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Optical coherence tomography (OCT) is a noninvasive cross-sectional imaging technique with micrometer resolution. The theoretical axial resolution is determined by the center wavelength and bandwidth of the light source, and the wider the bandwidth, the higher the axial resolution. The characteristics of OCT imaging depend on the optical wavelength used. In this paper, we investigated the wavelength dependence of ultrahigh-resolution (UHR) OCT using a supercontinuum for biomedical imaging. Wideband, high-power, low-noise supercontinua (SC) were generated at λ = 0.8, 1.1, 1.3, and 1.7 μ m based on ultrashort pulses and nonlinear fibers. The wavelength dependence of OCT imaging was examined quantitatively using biological phantoms. Ultrahigh-resolution imaging of a rat lung was demonstrated with λ = 0.8–1.0 μ m UHR-OCT. The variation of alveolar volume was estimated using three-dimensional image analysis. Finally, UHR-spectral domain-OCT and optical coherence microscopy at 1.7 μ m were developed, and high-resolution and high-penetration imaging of turbid tissue, especially mouse brain, was demonstrated.

Wideband, octave spanning, coherent supercontinuum (SC) with high flatness is very useful for many applications, but it has been difficult to achieve such ideal SC. In this paper, we investigated the octave spanning, coherent SC generation based on Er-doped ultrashort pulse fiber laser system. First, we developed a high power ultrashort pulse system using a single-wall carbon nanotube fiber laser and similariton amplifier. A 1.0–2.2 μ m octave-spanning coherent supercontinuum was successfully generated. Next, by use of a stabilized fiber laser comb system as the seed pulse, an octave spanning SC comb was demonstrated. An octave spanning, highly coherent comb with high flatness was generated with normally dispersive highly nonlinear fiber. The characteristics of the generated supercontinuum comb were examined via balanced heterodyne beat measurements with stable continuous-wave laser diodes. The SC comb was confirmed to have high coherence and low amplitude noise at the observed wavelengths. Finally, the SC generation in two kinds of highly nonlinear fibers with different dispersion properties and the characteristics of the generated comb were examined. The dependence on fiber length, wavelength, and dispersion properties were discussed.

An offset-free, wavelength-tunable, and midinfrared frequency comb working at 184 MHz based on difference frequency generation (DFG) in a PPMgSLT crystal was demonstrated. A coherent Raman soliton pulse and Yb-fiber laser output were used as the seed pulses of DFG, and wavelength tunability in the range 3.1–5.2 μ m was realized. A quantum cascade laser stabilized by the absorption line of N2O was used to measure the beat signal with an MIR comb. The MIR comb had a good spatial beam profile, and a high signal-to-noise ratio (SNR) of 70 dB at a wavelength of 4.52 μ m was confirmed. A stable and sharp beat signal was observed in the MIR region with a linewidth of 160 kHz, and the SNR was larger than 20 dB.

Co/Ni multilayers with a stack of Ta (2 nm)/[Co (tCo)/Ni (tNi)] N/Ta (30 nm) were prepared by dc magnetron sputtering, and their magnetization dynamics were measured by time-resolved magneto-optical Kerr effect (TRMOKE). The total thickness of the multilayer and perpendicular anisotropy were varied by changing the bilayer period d = tCo+ tNi and number of repeats N while tNi/tCo was kept at a constant of 2.5. The TRMOKE measurements show clear damped oscillation of the magnetization of Co/Ni multilayers after the pump pulse illumination, and the damping constant α of the Co/Ni multilayers was estimated from the TRMOKE waveform. The estimated α was found to be independent both on total thickness and anisotropy field of the multilayer and was estimated to be ~ 0.035 for all the multilayers. This means that the use of Ta capping and buffer layers is effective to evaluate intrinsic damping constant of the Co/Ni multilayer, and that independent control of α and perpendicular anisotropy are possible for the magnetic multilayers.

Using the optical fibers and ultrashort pulses, we can demonstrate highly functional optical control. We have succeeded in generation of low-noise, ultraflat, high-quality supercontinuum (SC). Recently, octave spanning high-quality SC is also generated. The carrier envelope offset frequency measurement and ultrahigh resolution optical coherence tomography are demonstrated using SC. We can also generate widely wavelength-tunable ultrashort soliton pulses using soliton self-frequency shift (SSFS). So far, several new techniques have been proposed and demonstrated using SSFS. The author has also discovered novel two-pulse trapping phenomena. The ultrafast all-optical switching is demonstrated using the pulse trapping techniques, and principles and characteristics are described.

We presented a three-dimensional optical memory using a newly developed compact fiber laser. The fiber laser can produce pulse with high peak power of 5.3 kW. This allows two-photon absorption recording for next generation optical recording media possible for consumer application. We demonstrated two-photon absorption recording on a photochromic materials; diarylethene and achieved multilayer recording using the developed fiber laser.

Time-domain magnetization dynamics of sputtered GdFeCo (30 nm) amorphous alloy films was measured by pump-probe method using high-power ultra-short pulse fiber laser. The effective g-factor $g_{\rm eff}$ and effective damping constant $α_{\rm eff}$ of the GdFeCo films were estimated by using a numerical calculation of Landau–Lifshitz–Gilbert equation. The precessional frequency took a maximum near the magnetization compensation composition $C_{\rm M}$ of the GdFeCo, while the estimated $g_{\rm eff}$ and $α_{\rm eff}$ increased around the angular momentum compensation composition $C_{\rm A}$. The compositional dependences of $g_{\rm eff}$ and $α_{\rm eff}$ were roughly described by a mean-field model. The $g_{\rm eff}$ and $α_{\rm eff}$ were also estimated from the ferromagnetic resonance (FMR) spectra, and the data from the FMR spectra agreed well with those from the pump-probe measurement except for the composition near $C_{\rm M}$. The FMR method was unable to excite the magnetization near $C_{\rm M}$ because of the small net magnetization.

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.

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.

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

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.

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 sech2 shape is generated, and its wavelength can be linearly shifted by varying merely the fiberinput power in the wide wavelength region of 1.56.1.78 μm for 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. Index Terms.Nonlinear wave propagation, optical fiber applications, optical fiber lasers, optical pulse generation, optical soliton, Raman scattering.