Vol.2 No.4 2010

23/68

Research paper : Portable national length standards designed and constructed using commercially available parts (J. Ishikawa)−253−Synthesiology - English edition Vol.2 No.4 (2010) earlier, to obtain the control signal by third derivative, the third harmonic detection is done. When the third harmonic is included in the vibration of the laser mirror, the first derivative signal component enters into the third derivative signal, and the laser wavelength shifts as a result. Therefore, the vibration of the laser mirror is required to be pure fundamental with extremely low harmonic distortion. Moreover, to maintain the sensitivity of the phase sensitive detection constant, the phase relationship of the driving sine wave and the vibration must not change. Extremely high level of amplitude stability, ultra low distortion, and phase stability are required for the wavelength modulation actuator. In many iodine stabilized He-Ne lasers including the conventional laser, ring-form piezo stack actuator with the same structure as the ones to control the resonator length are used for modulation. In the conventional laser, one ring-form piezo stack actuator was used for both modulation and control.The piezoactuator with stacked or multilayer structure is originally for control (DC operation), and the use with vibration (AC operation) is not assumed. The durability when it is vibrated is an issue. In conventional lasers, detachment of the adhesive surface of the piezoelement caused by modulating vibration occurred frequently. When detachment occurred, it cannot be repaired and the entire component must be replaced. The US made ring-form piezo stack actuator was expensive and the supply was unstable (because they were prioritized for military use), and this was a major barrier in using it in the iodine stabilized He-Ne laser.5.2.1 Modulation mechanism using single-layer piezoelementThe detached ring-form piezo stack actuator must be discarded. One day, I attempted disassembling it to understand how it failed. By soaking the stacked piezoactuator in acetone, all the adhesives detached and it came apart easily. I found out that the stacked piezoactuator was constructed by stacking 10 piezoelement discs to which metal was deposited on both sides to function as electrode. Since the stroke of the actuator is 10 m for applied voltage 1000 V, one element is 1/10 or 1 m.With the dissembled actuator in front of me, I thought of developing a single-layer modulation actuator using one element. There will be no detachment if it has only one layer. The amplitude necessary for modulation is about 4 nm as mentioned earlier, and the applied voltage equivalent to the displacement of the single-layer element is 4 V, a voltage that can be handled readily by circuit using ordinary op amp. Moreover, since the rigidity will increase greatly, the stability of operation can be expected to improve.Figure 12 is the structure of the modulation actuator that was first fabricated. It has a simple structure where a ring-shaped piezoelement, an aluminum laser mirror holder, and an O-ring were held down in a case between two brass securing ring screws. The voltage is applied between the laser mirror holder and the securing ring. However, when this mechanism was set in the iodine stabilized He-Ne laser and I attempted stabilizing the wavelength, large wavelength shift over 10 times the recommended uncertainty was observed. The baseline of the third derivative signal was so tilted that it could be discerned visually, and there was an inclusion of the first derivative signal, or the substantial occurrence of third harmonic distortion in the modulation mechanism. In this structure, when static, the soft O-ring distorted and functions as expected. However, in vibration, the brass securing ring supporting the piezo vibrates due to action and reaction. The brass securing ring is in contact with the main body by the thread of the screw. The contact points of ordinary screws are partial, and it is thought that the rigidity is low against small displacement and nonlinearity is high. I suspected that the structure where the screws directly supported the object caused distortion.5.2.2 Improvement of dynamic property of the modulation mechanismTo improve the dynamic property and to reduce the harmonic distortion of the modulation mechanism, it is essential that the mechanical contacts are carefully finished planes to maintain highly rigid and stable contact. Moreover, it is important that the holders that support the vibrating laser mirror are immobile as much as possible. Considering these two points, Fig. 13 shows the modulation actuator that reduces the harmonic distortion and improves the operational stability. The point that differs greatly with the modulation actuator in Fig. 12 is the mass of the brass support base. The mass of the brass support base is about 200 g, and is about 50 times the mass of the 4 g vibrating part including the laser mirror and the holder. Due to this large mass ratio, only the laser mirror and the holder move even if the ring-shaped piezoelement expands and contracts at high speed, and the brass base remains immobile. Particular care is taken for Fig. 12 Modulation actuator (before improvement, large harmonic distortion).1: Securing rings (2)2: O-ring3: Mirror holder4: Ring-form piezoelement5: Case123451

※このページを正しく表示するにはFlashPlayer9以上が必要です