Strain sensing at the 10-13 level, from the mHz to the kHz frequency range, is performed using a fiber Bragggrating
resonator interrogated by an optical-frequency-comb stabilized laser. The optical-comb synthesizer is a
fs-pulsed fiber laser phase-locked to a quartz oscillator, which provides for a very high laser-frequency
stability. The comb teeth span over the 1510-1580 nm spectral interval. Therefore, the OFS is also used as a
coherent radiation source for fiber-loop cavity ring-down evanescent-wave spectroscopy of analytes diluted in
liquid samples.
A novel fibre-cavity design based on highly reflective gold coatings, vapor-deposited to the two end faces of a
400um multimode waveguide, is presented. In contrast to common fibre-cavity approaches, the laser pulses are
not coupled through the reflective coatings into the cavity but through a micro hole in one of the fibre end
faces, which reduces the coupling losses from generally almost 100! to less than 1!. Since the decay function
of the back and forth reflected pulses is acquired through the same micro hole, a compact bi-directional module
can be used for pulse transmission and acquisition, consisting of a low power uncooled laser source and a fast
photodiode detector. By choosing the cavity length to be longer than the pulse width, wavelength tuning of the
pulses can be omitted resulting in a simplified hardware setup. Thus, the novel fibre-cavity design facilitates
ring-down experiments and considerably reduces the cost of the associated sensor applications.
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