Midfringe locked interferometer
SOTA low-noise measurement method
SOTA low-noise measurement method
K. Ishikawa et al. ``Low-noise optical measurement of sound using midfringe locked interferometer with differential detection,'' J. Acoust. Soc. Am. vol.150, no.2, pp.1514－1523, 2021. [Paper]
A midfringe locked interferometer with differential detection is proposed for non-contact optical sound measurement, and the equivalent noise level of approximately 0 dB SPL/Hz is achieved. The noise level of the proposed method is 30 dB lower than that of a very recent laser Doppler vibrometer and close to that of a quarter-inch measurement microphone. The midfringe locking stabilizes the optical interferometer against slow environmental fluctuations and enables detection of the acoustic signal directly from optical intensity. The differential detection method eliminates laser intensity noise, which is a dominant noise source in optical interferometers. The noise level of the constructed system was approximately 10 dB above the optical shot-noise (the classical detection limit attributed to the quantum nature of light) at frequencies higher than 2 kHz. Further noise reduction by several available methods could lead to optical measurements that are more sensitive than measurements by microphones. In addition, the constructed interferometer is used to reconstruct sound fields generated by a half-inch laboratory standard microphone used as a transmitter. The proposed method will be a powerful tool for measuring small-amplitude sound fields where it has been challenging to use existing methods.
Measurement system
Midfringe locked interferometer is a homodyne optical interferometer with a feedback control mechanism. A reference mirror position is actively controlled by a piezo-electric actuator to stabilize the operating point (phase difference between two waves) at midfringe. This stabilization ensures maximum sensitivity to the sound. We achieved state-of-the-art low-noise sound measurement by combining this mechanism with saturated operation and differential detection. Please refer to the paper for more details.
Experiment
The proposed method, d-MFLI (MFLI with differential detection), is compared with a quarter-inch measurement microphone (BK4939), a commercial laser Doppler vibrometer (VibroFlex, Polytec GmbH), and MFLI with single-ended detection. The sinusoidal wave of 50 dB SPL was measured by each method, and their power spectra were compared. The results revealed that the proposed method (d-MFLI) has a noise floor 30 dB lower than the LDV. The d-MFLI achieved the state-of-the-art low noise level of 0 dB SPL/Hz.