Norihiko Nishizawa

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).

Real-time, ultrahigh-resolution, optical coherence tomography with an all-fiber, femtosecond fiber laser continuum at 1.5 μm

Real-time, ultrahigh-resolution optical coherence tomography (OCT) is demonstrated in the 1.4-1.7-microm wavelength region with a stretched-pulse, passively mode-locked, Er-doped fiber laser and highly nonlinear fiber. The fiber laser generates 100-mW, linearly chirped pulses at a 51-MHz repetition rate. The pulses are compressed and then coupled into a normally dispersive highly nonlinear fiber to generate a low-noise supercontinuum with a 180-nm FWHM bandwidth and 38 mW of output power. This light source is stable, compact, and broadband, permitting high-speed, real-time, high-resolution OCT imaging. In vivo high-speed OCT imaging of human skin with approximately 5.5-microm resolution and 99-dB sensitivity is demonstrated.

All-polarization-maintaining Er-doped ultrashort-pulse fiber laser using carbon nanotube saturable absorber

We present an all-polarization-maintaining Er-doped ultrashort-pulse fiber laser using a single-wall carbon nanotube polyimide nanocomposite saturable absorber. The maximum average power for single-pulse operation is 4.8 mW, and the repetition frequency is 41.3 MHz. Self-start and stable mode-locking operation is achieved. The RF amplitude noise is also examined and it is confirmed that the noise figure is as low as that of a solid-state laser. Using a polarization-maintaining anomalous dispersive fiber, a 314 fs output pulse is compressed to 107 fs via higher-order soliton compression. The peak power of the compressed pulse is up to 1.1 kW.

Continuum generation in a novel photonic crystal fiber for ultrahigh resolution optical coherence tomography at 800 nm and 1300 nm

Ultrahigh resolution optical coherence tomography (OCT) is demonstrated at 800 nm and 1300 nm using continuum generation in a single photonic crystal fiber with a parabolic dispersion profile and two closely spaced zero dispersion wavelengths. Both wavelengths are generated simultaneously by pumping the fiber with ~78 mW average power at 1064 nm in a 52 MHz, 85 fs pulse train from a compact Nd:Glass oscillator. Continuum processes result in a double peak spectrum with > 110 nm and 30 mW average power at 800 nm and > 150 nm and 48 mW at 1300 nm. OCT imaging with < 5 mum resolution in tissue at 1300 nm and < 3 mum resolution at 800 nm is demonstrated. Numerical modeling of propagation was used to predict the spectrum and can be used for further optimization to generate smooth, broad spectra for OCT applications.

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.

Octave spanning high-quality supercontinuum generation in all-fiber system

Recently, widely broadened supercontinuum (SC) has been generated using ultrashort pulse and highly nonlinear fibers. However, inherent noise and fine structures have been the problem for the application of SC. We demonstrate wideband, low-noise, highly coherent, and ultraflat SC generation using soliton pulse and normal dispersion highly nonlinear fibers. Characteristics of generated SC are experimentally evaluated, and they are compared with those of the conventional SC. Octave spanning high-quality SC is also generated using high-power soliton pulse.

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.