Vol.2 No.4 2010

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Research paper : Portable national length standards designed and constructed using commercially available parts (J. Ishikawa)−249−Synthesiology - English edition Vol.2 No.4 (2010) third derivative signal I’’’() around the iodine molecule absorption component in the iodine stabilized He-Ne laser output observed on the oscilloscope. The laser wavelength is controlled so this third derivative signal becomes zero and stability is achieved.The wavelength of laser is proportional to the length of the laser cavity L in the range where mode hop does not occur, as mentioned earlier. Therefore, to change the laser wavelength for control and modulation, L may be changed. For example, 6 MHzp-p modulation, converted to optical frequency, is applied to the laser wavelength as modulation for third derivative signal detection. If L is 0.3 m, the change of L corresponding to the 6 MHz modulation is 3.8 nm. This means that if one of the laser mirrors is vibrated at amplitude 3.8 nm in the optical axis direction, modulation of needed for third derivative detection can be achieved. Also, the width of the third derivative signal (Fig. 5) used in the control of laser wavelength is about 5 MHz, converted to optical frequency. When the modulation range of the laser wavelength surpasses this width due to vibration or shock, control is lost (this is when “the frequency lock becomes unlocked”). To stabilize the laser wavelength, it is necessary to keep the change in L at about 1/10 of this width, or at about 500 kHz. The change of resonator length L equivalent to the change of laser frequency change of 500 kHz is 0.32 nm. This is approximately the diameter of an atom. For a stable operation of the iodine stabilized He-Ne laser, it is necessary to have the fine technology of keeping the change in the interval of the laser mirror due to vibration to about the diameter of an atom.Figure 6 is the mechanism of the first resonator of the iodine stabilized He-Ne laser used by the author. One of the laser mirrors is as is, while the other one is attached to a ring-shaped stacked piezoactuator. Each mirror is supported by endplates with mechanism for precise angle adjustment. The two endplates are supported facing each other with four spacer rods made of Super Invar that has thermal expansion close to zero. A laser tube and an iodine cell are set in the resonator. In actual operation, the effect of vibration was kept within the aforementioned condition range of 500 kHz by mounting the resonator on the anti-vibration plate supported by air springs and then covering it with a soundproof case. Only one multi-layer ring piezoactuator is shown in Fig. 6, and both the demodulation and the control signals were applied to this piezoactuator to accomplish simultaneous modulation and control.4 Road map for achieving the goalsAs mentioned in the earlier chapters, the performances and properties required from each mechanism of the iodine stabilized He-Ne laser are quite special. Special requirements cannot be achieved by using universal members. To achieve special performances and properties using universal parts, it is necessary to devise new and special usage. As for special usage, methods that cause danger or ones that significantly reduce the lifespan cannot be employed. By devising and realizing ways to achieve the required performances and properties within reasonable and feasible range, the employability of the universal parts can be widened even if they are used unconventionally.As an example, I shall explain the new bias adjustment circuit of the control mechanism. In the iodine stabilized He-Ne laser, it is necessary to smoothly change the voltage applied to the piezoactuator to search and select the absorption line. Conventionally, this change was done using the potentiometer, but there is no potentiometer that has sufficient voltage resolution over the entire range. Therefore, the range of change was limited to a narrow voltage range, and this made the operability poor. Figure 7 shows the voltage adjustment circuit that achieves extremely high voltage resolution using the DC gear motor and integrator circuit instead of a potentiometer. The DC gear motor is an actuator that rotates at rate proportional to the input Fig. 5 Actual third derivative signal observed on the oscilloscope.Fig. 4 Spike in laser output I() due to saturated absorption, signal for its first derivative I’, and signal for third derivative I’’’.××I (λ)λI'I'''0ff3fx3fFig. 6 Resonator mechanism of the iodine stabilized He-Ne laser (conventional type).Super Invar rod (4)Ring-form piezo stack actuatorEndplate with tilt adjustment mechanismEndplate with tilt adjustment mechanism

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