Jun Takayanagi

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).

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.

High-resolution time-of-flight terahertz tomography using a femtosecond fiber laser

High-resolution tomographic imaging is demonstrated using a reflection-type terahertz time-domain spectroscopy. To realize a practical system for general use, a robust all-fiber laser was used as the pump light source. Broadband terahertz waves were generated with the combination of optical pulses compressed to 17 fs using optical fibers and a DAST crystal. Using deconvolution signal processing, the wideband spectrum of the generated terahertz waves provided high-axial resolution leading to successful imaging of a multilayered structure containing a 2-microm-thin GaAs layer. To our knowledge, this is the first demonstration of terahertz tomographic imaging of such a thin layer.

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.

1.0–1.7-

A widely wavelength-tunable ultrashort-pulse fiber-laser source has been demonstrated using a high-power mode-locked ytterbium (Yb)-doped fiber laser and a highly nonlinear photonic crystal fiber (PCF). The Yb-doped fiber laser produces 66-fs ultrashort pulse at the wavelength of 1.05 mum. The pulse energy and the peak-power are 2 nJ and 20 kW, respectively. By injecting this pulse into the highly nonlinear PCF, soliton and anti-Stokes pulses are generated in the wavelength regions of 1.05-1.69 and 0.6-0.7 mum, respectively. The generated soliton pulse is 100-150-fs transform-limited sech2-shaped pulse

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We present generation of widely and flatly broadened, low-noise and high-coherence supercontinuum (SC). Pedestal-free high-power Raman soliton pulses are generated as pump pulses and a wideband and flat SC is generated in a normal and small dispersive highly nonlinear fiber. The average power and the full-width at half maximum (FWHM) bandwidth of the generated SC are 40 mW and 580 nm, respectively. The spectral shape is very flat and smooth. The flat spectrum with ±1 dB uniformity is obtained at wavelength region of 1370–1890 nm. The generated SC shows low relative intensity noise (RIN) and high degree of coherence.

m Wavelength-Tunable Ultrashort-Pulse Generation Using Femtosecond Yb-Doped Fiber Laser and Photonic Crystal Fiber

A broadband terahertz time-domain spectroscopy (TDS) system for frequencies of up to 15 THz, including the phonon resonance frequency range, has been developed using a transferred thin-film photoconductive switch (PCS) detector. The thin-film PCSs, based on low-temperature-grown GaAs, were fabricated using epitaxial layer transfer onto high-resistivity Si substrates. We observed a reduction of phonon resonant absorption, including between 7 and 10 THz, in a forward radiation configuration. Numerically calculated absorption spectra show good agreement with our experimental results. This technique will provide compact, broadband TDS systems.

Generation of widely and flatly broadened, low-noise and high-coherence supercontinuum in all-fiber system

Visible to near-infrared, widely broadened supercontinuum generation is demonstrated using an ultrashort-pulse fiber laser system. A 229 mW, 83 fs high-power ultrashort pulse was generated at 1550 nm in an Er-doped fiber-chirped pulse amplification system. An almost pedestal-free 107 fs second-harmonic pulse was generated at 780 nm using periodically poled LiNbO3. A 0.45-1.38 μm widely broadened supercontinuum was generated in a highly nonlinear photonic crystal fiber. In terms of spectral flatness, the total modulation bandwidth was within 14 dB. All of the fiber devices were fusion spliced so that the system showed good stability. The effect of a pulse-trapping phenomenon in the supercontinuum generation process was discussed from experimental considerations.

Reduction of phonon resonant terahertz wave absorption in photoconductive switches using epitaxial layer transfer

We present an all-fiber system for generating pedestal-free 22-fs ultrashort pulses with a single-peak spectrum. High-power Raman soliton pulses and a normally dispersive highly nonlinear fiber are used to generate a smooth, broadened single-peak spectrum. Then, the higher order dispersion is compensated for using a hybrid fiber composed of a reverse-dispersion fiber and a standard single-mode fiber, which allows pedestal-free ultrashort pulses to be successfully generated. To our knowledge, this is the first demonstration of pedestal-free 20-fs-class ultrashort pulse generation without spectral distortion using an all-fiber configuration.

Generation of 0.45-1.38 μm visible to near-infrared widely broadened supercontinuum using Er-doped ultrashort-pulse fiber laser system

Nondestructive analysis of tablet is of great importance from the aspect of productivity and safety. In terahertz (THz) region, however, the analysis of core of coated tablet has not been progressed. In this study, we have measured a flat-surface push-pull osmotic pump tablet, having no orifice, having bilayer core and a coating film layer. The bilayer core was made from the drug layer and the push layer, and acetaminophen was contained in the drug layer as a model drug. To study its structure and components, we have obtained reflection spectra from the drug layer side and the push layer side measurements using THz time-domain reflection spectroscopy (THz-TDRS). From these results, detection of the peak of acetaminophen in the spectrum from the drug layer side measurements was confirmed. We have made ridges approximated toward the peak using a general method of linear regression analysis in both spectra. Two-sample t-test was applied to their gradients, and significant difference between the drug layer and the push layer was shown. These results suggested that THz-TDRS is applicable to the analysis of structure and component of a coated tablet.