| 摘要 |
PURPOSE:To measure electric length of an optical fiber with high accuracy, by propagating a laser beam whose amplitude has been modulated by frequency (f), by the optical fiber, and using an approximate expression based on a propagation characteristic from a phase difference of modulated components at an input/ output position and the frequency (f). CONSTITUTION:A continuous light beam of a laser 5 is modulated as to its amplitude by a sine wave of frequency (f) of an oscillator 2, and is made incident to an optical fiber 1. An optical signal which is reflected by the projecting end of the fiber and is returned into the fiber is fetched through a half-mirror 8 of the incident end side and is detected by a photodetector 6. Subsequently, a phase difference (theta) between its detecting signal and a signal of the oscillator 2 is measured by a vector voltmeter 4, and on the other hand, exact frequency (f) of the oscillating signal is measured by a counter 3. Electric length L of the fiber is derived from the approximate expression (theta)=360(f)NL/C regarding said phase difference (provided that N and C denote a constant showing a group refractive index of the fiber, and a light velocity, respectively). |
