Masanao Kamata

Optical vibration sensor fabricated by femtosecond laser micromachining

We fabricated an optical vibration sensor using a high-repetition rate femtosecond laser oscillator. The sensor consists of a single straight waveguide written across a series of three pieces of glass. The central piece is mounted on a suspended beam to make it sensitive to mechanical vibration, acceleration, or external forces. Displacement of the central piece is detected by measuring the change in optical transmission through the waveguide. The resulting sensor is small, simple, and requires no alignment. The sensor has a linear response over the frequency range 20 Hz–2 kHz, can detect accelerations as small as 0.01m∕s2, and is nearly temperature independent.

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.

Optical waveguide fabrication with double pulse femtosecond lasers

We demonstrate the optical waveguide fabrication inside fused silica glasses using double pulse femtosecond lasers to decrease the optical loss which may be caused by nonuniformity of the refractive index change in the modified volume. The pulse separation time of the double pulses is varied from 500fsto200ps. An optical loss of less than 0.8dB∕cm is obtained under the conditions that the first pulse energy and the second pulse energy in the double pulse mode are 30 and 160nJ, respectively, and pulse time interval is 3ps. The weak first pulse would act as a pre-conditioner of the fused silica and then the second subsequent pulse effectively induces the uniform refractive index change. This optical loss is less than 1.3dB∕cm for the waveguides fabricated using a single 160nJ pulse femtosecond laser.

Optical waveguide fabrication in new glasses and PMMA with temporally tailored ultrashort laser

We have comparatively studied waveguide fabrication characteristics in various transparent materials by use of temporally shaped femtosecond laser pulses. The materials which we studied here are fused quartz, K-PG375 glass whose melting point is as low as 648 K, and polymethylmethacrylate (PMMA). We have measured amount of refractive index change, writing speed, and laser fluence threshold for waveguide writing in the above mentioned materials. To optimize the optical quality of internal modification of transparent materials by femtosecond laser pulses, we controlled the free electron density induced in the materials by tailoring energy injection as a function of time.

Fabrication of waveguide-based vibration sensors by femtosecond laser micromachining

We report on the fabrication of vibration sensors using femtosecond laser-micromachined waveguides. We micromachine an optically connected waveguide across three pieces of glass using a nJ high-repetition rate femtosecond laser oscillator in a transverse writing geometry. The three pieces are placed in close proximity of one another such that there is low loss across the segmented waveguide. The middle piece is mounted on a flexible beam and hence is affected by external forces. Vibration and acceleration are detected by monitoring the induced change in transmission through the assembly. These sensors have a linear frequency response to input vibrations and an acceleration sensitivity below 0.01 m/s/sup 2/. Moreover, the sensors exhibit a negligible temperature dependence, overcoming a serious limitation encountered in the widely used fiber Bragg grating optical sensors.

Waveguide-based Bragg filters inside bulk glasses integrated by femtosecond laser processing

In this paper, for the first time to our best knowledge, we report on an integration technique of waveguides and Bragg gratings for fabrication of waveguide-based Bragg filters.

Dual Wavelength Femtosecond Laser Materials Processing

Femtosecond laser ablation using the combination of 260 nm pulses and 780 nm pulses is explored for the high speed and high quality selected removal of insulating layers.

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.

Low-loss optical waveguide fabrication with double-pulse femtosecond lasers

Low-loss optical waveguides are realized by fabricating the waveguides inside a variety of glasses with double-pulse fs lasers with variable pulse interval of up to 200 ps.

Optical waveguide fabrication with tailored femtosecond laser pulses

One of the technical issues of waveguide fabrication technique inside transparent materials using femtosecond laser has been an optical loss which may be caused by a nonuniformity of the modified volume. In this paper we report on the optical waveguides fabrication inside fused silica glasses using double pulse femtosecond lasers to solve this issue. We investigate writing conditions of optical waveguides with adjusting pulse intervals and relative fluence of the double pulses. The pulse separation time of the double pulses is varied from 500 fs to 200 ps. The better optical property of the optical waveguides under the conditions that the first pulse energy and the second subsequent pulse energy of the double pulse mode are 30 nJ and 160 nJ, respectively and pulse separation time is 3 ps. This optical loss is smaller than that of the waveguides fabricated using a single pulse femtosecond laser.