Vol.2 No.4 2010

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Research paper : Portable national length standards designed and constructed using commercially available parts (J. Ishikawa)−248−Synthesiology - English edition Vol.2 No.4 (2010) stabilized He-Ne laser as the standard of the calibration service providers means the possession of the standard as a technology, as explained in the previous section.The traceability of artificial standards such as gauge blocks and weights can be maintained by assessment through calibration. However, the iodine stabilized He-Ne laser is a standard that extracts wavelength based on the quantum mechanical property of the iodine molecule, and the difference by device (uncertainty) depends on the technology for extracting the wavelength. In such a standard, in addition to evaluating the wavelength, the possession of technology for extracting the wavelength and its assessment, or the “traceability of technology” is essential. Universalization of the technology will promote conversion from possession of hardware to possession of technology, and the reliability in the mid-tier traceability such as among the calibration service providers will improve.3 Operating principle of iodine stabilized He-Ne laserHere, I shall explain the operating principle of the iodine stabilized He-Ne laser. Figure 2 is a schematic diagram of the structure of the iodine stabilized He-Ne laser. In ordinary He-Ne laser, a laser tube containing a mixture of helium and neon gases is placed between the laser cavity composed of two plane laser mirrors (more accurately, they are slightly concave). In the iodine stabilized He-Ne laser, the “iodine cell” where highly pure iodine molecule is sealed inside is placed in the laser cavity. The relationship between the optical interval (length of laser cavity) of the two laser mirrors L and wavelength can be expressed as Equation (1). = 2L / NHere, N is an integer. is proportional to L. Since the wavelength range in which the light amplification effect of the He-Ne laser tube is effective is extremely narrow, the laser wavelength remains at a certain limited range. When L is changed past the effective wavelength range, the integer N increases or decreases one at a time (mode hop), and remains at a certain range in which the light amplification is effective. When L is varied in the range where mode hop does not occur, the laser output I changes as shown in Fig. 3 (dashed line). I decreases at both ends where is close to mode hopping, and becomes highest at the center. When the iodine cell is present in the laser cavity in addition to the laser tube, the light absorption by the iodine molecules affects I, and the output curve changes as shown in Fig. 3 (solid line). Since a strong standing wave of light (10 mW) is present inside the laser resonator, the absorption by iodine molecules becomes saturated. Since the absorption weakens at the center of the absorption wavelength by saturated absorption, I increases slightly, and a spike appears on the output curve. Using this phenomenon, extremely high-resolution spectroscopy (saturated absorption spectroscopy) without the Doppler effect due to motion of the iodine molecules can be achieved. The iodine stabilized He-Ne laser uses the spike of saturated absorption that appears on the output curve as a marker, and the high precision is realized by controlling and stabilizing the laser wavelength to its center.Differential signal by phase sensitive detection is normally used as method for detecting the peak position of the output curve. Figure 4 shows the principle of differential signal by phase sensitive detection. To accomplish the differential signal, is slightly modulated. The modulation of changes the I, but the amplitude and phase are determined by the gradient of output curve I(). As shown in the figure, the first derivative I’() of I() is obtained when the DC component is extracted from the signal obtained by crossing the output signal and the demodulated signals. The laser wavelength stays at the peak position of I() by controlling the to keep this derivative at zero. However, in the iodine stabilized He-Ne laser, since the spike due to saturated absorption of iodine molecules is superimposed on the laser output curve I(), the effect on the gradient of the baseline cannot be avoided. To remove this effect and to detect the true center of the saturated absorption by iodine molecules, the third derivative detection is done in practice. The signal I’’’() of the third derivative can be obtained by using the threefold wave (frequency 3f) of the modulated signal (frequency f) as the demodulated reference signal of the phase sensitive detection. Since the laser output curve is gentler by far compared to the spike of saturated absorption, the effect is sufficiently removed by third derivative. Figure 5 is the Fig. 2 Schematic diagram of iodine stabilized He-Ne laser.Fig. 3 Relationship between laser cavity length L and laser output I (in single axial mode).Laser mirrorLaser tube(Discharge tube containing mixed gas of helium and neon)Iodine cell (Glass cell containing iodine molecule )LICapillaryLaser mirrorCrystal iodineILMode hopMode hopLaser output (without iodine cell)Laser output (with iodine cell)Saturated absorption spectral signal(1)

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