Long single-mode fiber optical lengths measured to micrometer-level precision

Oct. 13, 2015
A simple and more-precise method for measuring fiber length is based on the use of a gain-switched pulsed distributed-feedback (DFB) laser with optical delayed feedback.

Measuring the optical length of a relatively long section of single-mode optical fiber in situ, for example in a remote-sensing or communications application, can currently be done using the time-of-flight (TOF) method to a resolution of a few millimeters or so, limited by the speed of the photodetector response or by optical time-domain reflectometry (OTDR), although with even lower resolution over kilometers-long fibers. Now, researchers at Osaka Prefecture University (Osaka, Japan) have developed a simple and more-precise method for measuring fiber length based on the use of a gain-switched pulsed distributed-feedback (DFB) laser with optical delayed feedback—a method that, without the need for a fast photodetector, measured the optical length of a 1 km single-mode fiber immersed in water to be 1471.043915 m ±33 μm with a measurement time (over which many individual measurements are averaged) of approximately 1 min.

The laser itself generates a pulse train with a 1 GHz repetition rate, a pulse width of 28 ps, and a 0.7 nm spectral width at a 1550 nm wavelength. In the technique, a portion of the DFB laser's output pulse is fed back into the laser, producing fluctuations in the amplified spontaneous emission (ASE) noise intensity with respect to the modulation frequency of the gain switching. A cross-correlation trace is done between the pulse inside the laser cavity and the feedback pulse (which contains information about the fiber's optical length). In effect, the DFB laser is used as a time-gated picosecond detector. The researchers were able to measure variations in the optical length over a 41 h time span resulting from temperature variations, showing a variation of 280 μm/°C. Reference: K. Wada et al., Opt. Express (2015); doi:10.1364/OE.23.023013.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

Sponsored Recommendations

Brain Computer Interface (BCI) electrode manufacturing

Jan. 31, 2025
Learn how an industry-leading Brain Computer Interface Electrode (BCI) manufacturer used precision laser micromachining to produce high-density neural microelectrode arrays.

Electro-Optic Sensor and System Performance Verification with Motion Systems

Jan. 31, 2025
To learn how to use motion control equipment for electro-optic sensor testing, click here to read our whitepaper!

How nanopositioning helped achieve fusion ignition

Jan. 31, 2025
In December 2022, the Lawrence Livermore National Laboratory's National Ignition Facility (NIF) achieved fusion ignition. Learn how Aerotech nanopositioning contributed to this...

Nanometer Scale Industrial Automation for Optical Device Manufacturing

Jan. 31, 2025
In optical device manufacturing, choosing automation technologies at the R&D level that are also suitable for production environments is critical to bringing new devices to market...

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!