32 percent of the observed delay this makes the observed delay or change in length appear to be too small thus the measured delay should be increased by a factor of 1 32 to obtain the true change in length u03b4l to make strain measurements on an optical fiber cable two fibers are selected for the test one representing the transmitted path and the other the return path at the far end of the cable the same two fibers are fusion spliced together to complete the loop a five turn wrap on a 20 mm diameter mandrel is used on the transmitting fiber to obtain a more uniform modal power distribution both fibers are held in vacuum chucks on x y z positioners at the transmitting and receiving ends a base line measurement is then made to obtain a reference length the distance meter provides a direct readout of the distance as if it were measured in air the reading is then corrected for the index of refraction and the strain optic coefficient at the higher resolution u00b1 1 mm distance measurements can be made every 15 seconds and at the lower resolution u00b1 10 mm measurements can be taken every 2 seconds continuous length measurements may then be made while applying tension to the cable sheath by clamping the cable sheath onto the fibers in the vicinity of the ends of the cable the tension is thereby also transmitted to the optical fibers since the setup does not need to be adjusted after the initial alignment continuous measurements are possible as the strain progresses the meter may be controlled with a calculator to provide unattended measurements over long periods of time the foregoing principles will be more.