Eiji Toratani

Self-fabrication of void array in fused silica by femtosecond laser processing

We demonstrate self-fabrication of a submicrometer-sized void array in fused silica using a 100fs 0.2–3μJ Ti:Sapphire femtosecond laser and a high 0.9 numerical aperture (NA) objective lens. The effect of the focusing conditions of NA, laser energy, and pulse number on the shape of the fabricated void was investigated. The void has a linearly drawn shape in the direction of the laser irradiation when a single pulse is irradiated and an increasing number of incident pulses resulted in the break up of the long void into multiple spherical ones, leading to a periodically aligned void array. The void shape also varied with the depth of the focus point beneath the fused silica surface, because the amount of self-focusing has a significant effect on the generation of the voids. The void shape was narrower and longer when the laser pulse was focused with the higher NA (up to 0.9) objective lens in the deeper position (up to 70μm) in the fused silica.

Femtosecond laser processing of subwavelength-sized voids for compact optical devices

We have demonstrated femtosecond laser fabrication of submicrometer-sized voids in fused silica. Femtosecond laser pulses of 100 fs were focused into fused silica with a 0.9 numerical aperture (NA) objective lens under various incident conditions. The void shape is linearly drawn in the direction of the laser irradiation, when a single pulse is irradiated. The irradiation of multiple pulses induces multiple spherical voids which make a void array. The void shape also depended on the depth of the focus point beneath the fused silica surface, because the amount of self-focusing has a significant effect on the generation of the voids. The void shape was narrower and longer when the laser pulse was focused into the deeper position (up to 70 μm) in the sample. In addition, a 90 degree bend waveguide was fabricated in combination with a void array reflector. Since both reflector and optical waveguides were fabricated by femtosecond laser only, this technique would be useful to develop 3-dimensional optical devices.

Pulsewidth control of intense sub-picosecond laser using autocloned photonic crystal

A two-dimensional, autocloned, photonic crystal is used to control the pulse duration of a sub-picosecond Ti:sapphire laser for material processing applications. The photonic crystal consists of a periodic stack of 99.5-nm-thick Ta2O5 layers and 137.5-nm-thick SiO2 layers with a periodic surface corrugation deposited on a SiO2 substrate. The photonic crystal size is 10 ×10 mm2. It can withstand intense sub-picosecond laser pulses. Photonic crystals behave as a negative group velocity dispersion (GVD) device in a low incidence angle range and also as a positive GVD device in a high incidence angle range. By simply changing the incidence angle of the laser, pulsewidth can be controlled. With a 200-fs up-chirped pulse, the transmitted pulsewidth is compressed down to 121 fs at an incidence angle of 10°. At an incidence angle of more than 20°, pulsewidth is steeply stretched mainly because of the reduction in transmission spectral band in the photonic crystal. A down-chirped 200-fs pulse is stretched up to 232 fs because of the negative GVD of the photonic crystal at a normal incidence, whereas at a high incidence angle of 54°, pulse is compressed down to 111 fs because of the positive GVD of the photonic crystal. An autocloned photonic crystal can be used as a very simple GVD device to control an intense sub-picosecond laser pulse.

Void-array-type photonic crystal device fabricated by femtosecond laser

This study demonstrates self-assembling of a sub-micrometer-sized void array in fused silica using an amplified 1-kpps Ti:sapphire femtosecond laser. The effect of the focusing conditions of numerical aperture, laser energy, and pulse number on the shape of the fabricated void is also investigated

Self-fabrication of avoid array in fused silica by femtosecond laser

In this paper, we report on the self-fabrication of sub-micrometer-sized void array in fused silica using Ti:sapphire femtosecond lasers. We have investigated the effect of the focusing condition on the shape of the fabricated void. It is found that the shape of the voids was varied with the depth of a focusing point from the sample surface, because the self-focusing process has a significant effect on the generation of the voids

Waveguide Bragg gratings written inside bulk glasses by femtosecond laser

Fiber Bragg gratings are one of the key optical devices for optical communications and optical sensing. Usually fiber Bragg gratings are made holographically or with phase masks. However both techniques have inherent drawbacks as follows: in a holographic technique, the stability of the interference fringe patterns is a critical technical issue while in a phase mask technique, expensive masks are needed. Recently femtosecond lasers have demonstrated to make Bragg gratings inside a commercially available optical fiber. The first demonstration was to fabricate long period fiber Bragg gratings in single mode optical fibers b focused femtosecond lasers irradiation through a 0.46 NA objective lens. Another approach to make Bragg gratings in optical fibers was demonstrated using a phase mask technique. Additionally Bragg gratings have been fabricated inside bulk glasses using femtosecond lasers. However there have been no reports on waveguide Bragg grating fabrication inside bulk glasses using only femtosecond lasers. The fabrication of waveguide Bragg gratings inside bulk glasses using femtosecond lasers is very attractive for integrated photonic device fabrication. In this paper, we will present a novel integration technique of waveguides and Bragg gratings to fabricate waveguide Bragg gratings in glasses using femtosecond lasers. Femtosecond (fs) lasers can induce permanent refractive index changes inside various transparent materials via nonlinear absorptions. If fs lasers are focused on optical waveguides written previously by fs lasers to make waveguide Bragg gratings, it was hard to get enough refractive index difference between the waveguide and the Bragg grating because an amount of fs laser induced refractive index change is limited. Then the diffraction efficiency of the laser propagating through the waveguide by the grating is quite low. Moreover the light confinement in the fs laser written waveguides is weak due to the small refractive index difference ∆n, and consequently the Bragg grating written in the waveguide reflects an only small amount of the propagating laser power in the waveguides. In this paper, we present that one can induce long cylindrical refractive index changes as long as 200 μm that are produced with a 0.65 NA objective lens. Next, we integrate the waveguide and the Bragg grating by observing the emitted light from the laser induced plasma with a CCD camera in order to fabricate the waveguide Bragg gratings. Among the waveguide Bragg filters, waveguides are embedded in a volume type Bragg grating. To fabricate the Bragg gratings, we examined refractive index changes induced by fs laser pulses with a high NA objective lens. We used a Ti:sapphire fs chirped pulses amplification system that operates at 150 fs, a center wavelength of 800 nm and a repetition rate of 1 kHz in our experiment. The laser pulses are focused into borosilicate glass using a 0.65 NA microscope objective lens. If the fs laser pulses are focused using the 0.65 NA objective lens with a proper input energy to induce a refractive index change (around 1μJ in this case), the refractive index change is found as a cylindrical shape that is about 10 μm long along the laser axis. However if the effective NA is reduced down to 0.1, it is possible to increase the length of refractive index change as shown in Fig. 1. These refractive index changes are produced under the conditions that the effective NA is 0.1, puslewidth is 150 fs and irradiation time is 60 s without any translation of the target glass.

Pulse duration control of intense femtosecond laser by use of autocloned photonic crystal

The paper reports on the demonstration of pulsewidth control of intense femtosecond pulses by use of novel dispersive device consisting of an autocloned, two-dimensional (2-D) photonic crystal. 2-D autocloned photonic crystal that is used here for this application has dielectric multilayer coatings with periodical patterns on each layer.