Accurate Measurement of Fiber Length and Effective Index Using Equalized Ring-Down Enhancement and Pulse Sequence Methods

Distributed fiber-optic sensors are widely used for mining safety monitoring, civil engineering structural health monitoring and environmental monitoring. The position accuracy of measurements is of critical concern, as errors can lead to expensive decisions at the wrong location. Accurate fiber length measurement is key to calibrate distributed fiber-optic sensors. Existing methods suffer from time resolution or component delay errors. Optical time-domain reflectometry (OTDR) and its variants are widely used but needs calibration for practical applications. We propose two new methods that can self-calibrate, and can be used to calibrate other methods such as those based on OTDR. One based on a differential pulse loop using dual pulse statistical signal processing, and another is based on a differential pulse loop with multi-pulse sequence analysis, to enhance the timing accuracy. In addition, it is possible to determine the effective index of a single-mode fiber with known fiber length, which is important prerequisite information for practical applications in the field with uncertain laser wavelength and ambient temperature. When using a low-end oscilloscope with a nominal 26 MS/s sampling rate, compared with the measurement results of optical frequency domain reflectometer (OFDR) (LunaOBR4600), the double pulse method has an error of ±6 cm. For the multi pulse method, the measurement error can be as low as 1.75 mm for a fiber length of 2.049 km (relative error of 8.54 × 10⁻⁷).