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We report on a robust scheme for wideband variable-phase interferometer stabilization based on active modulation. In contrast to previous schemes, the correction signal is generated without using second harmonics, whose low amplitude often requires employing narrowband lock-in amplifiers. Resonances in the element modulating the phase are attenuated to enable high gain without high-frequency oscillations. Operation over a 3-kHz bandwidth is demonstrated.[Read More….]

source: © 2012 IEEE Photonics Technology Letters

(a) Schematic description of the experimental setup. ASE: amplified spontaneous emission. EDFA: erbium-doped fiber amplifier. ODL: optical delay line. FB: feedback. PZT: piezoelectric transducer. (b) Amplitude of frequency response of the PZT h^(f) . The first resonance is obtained at f=18kHz . (c) Spectrum of the signal detected by the photodiode with sine modulation at 70 kHz obtained for two values of ϕ . The first and second harmonics are marked by arrows.

A. Rosenthal, S. Kellnberger, G. Sergiadis, and V. Ntziachristos, “Wideband arbitrary-phase interferometer stabilization,” Phot. Technol. Lett. Vol. 24, pp. 1499 – 1501 (2012)

source: © 2011 Optical Society of America

A highly sensitive compact hydrophone, based on a pi-phase-shifted fiber Bragg grating, has been developed for the measurement of wideband ultrasonic fields. The grating exhibits a sharp resonance, whose centroid wavelength is pressure sensitive. The resonance is monitored by a continuous-wave (CW) laser to measure ultrasound-induced pressure variations within the grating. In contrast to standard fiber sensors, the high finesse of the resonance—which is the reason for the sensor’s high sensitivity—is not associated with a long propagation length. Light localization around the phase shift reduces the effective size of the sensor below that of the grating and is scaled inversely with the resonance spectral width. In our system, an effective sensor length of 270 μm, pressure sensitivity of 440 Pa, and effective bandwidth of 10 MHz were achieved. This performance makes our design attractive for medical imaging applications, such as optoacoustic tomography, in which compact, sensitive, and wideband acoustic detectors are required.
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Fig. 1 Schematic description of the detection scheme. A CW laser is used to monitor the reflection of an FBG.

A. Rosenthal, D. Razansky, and V. Ntziachristos, “High-sensitivity compact ultrasonic detector based on a pi-phase-shifted fiber Bragg grating,” Opt. Let., Vol. 36, pp. 1833-1835 (2011).