We demonstrate pulsewidth tunable Q-switched erbium fiber lasers using in-line high extinction ratio (> 30 dB) abrupt-tapered fiber Mach-Zehnder block filter and tunable Fabry-Perot filter in laser cavity. The pulsewidth can be tuning over 78 ns -23 ms. Q-switched pulse shape with pulsewidth and repetition rate at 1.84 μs, 60.84 kHz

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A differential gain-dependent phase shift between the core mode and the cladding mode can be all-optically tuned by a cw 975 nm pump light for Mach-Zehnder interferometers using two abrupt fiber tapers in a 1.2-cm-long highly Er/Yb codoped fiber. The highly doped fiber has large absorption and emission cross sections to efficiently introduce the phase shift when the inversion rate is changed by varying the pump power. The first abrupt taper converts part of core mode into the cladding mode, while the second abrupt taper recombines both modes to interfere after propagating for a length of a few millimeters.

All-fiber inclinometers and displacement sensors using tunable Mach-Zehnder interferometers with an extinction ratio of > 30.4 dB are produced by introducing two abrupt tapers in a short length (<1.4 cm) of highly Er/Yb co-doped fiber. The abrupt tapers partially convert the core mode into cladding modes and interference occurs when the phase difference between the core and cladding modes accumulate to π after a certain propagation length. The maximal wavelength shift for the resonant transmission dips can reach 40.8 nm (from 1648 to 1607.2 nm) when the inclination angle, θ, varies from 0°to 32°. The wavelength shift can reach 26 nm (from 1404.4 to 1430.4 nm) when the compressive strain ε is 0.004.

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