Vol.2 No.4 2010

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Research paper : Portable national length standards designed and constructed using commercially available parts (J. Ishikawa)−252−Synthesiology - English edition Vol.2 No.4 (2010) preload occurs in the central part. As a result, the rigidity against the shift can be maintained but the rigidity against the tilt is insufficient. In the manufacturer’s explanation, it is recommended that two splines should be used linearly in place where there is side moment.5.1.2 Linear movement mechanism by cross roller guideIt was mentioned earlier that the position shift accompanying the movement of the support point position couldn’t be avoided in the finite stroke rolling linear motion guide. In principle, this position shift is thought to gradually progress throughout the stroke. On the other hand, the stroke needed for stabilizing the laser wavelength is around 0.05 mm since the purpose is to correct the expansion and contraction of the laser cavity length due to heat expansion. This is 1/100 or less of the maximum stroke of the cross roller guide[5], which is a representative limited distance rolling guide, as well as for the smaller versions. For the position shift for the entire stroke of the cross roller guide, the position shift against the movement of 1/100 stroke is expected to be sufficiently small, while it may not fulfill the performance required for the linear mechanism of the iodine stabilized He-Ne laser. Moreover, in the finite stroke rolling guide, since there is no entry or exit of the rolling body into the guide, it is not necessary to widen the ends of the guide as in the infinite stroke rolling guide. Therefore, it is expected to have high rigidity against tilt. After considering the conditions and required performances of the linear motion guide for cavity length control, as well as the performance of the linear motion guide used under that condition, cross roller guide emerged as an optimal linear motion guide.The shape of the cross roller guide is suitable for a movement stage, and application to circular mechanism is difficult. With employment of the cross roller guide, the structure of the laser cavity was greatly changed from the conventional endplate and rod to a housing combining the plate members as shown in Fig. 10. Moreover, making use of the separation of the movement control of the laser mirror and the maintenance of linearity, I devised a structure where the linear piezoactuator was fixed with the Super Invar rod, and the other side of the Super Invar rod was fixed to the housing at the position of the other laser mirror, as shown in Fig. 11. With this structure, the resonator length is determined by the piezoactuator and the Super Invar rod. Since it is not necessary to consider the heat expansion of the housing material, aluminum could be used. Since aluminum has high heat expansion, it was not conventionally used as the housing of the laser resonator. However, it also has high heat conductivity, and it has excellent characteristic where the heat strain due to heat gradient is not likely to occur. Compared to Super Invar rod, the heat expansion coefficient of aluminum is 50 times more, but the heat conductivity coefficient is also close to 20 times, and the effect of the heat strain will remain within 3 times.The new laser with the cross roller guide, aluminum housing, and control mechanism for resonator length by linear piezoactuator and Super Invar rod has extremely small tilt angle change of the laser mirror. Stable operation with small temporal change and environmental dependency has been achieved. Currently, among the five new lasers owned as the national standard of Japan, the difference in laser wavelength is held within ± 3 kHz converted to optical frequency by conducting appropriate adjustment. This is within 1/3 of the ± 10 kHz uncertainty recommended by CIPM. It is also stable against the changes in environmental temperature, and shows excellent performance where the change of the laser wavelength stays within the deviation of ± 5 kHz converted to optical frequency (relative 1 × 10−11) even against large temperature shift of 25 ± 5 ºC.5.2 Modulation mechanism of the laser resonatorIn the iodine stabilized He-Ne laser, modulation of 6 ± 0.3 MHz converted to optical frequency is applied to laser wavelength to detect the saturated absorption signal of the iodine molecule. This modulation is accomplished by vibrating one of the laser mirrors by sine wave in the optical axis direction. In the case where the resonator length is 30 cm, the amplitude is (3.81 ± 0.19) nm. As mentioned Fig. 10 Box-shaped laser cavity mechanism of the iodine stabilized He-Ne laser (new type).Fig. 11 Control mechanism for cavity length.Position of laser mirrorCross roller guideSuper Invar rod

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