Vol.2 No.4 2010

24/68

Research paper : Portable national length standards designed and constructed using commercially available parts (J. Ishikawa)−254−Synthesiology - English edition Vol.2 No.4 (2010) the finish of the contact planes of the brass support base, the ring-shaped piezoelement, the laser mirror holder, and the laser mirror. Lapping finish is done to increase the contact surface area to reduce the harmonic distortion. Also, the ring-shaped piezoelement and the laser mirror holder are placed on the brass support before assembly. Alcohol is poured on the contact plane, the brass support is flipped over, and firm planar contact is confirmed by seeing that the ring-shaped piezoelement and the mirror holder are fast and do not fall off.The mirror holder is preloaded by a thin-walled phosphor-bronze cylindrical spring and a brass round nut, and is placed in contact with the ring-shaped piezoelement and the brass support base. The brass base, the bronze thin-walled cylindrical spring, and the brass round nut are electrically insulated using a nylon insulator. The driving voltage of the piezoactuator is applied between the brass round nut (+) and the brass base (G).When designing, the purpose of the nylon insulator was electrical insulation, but it was found that it had another important function. When I used an aluminum insulator coated with insulating alumite to test the insulating materials to replace the nylon insulator, the modulation became unstable although there was no problem in the insulating property (substantial temporal change in amplitude occurred). The cause was the vibration of the mirror holder transferred to the phosphor-bronze thin-walled cylindrical spring and to the brass round nut through strong nonlinear screw contact. This resulted in complex resonation. The soft nylon insulator functioned as a damper that greatly reduces this resonance.5.3 Anti-vibration mechanismAs mentioned earlier, to achieve a stable function of the iodine stabilized He-Ne laser, the laser resonator length change caused by vibration and acoustics of the external environment must be kept under the diameter of an atom (0.3 nm) or less. As an assumption for development of the vibration mechanism, it is necessary to establish a method for detecting and evaluation the slight changes of the resonator length. Following is a brief explanation of the slight change in resonator length and the anti-vibration mechanism.The optical intensity change of laser output is measured for the stabilization of the laser wavelength, and this is amplified and connected to loudspeakers. The intensity change caused by wavelength modulation (6 MHz converted to optical frequency) of frequency f (3 kHz) can be heard as sound. The change of wavelength by wavelength modulation is converted to intensity change at laser wavelength – output curve I() and is heard as sound. At the same time, the resonator length change caused by external vibration also causes wavelength change, and that also can be heard. The amplitude of sound is proportional to the wavelength change or change in the resonator length. Therefore, if the sound of the external vibration can be kept to 1/10 of the amplitude of sound caused by wavelength modulation (1/100 in intensity, -20 dB), stable operation of the iodine stabilized He-Ne laser can be achieved. The metal housing of the iodine stabilized He-Ne laser was placed directly on the optical table with a honeycomb structure (the surface is 5 mm thick stainless plate). The sound from the loudspeaker when the table surface is scratched with a coin or tapped lightly was similar to the sound heard when a person pressed an ear against the table surface.To assess the performance of anti-vibration rubber, aluminum plate with thickness of 1 cm (same as the bottom plate of the laser body) was placed on the table, and the sound heard when the table surface was scratched or tapped while pressing the ear against the plate was checked. Next, the anti-vibration rubber to be assessed was placed beneath the metal plate, and the degree of reduction of sound was checked.Three hemisphere rubber feet with radius of 7 mm made of polyurethane, which showed excellent sound insulation in the above simple experiment, were placed at the bottom of the housing of the new iodine stabilized He-Ne laser (Fig. 14). When it was placed directly on the table and operations such as optical axis adjustment were done, the wavelength stabilization became unlocked due to vibration, but with the anti-vibration effect of the rubber feet, there was hardly any effect on wavelength stabilization due to ordinary operations on the table.Fig. 13 Modulation actuator (after improvement, structure with low harmonic distortion).1: Thin-walled cylindrical spring2: Mirror holder3: Ring-form piezoelement4: Brass support base5: Nylon insulator6: Brass round nut123456Fig. 14 Sub-nanometer vibration-proof mechanism (rubber feet).Polyurethane rubber feet

※このページを正しく表示するにはFlashPlayer9以上が必要です