Subwavelength Optical Fiber Sensitive to Sound

Subwavelength optical fibers that produce a newly identified type of Brillouin light scattering could guide the development of highly sensitive sensors.

A team at the Femto-ST Institute, in collaboration with the Institute of Optics’ Charles Fabry Laboratory, made this discovery upon heating and stretching optical fibers used in telecommunications that measured 125 µm in diameter, and then injecting a laser beam into silica microwires.

Brillouin light scattering was observed during the process. In conventional optical fibers, the phenomenon induces “forward-guided acoustic wave Brillouin light scattering and backward-stimulated Brillouin scattering,” the researchers wrote in a study. Typically, light travels in the core of a 10-µm optical fiber and does not generate surface waves.

A red HeNe laser beam passes through a 1-µm-diameter optical fiber encapsulated in an airtight system to avoid oxidation. Courtesy of Thibaut Sylvestre/Institut Femto-ST/CNRS.

In optical fibers with subwavelength diameter, however, the researchers have produced the new type of Brillouin scattering from surface acoustic waves.

The diameter of these fibers is smaller than the wavelength of the IR light used (1.5 µm), making the light extremely confined inside. As it traveled, it shook the wire an immeasurable amount, displacing it by a few nanometers. This distortion induced the acoustic wave that traveled along the fiber surface at a velocity of 3400 m/s with a Doppler shift of about 6 GHz, according to the researchers.

The acoustic wave in turn affected the propagation of the light, as part of the light radiation was sent back with a different wavelength in the opposite direction.

Since the waves generated by the confinement of the light can travel along the surface of the microfibers, the researchers said they are sensitive to environmental factors such as temperature, pressure and ambient gas.

The findings could lead to highly sensitive, compact optical sensors for a variety of sensing applications. This could also better explain the fundamental interaction between light and sound at the microscale.

The work was published in Nature Communications (doi: 10.1038/ncomms6242).

For more information, visit www.femto-st.fr.