A fast and highly sensitive reflective fiber-optic hydrophone for measuring pressure waves in liquids, wherein it comprises at least: - a source of probe light coupled into a first single-mode fiber , - the first single-mode fiber coupled into a first fiber-optic element for transmitting at least a part of the incoming probe-light propagating in the forward direction from the first single-mode fiber into a second single-mode fiber and for transmitting at least a part of a reflected light from a probe propagating in the backward direction into a third single-mode fiber , characterized in that the hydrophone further comprises - the single-mode fiber-optic probe having a perpendicularly-cleaved or polished single-mode fiber tip, so that the surface of the fiber tip is perpendicular to the probe-light propagation direction, wherein the probe is connected to the second single-mode fiber , - a source of pump light coupled into a fourth optical fiber with a wavelength chosen such that it allows for pumping an optical amplifier, - a second fiber-optic element for combining the reflected probe light from the third single-mode fiber and the pump light into a fifth optical fiber , - a single-mode fiber-optic amplifier for amplifying optical signal from the fifth optical fiber , said amplifier comprising a doped single-mode fiber with length between 20 cm and 5 m to allow for sufficient absorption of the pump light and sufficient amplification of the probe light, wherein the dopant is chosen according to the chosen probe light wavelength, preferably ytterbium-doped or erbium-doped fiber is used, and - a photodetector with sensitivity above 0.01 A/W and bandwidth above 1 GHz to measure the previously amplified probe light. - a source of probe light coupled into a first single-mode fiber , - the first single-mode fiber coupled into a first fiber-optic element for transmitting at least a part of the incoming probe-light propagating in the forward direction from the first single-mode fiber into a second single-mode fiber and for transmitting at least a part of a reflected light from a probe propagating in the backward direction into a third single-mode fiber , - the single-mode fiber-optic probe having a perpendicularly-cleaved or polished single-mode fiber tip, so that the surface of the fiber tip is perpendicular to the probe-light propagation direction, wherein the probe is connected to the second single-mode fiber , - a source of pump light coupled into a fourth optical fiber with a wavelength chosen such that it allows for pumping an optical amplifier, - a second fiber-optic element for combining the reflected probe light from the third single-mode fiber and the pump light into a fifth optical fiber , - a single-mode fiber-optic amplifier for amplifying optical signal from the fifth optical fiber , said amplifier comprising a doped single-mode fiber with length between 20 cm and 5 m to allow for sufficient absorption of the pump light and sufficient amplification of the probe light, wherein the dopant is chosen according to the chosen probe light wavelength, preferably ytterbium-doped or erbium-doped fiber is used, and - a photodetector with sensitivity above 0.01 A/W and bandwidth above 1 GHz to measure the previously amplified probe light. <EOS>