A self-mixing laser-diode interferometer for measuring basilar membrane vibrations without opening the cochlea

Area of Science:

• * Physics
• * Optics
• * Biomedical Engineering

Background:

• * Traditional interferometers struggle with poorly reflective surfaces.
• * Measuring nanoscale displacements in delicate biological structures is challenging.
• * The self-mixing effect in laser diodes offers a potential solution for compact, cost-effective interferometry.

Purpose of the Study:

• * To develop and validate a displacement-sensitive homodyne interferometer using a laser diode.
• * To demonstrate its capability for measurements on poorly reflective surfaces.
• * To apply the interferometer for in-vivo measurements of inner ear mechanics.

Main Methods:

• * A laser diode was employed as the core component of a homodyne interferometer.
• * The self-mixing effect, where reflected light re-enters the laser cavity, was utilized.
• * Real-time calibration was performed using a piezo positioner with known displacement.
• * A signal-processing algorithm was developed to handle high-amplitude noise.

Main Results:

• * The interferometer successfully measured sub-nanometer displacements from poorly reflective surfaces.
• * Real-time calibration confirmed the interferometer's sensitivity.
• * The developed algorithm enabled measurements despite significant noise.
• * Sound-induced basilar membrane displacements were measured in rodent cochleae through the round window.

Conclusions:

• * The laser-diode interferometer is a viable tool for sub-nanometer mechanical measurements.
• * It is suitable for applications involving poorly reflective surfaces and biological tissues.
• * This technique allows non-invasive measurements in the intact cochlea of small mammals.