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 (1, 1') coupled into a first single-mode fiber (1a, 1a'), - the first single-mode fiber (1a, 1a') coupled into a first fiber-optic element (2, 2') for transmitting at least a part of the incoming probe-light (2a, 2a') propagating in the forward direction from the first single-mode fiber (1a, 1a') into a second single-mode fiber (2b, 2b') and for transmitting at least a part of a reflected light from a probe (4, 5') propagating in the backward direction into a third single-mode fiber (2c, 2c'), characterized in that the hydrophone further comprises - the single-mode fiber-optic probe (4, 5') 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 (2, 2') is connected to the second single-mode fiber (2b, 2b'), - a source (5, 6') 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 (6, 7') for combining the reflected probe light from the third single-mode fiber (2c, 2c') and the pump light into a fifth optical fiber, - a single-mode fiber-optic amplifier (7, 8') for amplifying optical signal from the fifth optical fiber, said amplifier (7, 8') 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 (8, 9') with sensitivity above 0.01 A/W and bandwidth above 1 GHz to measure the previously amplified probe light. - a source of probe light (1, 1') coupled into a first single-mode fiber (1a, 1a'), - the first single-mode fiber (1a, 1a') coupled into a first fiber-optic element (2, 2') for transmitting at least a part of the incoming probe-light (2a, 2a') propagating in the forward direction from the first single-mode fiber (1a, 1a') into a second single-mode fiber (2b, 2b') and for transmitting at least a part of a reflected light from a probe (4, 5') propagating in the backward direction into a third single-mode fiber (2c, 2c'), - the single-mode fiber-optic probe (4, 5') 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 (2, 2') is connected to the second single-mode fiber (2b, 2b'), - a source (5, 6') 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 (6, 7') for combining the reflected probe light from the third single-mode fiber (2c, 2c') and the pump light into a fifth optical fiber, - a single-mode fiber-optic amplifier (7, 8') for amplifying optical signal from the fifth optical fiber, said amplifier (7, 8') 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 (8, 9') with sensitivity above 0.01 A/W and bandwidth above 1 GHz to measure the previously amplified probe light.