Focused infrared femtosecond laser pulses (wavelength ~800 nm, emission pulse duration 100 fs) were employed to
fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F<sub>2</sub> color
centers of about 2x10<sup>18</sup> cm<sup>-3</sup> and refractive index change of about 1% at 633 nm were induced by the fs-laser irradiation.
This technique was applied to fabricate a distributed-feedback (DFB) F<sub>2</sub> color center laser structure inside LiF single
crystal. The LiF DFB laser exhibited laser oscillation at 707 nm at room temperature. The slope efficiency of ~10% and
beam divergence of ~20 mrad were achieved.
Single shot irradiation of interfered fs laser pulses allows for encoding non-erasable periodic nano-structures in almost all kinds of materials. However, strong absorption of the intense laser pulses at the material surface and the possible distortion of the pulse due to the non-linear interaction with materials make it very difficult to fabricate embedded structures inside inorganic dielectric materials. We have overcome this difficulty by using chirped pulses from Ti:sapphire fs pulse laser. The chirping stretches laser pulse width from 100 fs to 5 ps without changing the total energy, thus reducing the peak energy of the pulse. Multiple micro-gratings were encoded vertically inside silica glass at a depth of ~5 mm from the surface by using the pulse with an optimized pulse width of 500 fs. The present technique provides unique opportunity to fabricate embedded diffractive devices such as fiber gratings of pure silica fiber, optical couplers in planar waveguides and diffractive gratings in DFB lasers.
The ability of UV femtosecond laser pulse to fabricate the fine-pitched microgratings on fused silica or ZnO surfaces has been demonstrated through a two-beam laser interference technique. A pump and probe method has been developed to find the time coincidence of the two UV pulses through a laser-induced optical Kerr effect or transient transmission change. The UV pulses achieve to narrow the grating pitches as small as 290nm. The establishment of the technique provides a novel opportunity for the fabrication of periodic nanoscale structures in various materials.
Non-erasable fine pitched grating structures were successfully encoded in amorphous SiO<SUB>2</SUB> glasses and amorphous SiO<SUB>2</SUB> thin films on silicon wafers by colliding a pair of focused pulses split from a single femtosecond pulse from a 10 Hz mode-locked Ti:sapphire laser with a regenerative amplifier (wavelength: 800 nm; pulse duration: 100 fs; pulse energy: approximately 3 mJ/pulse). This pattern was not observed for cases in which the relative time delay of the two pulses was over 0.2 ps. The encoded periodic spacing was changed by varying the angle between the two crossed pulses. A minimum periodic spacing of approximately 430 nm was achieved for a laser wavelength of 800 nm. Structural alternations of silica network induced by intense fs-laser irradiations were observed. Laser ablation processes and volume compaction in amorphous SiO<SUB>2</SUB> are origin of the formation of grating structures.
Integrated Services Digital Network (ISDN) communication technology has made it possible to conduct broadband teleconferencing and teleoperation in a method which is much less costly than satellite systems. Fujita Research has taken advantage of this medium to conduct teleoperation tests of a robot system across the Pacific Ocean. Through our application, researchers in Encino, California were able to conduct simple pick and place tasks using a 6 degree-of-freedom robot in Tokyo, Japan. Problems of time delay and low update rates prompted preliminary investigations on the use of visual enhancements. The use of predictive overlays and stereo visual cues were investigated using the ISDN set-up and results confirm past studies which indicate their usefulness in improving teleoperation task performances. This research may eventually be applied to remote monitoring and operation systems of robots on several construction sites or in hazardous environments.