A monitoring device for monitoring a fluid in subsea equipment, comprising: a waveguide with a sensing element in contact with the fluid, said waveguide comprising an optical fiber with a core; a plurality of radiation sources (6, 7, 8) in communication with the sensing element via a plurality of first transmission lines (17, 18, 19) and a first coupling element designed to merge the plurality of first transmission lines (17, 18, 19) from the plurality of radiation sources (6, 7, 8) into a part of the waveguide which comprises the sensing element; a plurality of detectors (16, 21) in communication with the sensing element via an optical splitter and a plurality of second transmission lines (23, 24); a control unit configured to operate the plurality of radiation sources (6, 7, 8) and the plurality of detectors (16, 21) and configured to put the plurality of radiation sources (6, 7, 8) on standby or turn the plurality of radiation sources (6, 7, 8) off for prolonged periods of time and activate the plurality of radiation sources (6, 7, 8) just when monitoring of the fluid is required; the waveguide, operatively coupled to the first coupling element and the optical splitter; wherein said plurality of radiation sources (6, 7, 8) feeding a plurality of optical signals into said waveguide and wherein each one of the plurality of radiation sources (6, 7, 8) is designed to feed the optical signals into the waveguide over a range of wavelengths, wherein the range of wavelengths corresponds at least partially to wavelengths being to a larger extent attenuated by a specific contaminant in the fluid than by other contaminants; wherein the sensing element is designed as a region of the waveguide in which said core is at least partially free of a cladding, wherein the plurality of detectors (16, 21) is designed to detect the optical signals of a predetermined number of wavelengths and an attenuated optical signal fed into the waveguide, the attenuation being caused by evanescent field absorption by the fluid, wherein the optical splitter is designed to distribute the attenuated optical signals of the plurality of radiation sources (6, 7, 8) to the plurality of detectors (16, 21), and wherein the control unit is designed to be partially submerged in the fluid and to withstand a pressure of 300 bar existing in the fluid. a waveguide with a sensing element in contact with the fluid, said waveguide comprising an optical fiber with a core; a plurality of radiation sources (6, 7, 8) in communication with the sensing element via a plurality of first transmission lines (17, 18, 19) and a first coupling element designed to merge the plurality of first transmission lines (17, 18, 19) from the plurality of radiation sources (6, 7, 8) into a part of the waveguide which comprises the sensing element; a plurality of detectors (16, 21) in communication with the sensing element via an optical splitter and a plurality of second transmission lines (23, 24); a control unit configured to operate the plurality of radiation sources (6, 7, 8) and the plurality of detectors (16, 21) and configured to put the plurality of radiation sources (6, 7, 8) on standby or turn the plurality of radiation sources (6, 7, 8) off for prolonged periods of time and activate the plurality of radiation sources (6, 7, 8) just when monitoring of the fluid is required; the waveguide, operatively coupled to the first coupling element and the optical splitter; wherein the sensing element is designed as a region of the waveguide in which said core is at least partially free of a cladding, wherein the plurality of detectors (16, 21) is designed to detect the optical signals of a predetermined number of wavelengths and an attenuated optical signal fed into the waveguide, the attenuation being caused by evanescent field absorption by the fluid, wherein the optical splitter is designed to distribute the attenuated optical signals of the plurality of radiation sources (6, 7, 8) to the plurality of detectors (16, 21), and wherein the control unit is designed to be partially submerged in the fluid and to withstand a pressure of 300 bar existing in the fluid.