Kenichi Hirosawa

Demonstration of a Ti:sapphire mode-locked laser pumped directly with a green diode laser

We demonstrate a Ti:sapphire laser pumped directly with a green diode laser. A single 1 W InGaN diode laser operating at 518 nm is used as the pump source. Pulse durations as short as 62 fs and average output powers of up to 23.5 mW are obtained with chirped-mirror-based dispersion compensation and a semiconductor saturable absorber mirror (SESAM).

Sequentially timed all-optical mapping photography (STAMP) utilizing spectral filtering

We propose and experimentally demonstrate a new method called SF-STAMP for sequentially timed all-optical mapping photography (STAMP) that utilizes spectral filtering. SF-STAMP is composed of a diffractive optical element (DOE), a band-pass filter, and two Fourier transform lenses. Using a linearly frequency-chirped pulse and converting the wavelength to the time axis, we realize single-shot ultrafast burst imaging. As an experimental demonstration of SF-STAMP, we monitor the dynamics of a laser ablation using a linearly frequency-chirped broadband pulse (>100 nm) that is temporally stretched up to ~40 ps. This imaging method is expected to be effective for investigating ultrafast dynamics in a diverse range of fields, such as photochemistry, plasma physics, and fluidics.

Spatiotemporal control of femtosecond plasmon using plasmon response functions measured by near-field scanning optical microscopy (NSOM).

Spectral interferometry combined with near-field scanning optical microscopy is applied in the spatiotemporal characterization of femtosecond plasmon localized at gold nanostructures and surface plasmon polariton in an air-gap waveguide. Based on the plasmon response function in both the amplitude and the phase obtained from the measurements, we deterministically tailored the femtosecond plasmon pulse by shaping the femtosecond excitation laser pulses.

Saturation of 640-nm absorption in Cr⁴⁺:YAG for an InGaN laser diode pumped passively Q-switched Pr³⁺:YLF laser.

We measure the absorption recovery time, the ground- and excited-state absorption cross sections of a Cr4+:YAG crystal at 640 nm for the first time. A pump-probe measurement reveals the existence of two recovery times of 26 ns and 5.6 μs. By a Z-scan experiment, the ground- and excited-state absorption cross sections are estimated to be 1.70 - 1.75 × 10(-17) and 0.95 - 1.00 × 10(-17)cm2, respectively. The adequacy of the proposed model and the accuracy of the estimated parameters of the saturable absorber are verified by reproducing the experimentally obtained performance of a passively Q-switched Pr3+:YLF laser with the Cr4+:YAG saturable absorber from rate equation analysis.

Two-dimensional spatiotemporal focusing of femtosecond pulses and its applications in microscopy

We demonstrate and theoretically analyze the two-dimensional spatiotemporal focusing of femtosecond pulses by utilizing a two-dimensional spectral disperser. Compared with spatiotemporal focusing with a diffraction grating, it can achieve widefield illumination with better sectioning ability for a multiphoton excitation process. By utilizing paraxial approximation, our analytical method improves the axial confinement ability and identifies that the free spectra range (FSR) of the two-dimensional spectral disperser affects the out-of-focus multiphoton excitation intensity due to the temporal self-imaging effect. Based on our numerical simulation, a FSR of 50 GHz is necessary to reduce the out-of-focus two-photon excitation by 2 orders of magnitude compared with that in a grating-based spatiotemporal focusing scheme for a 90-fs excitation laser pulse. We build a two-dimensional spatiotemporal focusing microscope using a virtually imaged phased array and achieve an axial resolution of 1.3 μm, which outperforms the resolution of conventional spatiotemporal focusing using a grating by a factor of 1.7, and demonstrate better image contrast inside a tissue-like phantom.

Phase locking in a Nd:YVO 4 waveguide laser array using Talbot cavity

We demonstrated phase-locking in a laser-diode-array-pumped Nd:YVO₄ laser array (15 emitters) using a Talbot cavity. The Nd:YVO₄ slab crystal was coated by dielectric material for claddings and formed a planar waveguide for the vertical mode. To stabilize the horizontal array mode, periodical thermal lenses were generated by controlling the heat flow. The phase-locked waveguide array generated 1.65-W output power, while 2.02 W was available in a standard cavity. Two-peak supermode was demonstrated with the Talbot cavity and was converted to a single peak with a spatial light modulator. We also experimentally and numerically analyzed the characteristics of Talbot phase-locking.

Purification of Squeezed Vacuum Pulse Generated from a Sagnac Loop Fiber Using Linear Optics and Conditional Homodyne Detection

Purification of quadrature squeezed vacuum pulses at the wavelength of 1.5 µm generated from a Sagnac loop fiber interferometer is experimentally demonstrated using linear optics and conditional homodyne detection. The anti-squeezed quadrature variance degraded by guided acoustic wave Brillouin scattering in the fiber is significantly improved from 12.6 to 4 dB after the post selection, while the squeezed quadrature variance of -0.7 dB slightly decreases to -0.3 dB due to the linear optical loss caused by a beam splitter. This result agrees well with the theory.

2D simultaneous spatial and temporal focusing multiphoton microscopy for fast volume imaging with improved sectioning ability

Simultaneous spatial and temporal focusing (SSTF) multiphoton microscopy offers us widefield imaging with sectioning ability. As extending the idea to 2D SSTF, people can utilize a 2D spectral disperser. In this study, we use a 2D spectral disperser via a virtually-imaged phased-array (VIPA) and a diffraction grating to fulfill the back aperture of objective lens with a spectrum matrix. This offers us an axial resolution enhanced by a factor of ~1.7 compared with conventional SSTF microscopy. Furthermore, the small free spectral range (FSR) of VIPA will reduce the temporal self-imaging effect around out-of-focus region and thus will reduce the out-of-focus multiphoton excited fluorescence (MPEF) signal of 2D SSTF microscopy. We experimentally show that inside a sample with dense MPEF, the contrast of the sectioning image is increased in our 2D SSTF microscope compared with SSTF microscope. In our microscope, we use a 1 kHz chirped amplification laser, a piezo stage and a sCMOS camera integrated with 2D SSTF to realize high speed volume imaging at a speed of 50 volumes per second as well as improved sectioning ability. Volume imaging of Brownian motions of fluorescent beads as small as 1μm has been demonstrated. Not only the lateral motion but also the axial motion could be traced.

In-phased second harmonic wave array generation with intra-Talbot-cavity frequency-doubling

The Talbot cavity is one promising method to synchronize the phase of a laser array. However, it does not achieve the lowest array mode with the same phase but the highest array mode with the anti-phase between every two adjacent lasers, which is called out-phase locking. Consequently, their far-field images exhibit 2-peak profiles. We propose intra-Talbot-cavity frequency-doubling. By placing a nonlinear crystal in a Talbot cavity, the Talbot cavity generates an out-phased fundamental wave array, which is converted into an in-phase-locked second harmonic wave array at the nonlinear crystal. We demonstrate numerical calculations and experiments on intra-Talbot-cavity frequency-doubling and obtain an in-phase-locked second harmonic wave array for a Nd:YVO₄ array laser.

Generation of squeezed pulses with a Sagnac loop fiber interferometer using a non-soliton femtosecond laser pulse at 800 nm

We experimentally demonstrate generation of a squeezed vacuum at 800 nm with a Sagnac loop fiber interferometer. When negative dispersion is properly added to an input laser pulse to compensate for the fiber dispersion, the level of squeezing is improved. A squeezed vacuum of 0.45 dB is obtained at a dispersion of -0.0157 ps(2) for the 1.5 m-long fiber loop. Since the squeezed vacuum is degraded by guided acoustic-wave Brillouin scattering (GAWBS), the noise level of the squeezing is improved by -0.3 dB at a liquid nitrogen temperature. We also demonstrate generation of photon number squeezing at -1.3 dB.