Optoacoustic tones generated by nanosecond laser pulses can cover the entire human hearing range

Abstract

The aim of this work is to generate defined tones that cover the human hearing range in aqueous media for a later application in middle or inner ear implants. In our experiments, we investigated the characteristics of single laser pulses and pulse trains with different laser repetition rates of nanosecond laser pulses that were focused into aqueous media in a small volume. The frequency of the generated tones was limited by the spectral properties of the single acoustic pulses, which depended on the medium. Tones with fundamental frequencies above 8 kHz were generated using laser pulses focused into water. By replacing water with gel, tones between 500 Hz and 20 kHz could be produced. The generation of tones in the low‐frequency range was only possible when laser pulse trains with pulse density modulated pulse patterns were applied in gel. This enabled the generation of tones between 20 Hz and 2 kHz. Consequently, the combination of different pulse patterns for the different frequency ranges allows generating optoacoustic tones between 20 Hz and 20 kHz in gel. Thus, we can cover the complete range of human hearing through optoacoustically generated tones.

Acoustic stimulation of the human round window by laser-induced nonlinear optoacoustics

“…To achieve an optical breakdown 20 , a nanosecond pulsed near infrared (NIR) laser system (HELIOS 1064-5-50, Coherent, Inc.) with 1064 nm wavelength, a pulse duration of 0.7 ns and a maximum pulse energy of 100 µJ was used. Our setup used in a previous publication 13 was modified in order to place the focus of the laser pulses inside a gel volume in direct contact to the round window membrane 24 . By the nonlinear optoacoustic effect, sequences of pressure waves were generated inside the gel and transmitted acoustically into the cochlear fluid.…”

“…The integral pulse density modulation (IPDM) of the laser pulses is inspired by Li and Jones 27 . It is a further development from an earlier pulse density modulation (PDM) method that was already used in a previous study of our group 13 , 28 . In both methods, the uniform laser pulses can be spaced with varying time gaps between pulses to modulate an arbitrary signal.…”

“…Another optical stimulation method utilizes near-infrared (NIR) laser pulses to directly stimulate neurons such as cortical neurons or the sciatic nerve 2 , 9 – 11 . However, Kallweit et al showed that the stimulation of the cochlea via NIR laser radiation is not directly caused by neuronal stimulation but by the optoacoustic effect, which evokes a pressure wave and consequently an activation of the inner hair cells in residually hearing animals 12 , 13 .…”

“…Wenzel et al in hearing aid applications and for inner ear stimulation by Kallweit et al 12 , 14 . In a recent study, our group demonstrated the laser-induced tone generation by nonlinear optical breakdown in water as well as in a gel volume 13 . In this study the same technique is applied for sound generation in human inner ear specimens.…”

“…This optoacoustic method can be potentially combined with electrical stimulation by cochlear implants for EAS applications to cover low frequencies or with higher pulse repetition rates the entire human hearing range 13 . In our present work, we used an integral pulse density modulation (IPDM) as opposed to pulse density modulation (PDM) from our previous study 13 and translated the application to a fiber-optical setup to stimulate the round window in human temporal bones (TBs) acoustically. The stimulation was produced by modulated sequences of optoacoustically evoked clicks in a gel volume in direct contact with the round window of the cochlea.…”