Ken-ichi Hayashi

Optical seizing and merging of voids in silica glass with infrared femtosecond laser pulses

We demonstrate that one can seize and translate voids formed by IR femtosecond laser pulses inside silica glass and can also cause two voids to merge into one. We also present clear evidence of a void and its surrounding region by showing scanning electron microscope images of cleaved voids, which we produced by cleaving through the glass along a plane that included a thin laser-ablated line on the surface of the glass and the voids formed inside.

Deep ultraviolet light generation at 266 nm by quasi-phase-matched quartz

We demonstrated the finest twin structure ever reported in crystal quartz with a period of 17.8 mum. Second harmonic 266 nm light of 0.10 mW was obtained by the third-order QPM in quartz, from a ns-pulsed doubled Nd:YVO4.

Fine twin structure in crystal quartz for quasi-phase-matched deep ultraviolet generation

We succeeded in fabrication of the finest twins in crystal quartz with a period of 11.9 mum and generation of 266 nm light with 2.2 mW by 2nd-order-QPM through precise control of the temperature and pulsed stress.

High luminance application of orange fiber laser

We developed an orange fiber laser as the source for photocoagulation in ophthalmic applications. While the beam quality (M2=1.2) is excellent in comparison with the existing solid state laser (M2=7~8), there is the unfortunate necessity of an additional optical system to make the focusing beam uniform after transmission into the 50μm core diameter multimode fiber used in the existing photocoagulator. The purpose of this paper is to obtain a high luminance spot while maintaining the uniformity of the beam. We used 4.7μm Mode field diameter, single mode fiber (SMF) to focus the laser beam emitted from the developed 580nm orange fiber laser source. A coupling efficiency of 70% was obtained at the input power of 500 mW. Moreover, the beam quality of M2=1.27 was achieved after fiber coupling. No additional optical system was necessary to make the beam uniform owing to the SMF characteristics. As a result, the beam diameter at the irradiation point became 12.2μm, and the power density was calculated to be about 25 times higher than that of the existing photocoagulator. Significantly high luminance beams were obtained in the method described above. If the laser spot can be focused near the diffraction limit using adaptive optics in the future, it can be used in new operative procedures such as microsurgery in the macular region. When this method is used in photocoagulation, improvement in the quality of vision of patients is expected due to minimizing any damage to the retina. The effects of high luminance laser irradiation on pseudobiological tissue will be examined in a future paper.

Photofabrication for microphotonics in glass

Large-scale integration of compact photonic devices may be the key technology for the future information age. If optical-waveguide structures with small radius of curvatures are integrated inside a small glass tip along with active devices, a paradigm of microphotonics inside glass may become possible. We will review the fabrication techniques for the waveguides and photonic-band structures inside glass with the ultrashort laser pulses and present our recent results of fabrication experiments. The experimental studies include fabrication of birefringent waveguides, formation of small vacancies called voids, and drilling of a narrow but long hole from the rear side of a glass. We will show the birefringence properties of waveguides that are induced by the lienarly-polarized ultrashort laser pulses of approximately 100 fs duration. Birefringence of the waveguides depends on the polarization states of the fabrication beam. The experimental studies on the stabilities of a void during the fabrication process are also presented. We will show that the voids move during the fabrication process although the beam and sample are fixed. It will be shown that the void moves from pulse to pulse toward the upstream direction along the optical axis. We can currently drill a long hole of more than 200 microns with a diameter of several microns. We also present the results of numerical analysis of optical propagation through photonic structures that are based on the array of voids.

Novel laser process for removing polymeric film by solid-state lasers

In this paper, we demonstrate removal of a polymeric layer from a multilayered print board by an IR laser at a wavelength of 1047 nm, at which the polymeric layer is transparent. The polymeric layer was removed completely with a single IR shot.

Observation of voids and optical seizing of voids in silica glass with infrared femtosecond laser pulses

Many researchers have investigated the interaction of femtosecond laser pulses with a wide variety of materials. The structural modifications both on the surface and inside the bulk of transparent materials have been demonstrated. When femtosecond laser pulses are focused into glasses with a high numerical-aperture objective, voids are formed. We demonstrate that one can seize and move voids formed by femtosecond laser pulses inside silica glass and also merge two voids into one. We also present clear evidence that a void is a cavity by showing a scanning-electron-microscope image of cleft voids: we clove through the glass along a plane that includes the laser-ablated thin line on the surface and the voids formed inside. The optical seizing and merging of voids are important basic techniques for fabricate micro-optical dynamic devices, such as the rewritable 3-D optical storage.

Orange fiber laser for ophthalmology

For the light source of photocoagulators for ophthalmology, orange laser is more suitable than green laser because of low scattering loss by the crystalline lens, and low absorption by xanthophylls in the retina. We developed two orange fiber lasers (580 nm and 590 nm) to investigate the effect depending on the difference in the range of orange. The 580nm laser is composed of a 1160 nm fiber laser and a Periodically Polled Lithium Niobate (PPLN) crystal for second harmonic generation. The 1160 nm fiber laser beam is focused into the MgO-doped PPLN crystal whose length is 30 mm with 3-pass configuration. Continuous-wave 1.3 W output power of 580 nm was obtained with 5.8 W input power of 1160nm for the first time. The conversion efficiency was 22%. The band width of the second harmonic was 0.006 nm (FWHM). The 590 nm laser is almost the same as 580 nm laser source. In this case we used a Raman shift fiber to generate 1180 nm, and the output power of 590 nm was 1.4 W. We developed an evaluation model of photocoagulator system using these two laser sources. A 700 mW coagulation output power was obtained with this orange fiber laser photocoagulator system. This is enough power for the eye surgery. We have the prospect of the maintenance-free, long-life system that is completely air-cooled. We are planning to evaluate this photocoagulator system in order to investigate the difference between the two wavelengths at the field test.

Photo-induced microstructures in transparent materials with femtosecond laser pulses

We report the observation that a void moves inside transparent materials under irradiation by femtosecond laser pulses in a fixed optical system. We also show birefringent structures induced by self-trapped filaments in silica glass.