Precise length measurements in multimode optical fibers.

Area of Science:

• Optical physics
• Fiber optics engineering
• Metrology

Background:

• Accurate characterization of optical fiber properties is crucial for telecommunications and sensing.
• Existing methods for measuring group index and group delay in multimode fibers have limitations in precision.
• Multimode optical-time-domain reflectometers (OTDRs) require precise calibration for accurate distance measurements.

Purpose of the Study:

• To develop and demonstrate a precise method for determining the group index and group delay of on-axis modes in multimode fibers.
• To validate the technique by measuring these parameters for various fiber types at 1310 nm.
• To apply the measurements for accurate length determination of optical fibers and to improve OTDR performance.

Main Methods:

• Utilizing selective optical excitation to isolate and analyze on-axis modes.
• Employing a fiber Michelson interferometer setup for group index measurement at 1310 nm.
• Measuring group delays via the transit time of short-duration optical pulses.

Main Results:

• High-precision values for group index and group delay of multimode fibers were obtained.
• Group indices for several fiber types were measured and tabulated.
• Length calculations for ~2 km reference fibers achieved an accuracy of approximately 0.04%, limited by group index uncertainties.

Conclusions:

• Selective optical excitation offers a highly accurate method for characterizing multimode fiber optical properties.
• The precise group index and group delay data enable accurate fiber length measurements.
• The developed technique provides a pathway to minimize uncertainties in distance measurements using multimode OTDRs.