Femtosecond laser carves refractive-index-sensing F-P cavity in optical fiber

Jan. 29, 2014
Harbin Institute of Technology researchers have created a high-quality Fabry-Perot interferometer (FPI) cavity with flat axial surfaces suitable for sensing the refractive index of liquids.

Yi Liu and Shiliang Qu of the Harbin Institute of Technology (Weihai, China) have used femtosecond-laser-induced water-breakdown machining inside a silica optical fiber to carve out a walled, partially enclosed cavity, then using an arc discharge to anneal the configuration, creating a high-quality Fabry-Perot interferometer (FPI) cavity with flat axial surfaces suitable for sensing the refractive index of liquids. The FPI cavity has a fringe visibility of almost 30 dB and a reflection loss of only -3 dB. The resulting sensor has a sensitivity of 1147. 48 nm/RIU (refractive-index units), a measurement accuracy of 1.29 × 10-4 RIU, and very little temperature sensitivity.

More Laser Articles

Femtosecond laser carves refractive-index-sensing F-P cavity in optical fiber

Low-noise fiber lasers enable far-reaching optical sensing

Stack-and-draw technique creates ultrasmall-diameter endoscopes

The initial cavity was hollowed out using a Ti:sapphire regenerative amplified laser by Coherent (Santa Clara, CA) that had a 1000 Hz repetition rate, an 800 nm central wavelength, and a 120 fs pulse duration; an aperture, optical attenuator, and mechanical shutter provided additional control of the beam, which was focused with a microscope objective. The single-mode fiber being machined was immersed in water to cause a bubble-, shock-wave-, and high-speed-jet-induced breakdown effect. The FPI cavity was then annealed with an electric arc for 0.2 s. The final cavity had an interference length of about 40 μm; its index-sensing ability was tested using solutions of glycerol and water in differing proportions. Contact Shiliang Qu at [email protected].

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.

Voice your opinion!

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