Shinki Nakamura

Simulating and designing Brillouin gain spectrum in single-mode fibers

For many fiber applications, the Brillouin gain spectrum (BGS) contains important information including the Brillouin frequency shift, the Brillouin spontaneous linewidth, and the Brillouin gain coefficient. This paper is the first, to the best of our knowledge, to present an accurate numerical simulation of the BGS in single-mode fibers. The simulated and measured BGS were in good agreement. Through repeated numerical simulations, we revealed a tendency of the peak Brillouin gain coefficient that determines the stimulated Brillouin scattering threshold.

Ablation of polymer films by a femtosecond high‐peak‐power Ti:sapphire laser at 798 nm

Femtosecond infrared pulses from an ultrashort high‐peak‐power Ti:sapphire laser were successfully utilized for ablation of polymer films such as polytetrafluoroethylene, tetrafluoro‐ ethylene‐hexafluoropropylene copolymer, and polyimide. Good agreement between experimental data and calculations from a model suggested that simultaneous absorption of three photons for polyimide and five photons for tetrafluoroethylene‐hexafluoropropylene copolymer predominated over the other absorptive channels and that such a multiphoton absorption was induced efficiently.

Comparison between theory and experiment of nonlinear propagation for a-few-cycle and ultrabroadband optical pulses in a fused-silica fiber

Wave-propagation equations, including effectively the second derivative in time under the condition of a small difference between the group and phase velocities and the first derivative in position /spl xi/ in the group velocity coordinate, are derived based on the slowly evolving wave approximation. These can describe ultrabroadband optical pulse propagation with not only self-phase modulation (SPM), but also induced-phase modulation (IPM) in the monocycle regime in a fiber. It is shown that linear dispersion effects can be rigorously included in the numerical calculations. Calculations including SPM in a single-mode fused-silica fiber with the Raman effect are performed and compared with experimental results. Also, calculations including IPM in the fused-silica fiber are compared with experimental results. The effects of each term in the calculations on spectra are analyzed and it is shown that inclusion of the Raman effect and the dispersion of the effective core area is important for obtaining better agreement with experiments. It is shown that inclusion of more than third-order dispersion terms is necessary for calculations of monocycle pulse propagation.

Carrier-envelope-phase stabilized chirped-pulse amplification system scalable to higher pulse energies

We have demonstrated a carrier-envelope phase (CEP) stabilized chirped-pulse amplification (CPA) system employing a grating-based pulse stretcher and compressor and a regenerative amplifier for the first time. In addition to stabilizing the carrier-envelope offset phase of a laser oscillator, a new pulse selection method referenced to the carrier-envelope offset beat signal was introduced. The pulse-selection method is more robust against the carrier-envelope offset phase fluctuations than a simple pulse-clock dividing method. We observed a stable fringe in a self-referencing spectrum interferometry of the amplified pulse, which implies that the CEP of amplified pulse is stabilized. We also measured the effect of the beam angle change on the CEP of amplified pulses. The result demonstrates that the CEP stabilized CPA is scalable to higher-pulse energies.

Dual-wavelength mode-locked Yb: YAG ceramic laser in single cavity

We demonstrated a 380 fs dual-wavelength independently mode-locked Yb∶YAG ceramic laser at 1033.6 and 1047.6 nm. To the best of our knowledge, this is the first dual-wavelength mode locking achieved in Yb-doped solid-state lasers.

Diode-pumped mode-locked Yb:YAG ceramic laser.

A diode-pumped passively mode-locked Yb:YAG ceramic laser was demonstrated. 417 and 286 fs pulses with average powers of 250 and 25 mW were obtained at 1030 nm using 1 and 0.1% output couplers, respectively. 233 fs pulses with an average power of 20 mW were also obtained at a center wavelength of 1048.3 nm using a 0.1% output coupler. To the best of our knowledge, this is the first demonstration of a diode-pumped mode-locked Yb:YAG ceramic laser.

Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.

High-Power High-Efficiency Yb3+-Doped Y3Al5O12 Ceramic Laser at Room Temperature

A diode-end-pumped Yb3+:Y3Al5O12 (Yb:YAG) ceramic laser with high efficiency and high power was demonstrated. A 5.5 W cw output power was obtained with a slope efficiency of 52%. To the best of our knowledge, this is the highest reported efficiency for >5 W Yb:YAG ceramic lasers.

Finite-difference time-domain calculation with all parameters of Sellmeier's fitting equation for 12-fs laser pulse propagation in a silica fiber

In order to both experimentally and numerically investigate nonlinear femtosecond ultrabroadband-pulse propagation in a silica fiber, we have extended the finite-difference time-domain (FDTD) calculation of Maxwells equations with nonlinear terms to that including all exact Sellmeier-fitting values. We have compared results of this extended FDTD method with experimental results, as well as with the solution of the generalized nonlinear Schrodinger equation by the split-step Fourier method with a slowly varying-envelope approximation. To the best of our knowledge, this is the first comparison between FDTD calculation and experimental results for nonlinear propagation of a very short (12 fs) laser pulse in a silica fiber.

Effect of Pulse Duration on Ablation Characteristics of Tetrafluoroethylene-hexafluoropropylene Copolymer Film Using Ti:sapphire Laser

Pulse duration dependence of ablation characteristics of tetrafluoroethylene-hexafluoropropylene copolymer (FEP) film has been investigated with femtosecond and picosecond Ti:sapphire laser pulses at 798 nm. Laser fluence dependence of ablation rates was examined for the laser pulse duration from 170 fs to 12 ps. Surface morphology of ablated holes was investigated using a scanning electron microscope. A great difference has been found in the ablation characteristics for laser pulse durations around 170-fs and longer than 500 fs. From the analysis of the fluence dependence of ablation rates and observation of the ablated surface, we have found that five-photon absorption predominated over other processes only for 170-fs pulses. It has also been found that the ablation threshold intensity was inversely proportional to the square root of the pulse duration in the region from 170 fs to 200 ps.