Abstract
A low-noise transducer based on a fiber Fabry-Perot (FFP) cavity was used as a pickup for an acoustic guitar. A distributed feedback (DFB) laser was locked to a 25 MHz-wide resonance of the FFP cavity using the Pound-Drever-Hall method. The correction signal was used as the audio output and was preamplified and sampled at up to 96 kHz. The pickup system is largely immune against optical noise sources, exhibits a flat frequency response from the infrasound region to about 25 kHz, and has a distortion-free audio output range of about 50 dB.
Highlights
Vibration and strain sensors based on fiber Bragg gratings (FBGs) exploit that the FBG reflection spectrum is narrow and shifts as strain is applied to the optical waveguide [1]
In-fiber Fabry-Perot (FFP) cavities made from two identical FBGs can be used for vibration and strain measurements, since the resonances in the cavity spectrum depend on the cavity length and thereby, again, on the strain applied to the waveguide [1]
The FFP cavity was interrogated using the PDH-frequency locking method and was thereby rendered largely immune to noise arising from optical intensity fluctuations
Summary
Vibration and strain sensors based on fiber Bragg gratings (FBGs) exploit that the FBG reflection spectrum is narrow and shifts as strain is applied to the optical waveguide [1]. In-fiber Fabry-Perot (FFP) cavities made from two identical FBGs can be used for vibration and strain measurements, since the resonances in the cavity spectrum depend on the cavity length and thereby, again, on the strain applied to the waveguide [1] Both types of fiber strain sensors have been applied recently as transducers for musical instruments [7, 8]. By detection and audio sampling of the reflected light from either an FBG or a low finesse in-fiber Fabry-Perot cavity our group has realized compact and very lightweight transducers (“pickups”) for guitars, violins, harmonicas and other musical instruments, in which the sound is generated to a large extent by the vibration of a soundboard. Since the strain information is contained in the frequency response of the transducer and is largely independent of the reflected light intensity, a guitar pickup based on a PDH method is expected to be immune to most optical noise sources. Due to, e.g., thermal expansion or material fatigue will be compensated by the feedback mechanism
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