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Research paper : Portable national length standards designed and constructed using commercially available parts (J. Ishikawa)−250−Synthesiology - English edition Vol.2 No.4 (2010) voltage, but when it is rotated by external force, it also functions as a generator and produces voltage proportional to the angular velocity (2 × number of rotations). When the generated voltage is input to the integrator circuit, the output is voltage proportional to the rotational angle of the gear motor. The constant of proportion of the rotational angle and the integrator circuit output voltage can be set arbitrarily by selecting the input resistance of the integrator circuit and the capacitor. Moreover, this gear motor can be rotated without limit, unlike a potentiometer, and the voltage can be adjusted throughout the process. For example, it can be set at a rotation of 100 times or more.Achievement of special functions using universal parts was not conducted in a planned or systematic manner. While the development of parts directly affecting the laser performance as well as parts that may be difficult to procure or to maintain should be priority, the order at which ideas arose and problems were solved did not necessarily correspond to the priorities. Therefore, in conducting this development, there was no planned or systematic scenario in the beginning. After I succeeded at several universalizations, I became capable of seeing the bottleneck accurately. By recognizing the importance of the issue and then placing it somewhere in the corner of my mind, I was able to engage in improvements and developments whenever a solution came up while doing some other work. Under such situations, the set of universalizations was completed in about 10 years.The example of the above voltage adjustment circuit improved the operability, and it was possible to improve the basic performance (uncertainty and stability) by using the new mechanism with universal parts. The following chapters explain the control mechanism of the laser cavity length that determines the basic performance of the laser.5 Universalization of laser cavity length control mechanismThe laser resonator length control mechanism is the most important part that determines the basic performance of the iodine stabilized He-Ne laser. Conventionally, special parts such as ultra-low heat expansion materials and piezoactuators were used. The resonator length control mechanism is composed of the linear movement mechanism to control the wavelength and the modulation mechanism to obtain the third derivative signal. The universalization of the two mechanisms is explained below. Also, I shall explain the anti-vibration mechanism that is important to achieve stable resonator length control.5.1 Linear movement mechanism of the laser mirrorThe operating parameters of the iodine stabilized He-Ne laser are listed as the recommendation of the Comité International des Poids et Mesures (CIPM)[2]. Table 1 shows the operating parameters of the iodine stabilized He-Ne laser as recommended by CIPM. Of the four recommended parameters, only the depth of the modulation (6 ± 0.3 MHz converted to optical frequency) as mentioned earlier is directly related to the resonator mechanism, but since the one-directional optical intensity (10 ± 5 mW) within the resonator is affected by the laser mirror angle, it can also be considered as an issue of the resonator mechanism. In the conventional iodine stabilized He-Ne laser, the resonator length was controlled by a ring-shaped stacked piezoactuator. However, the piezoelement is not necessarily made of homogenous materials, and unevenness where the expansion and contraction may differ in places may occur even when constant voltage is applied. This causes a phenomenon where the tilt angle of the laser mirror slightly varies. When the tilt of the laser mirror changes, the loss of laser cavity changes so that the laser output also changes. The laser output is determined by the sum of the one-directional optical intensity within the resonator and the transmissivity of the laser mirror. Table 1 shows that the tolerance of change is large at ± 50 % of the designated output (one-directional optical intensity in the resonator is 10 mW), and the output change due to change in the tilt of the piezoactuator normally falls within this range.However, from actual experiments, it was absolutely necessary to keep the change of the tilt of the laser mirror during control as small as possible, to realize the iodine stabilized He-Ne laser with excellent reproducibility and stability of laser wavelength. On this point, the recommendation of CIPM is deficient on the major parameter that affects the laser wavelength. The author has conducted measurements while maintaining the laser power I almost constant while changing the angle of the laser mirror, to assess the dependency of the laser wavelength on the inclination of the laser mirror[3]. As a result, it was experimentally confirmed that the laser wavelength may Fig. 7 Wide-range voltage micro-adjustment circuit using DC gear motor and integrator circuit.DC gear motorVm = kθ'Handle-+Vo = kθ/ CRRotational angle:θIntegrator circuitCROutput voltage :Output voltage :Table 1 Operational parameters of the iodine stabilized He-Ne laser (recommendation of the CIPM).*The coefficient of one-way intracavity beam power of the optical frequency must be 1.4 kHz/mW or less. mW(10±5)・One-way intracavity beam powerMHz(6±0.3)・Frequency modulation width peak-to-peak(P-P)℃(15±0.2)・Cold-finger temperature of iodine cell℃(25±5)・Cell-wall temperature of iodine cell

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