different properties. The SC spectra can be properly controlled by the optimal design of the hybrid fiber based on the numerical analysis. The generated SC pulse shows the low relative intensity noise (RIN).

Experimental and numerical analysis of widely broadened supercontinuum generation in highly nonlinear dispersion-shifted fiber with a femtosecond pulse

We report the analysis of ultrawideband supercontinuum generation in a highly nonlinear dispersion-shifted fiber. A >1000-nm-spanning white-light continuum is generated by pumping the femtosecond fiber laser pulse at λ=1.56 μm into the extreme vicinity of the zero-dispersion wavelength of the fiber. The supercontinuum pulses are characterized with the experimentally observed sonogram traces. The numerical calculation based on the nonlinear Schrodinger equation is used to investigate the mechanism of the supercontinuum generation, and these results are in good agreement with experiment. We show that there are two stages with different spectral-broadening processes in the propagation evolution. Self-phase modulation and group-velocity dispersion play an important role in the first spectral broadening. Through an increase the propagation distance, further spectral broadening occurs due to the soliton self-frequency shift and the trapping effect by the redshifted soliton pulse through cross-phase modulation. Additionally, we show that the temporal and spectral interferences between the generated supercontinuum components cause the oscillating fine structure on the temporal waveform and the spectrum.

Widely Broadened Super Continuum Generation Using Highly Nonlinear Dispersion Shifted Fibers and Femtosecond Fiber Laser

1.25–1.95 µm widely broadened and almost flat super continuum is generated using only 5-m-long polarization maintaining highly nonlinear dispersion shifted fiber (PM-HN-DSF) and passively mode-locked Er-doped fiber laser. When the fiber length is 200 m, although there is a large depression around 1.5 µm, the optical spectra are widely broadened from 1.1 to 2.1 µm. The temporal distributions of the spectral components in the super continuum are observed using the technique of cross correlation frequency resolved optical gating. The initial generation process of the super continuum is directly observed.

Pulse trapping by ultrashort soliton pulses in optical fibers across zero-dispersion wavelength

A new phenomenon of pulse trapping by the ultrashort soliton pulse of an optical fiber has been experimentally observed. The trapped pulse in the normal-dispersion region copropagates with the soliton pulse in the anomalous-dispersion region along the fiber, and the wavelength of the trapped pulse is shifted to satisfy the condition of group-velocity matching. The wavelengths of the soliton pulse and the trapped pulse change almost continuously as the power of the soliton pulse is varied. Almost perfect conversion efficiencies are observed for soliton self-frequency shift and pulse trapping.

Widely wavelength-tunable ultrashort pulse generation using polarization maintaining optical fibers

Characteristics of widely wavelength tunable ultrashort pulse generation using several types of polarization maintaining fibers have been experimentally analyzed. Using the diameter reduced type of polarization maintaining fibers, the wavelength tunable soliton pulse is generated from 1.56 to 2.03 /spl mu/m. It is confirmed that the almost transform-limited 340-fs soliton pulse is generated at a wavelength of around 2 /spl mu/m using a frequency-resolved optical gating method. When low-birefringence fibers are used, it is observed that the orthogonally polarized small pulse spectrum is trapped by the soliton pulse and is also shifted toward the longer wavelength side in the process of soliton self-frequency shift. The wavelength of the orthogonally polarized pulse spectrum is 40-50 nm longer than that of the soliton pulse, and the birefringence of the fiber is compensated by the chromatic dispersion. Finally, a polarization maintaining highly nonlinear dispersion-shifted fiber is used as the sample fiber. When the fiber input power is low, the wavelength-tunable soliton and anti-Stokes pulses are generated. As the fiber input power is increased, the pulse spectra are gradually overlapped and the 1.1-2.1 /spl mu/m widely broadened supercontinuum spectra are generated by only 520 pJ pulse energy.

Generation of high-power femtosecond pulse and octave-spanning ultrabroad supercontinuum using all-fiber system

We present the all-fiber system for amplification of high peak power femtosecond pulses. The 260-fs pulses are generated in the passively mode-locked Er-doped fiber (EDF) laser and amplified using the EDF amplifier system. The average and peak powers of the generated pulses are 215 mW and 43.2 kW, respectively, and the pulsewidth is 42.3 fs. Then the amplified pulses are coupled into polarization-maintaining highly nonlinear dispersion-shifted fiber and octave-spanning supercontinuum is generated. The spectral range is widely expanded from 980 to 2570 nm. To the best of our knowledge, this bandwidth is the maximum one in this wavelength region.

Experimental analysis of ultrashort pulse propagation in optical fibers around zero-dispersion region using cross-correlation frequency resolved optical gating

Ultrashort pulse propagation at =1.55 microm in polarization maintaining dispersion shifted fiber is experimentally analyzed using cross-correlated frequency resolved optical gating (X-FROG). The generated soliton pulse is picked out with the spectral filter and is used as the probe pulse. The temporal distributions of the spectral components in the generated pulses are observed. The initial process of the pulse breakup is directly observed for the first time. The results of X-FROG traces are in agreement with the measured cross-correlation traces and the optical spectra. It is shown that the generated soliton pulse and the anti-stokes pulse are partially overlapped and almost copropagate along the fiber.

Ultrafast all optical switching by use of pulse trapping across zero-dispersion wavelength

Ultrafast all optical switching by use of pulse trapping across zero dispersion wavelength in optical fiber is demonstrated both experimentally and numerically for the first time. Only an arbitrary single pulse among four pulses with temporal separation of about 1.5 ps is successfully picked off with almost perfect extinction ratio. The spectrogram of the optical switching is directly observed using the X-FROG technique. The characteristics of all optical switching are analyzed numerically by the use of strict coupled nonlinear Schrödinger equations and the numerical results are in agreement with the experimental ones. It is interesting to note that although the other pulses are also overlapped with the soliton pulse, they are not trapped.

Simultaneous generation of wavelength tunable two-colored femtosecond soliton pulses using optical fibers

Wavelength tunable two-colored femtosecond (fs) soliton pulse generation is proposed and demonstrated for the first time, using passively mode-locked fs fiber laser and polarization maintaining fibers. The wavelengths of the two soliton pulses can be changed arbitrarily by varying the power and polarization direction of the fiber-input pulse. Ideal two colored soliton pulses in which the pulsewidths are about 200 fs are generated in the wavelength region of 1.56-1.70 /spl mu/m for 110-m fiber. The generated pulses are almost transform-limited ones.