Method and apparatus for measuring distribution of elongation in an

Abstract

Elongation distortion at any location of an optical fiber cable is measured by using the principle of the stimulated Raman effect in which a stokes beam with a little frequency difference from that of a pumping beam is generated when a pumping beam and a seed beam with the same frequency as that of the stokes beam meet in a Raman material like an optical fiber cable. Due to the fact that the group refractive index of an optical fiber cable for a pumping beam is different from that for a stokes beam, elongation of a cable is measured by observing time difference of the arrival of pumping beam and stokes beam. The location that the pumping beam meets with the stokes beam can be adjusted merely by shifting the pumping pulse, then, elongation distortion at any location can be measured. According to the invention, the pumping pulse is applied to the cable to be tested from both the ends of the cable with one of the pumping pulse delayed according to the desired location for meeting of two pulses. At one end of the cable, the arrival time t.sub.1 and t.sub.2 is observed, where t.sub.1 is the arrival time of the stokes pulse and t.sub.1 +t.sub.2 is the arrival time of the pumping pulse. The distribution of elongation (d.DELTA.z/dz) is obtained by calculating the equations z=k.sub.1 t.sub.1 -k.sub.2 t.sub.2, and .DELTA.z=k.sub.3 t.sub.2 -k.sub.4 t.sub.1, where k.sub.1, k.sub.2, k.sub.3 and k.sub.4 are constants, z is the length between the end and the location to be tested, and .DELTA.z is the elongation of the cable of the length z.