Interferometric determination of the diameter of a silicon sphere using a direct optical frequency tuning system

A new interferometer with a direct optical frequency tuning system using a phase-lock loop (PLL) offset locking has been developed to accurately measure the volumes of 1-kg single crystal silicon spheres. Three lasers are used in this system. The first one is an iodine-stabilized He-Ne laser, which is used as a reference laser. The second one is an offset-locked He-Ne laser whose optical frequency is offset-locked to the reference laser. The third one is an external cavity diode laser whose optical frequency is offset-locked to the offset-locked He-Ne laser by a PLL circuit. By changing the offset frequency between the diode laser and the offset-locked He-Ne laser, the optical frequency of the diode laser can be tuned continuously over a frequency range of 19 GHz and stabilized at a desired frequency with an uncertainty of 900 kHz within an integration time of 1/30 s. Accurate optical frequency changes in the diode laser produced by this system have been used to measure the sphere diameter by phase-shifting interferometry. Outlines of the interferometer and the frequency tuning system are presented. Preliminary results of the diameter measurement for a silicon sphere are also presented.