Building a 3.5m prototype interferometer for the vacuum birefringence experiment and high precision ellipsometry

• Main Authors: Jeah-Sheng Wu, 吳誌笙
• Other Authors: Wei-Tou Ni
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/07272367374867497438

博士 === 國立清華大學 === 物理學系 === 90 === A 3.5m prototype interferometer was formed using a high finesse Fabry-Perot interferometer. Taking the advantage of Fabry-Perot interferometer, the phase difference of incident light with different polarization will be enhanced. Each mirror of the 3.5m prototype interferometer was sited on a vacuum chamber with suspension system inside each chamber. X-pendulum plus double pendulum offers briliant rejecting power from seismic noise higher than a few Hz. The resonant frequency of X-pendulum suspension system was measured to be around 0.2 Hz. Laser frequency was locked on the axial mode of the 3.5m prototype interferometer using a Pound-Drever scheme. Polarizers were the key component of ellisometry. The polarizer used in our experiment was Glan-Thompson type with extinction ratio 2.6x10^{-7} Rad, while the analyzer was Glan-Laser type with extinction ratio measured 9x10^{-7}. Due to it''s high sensitivity, the prototype interferometer could be used to probe the anisotropy of dilute material, say air or pure gas. The axial magnetic field was generated by a solenoid. When applying a current to the solenoid, the response of the axial magnetic field was 21 Gauss/Amp. In the experiment of measuring Verdet constant, we apply both DC and 100Hz AC current to the solenoid. Verdet constant of air was measured C_v=3.9x10^{-10} Rad/Gauss-cm under room temperature and normal pressure. The measurement of Cotton-Mouton constant follows in similar way. A dipole electromagnet with maximum field of 12000 Gauss and effective length 21 cm was located in the middle of prototype interferometer. Modulate the magnetic field by varing the supply current with 50 mHz frequency. Also take into consideration of the coupling contribution from non-transverse magnetic field. We conclude that the Cotton-Mouton constant of air under room temperature was measured to be C_{CM}=7.5x10^{-17} Rad/Gauss^2-cm. The present sensitivity of 3.5m prototype interferometr was determined using double modulation scheme. Polariztion modulation of output signal was formed by a Faraday cell with the modulation depth response 0.019 Rad/Amp. Together with modulated magnetic field the sqrt{Hz} sensitivity of 3.5m prototype interferometer was 5.1x10^{-6} Rad/sqrt{Hz}. With integration time 44 min,the sensitivity reaches 1x10^{-7}Rad in polarization.