Abstract: A device and a method for detecting a water level of a water trap in a fuel cell can accurately output a water level of water collected in the water trap by reaction of the fuel cell. The device and the method for detecting a water level of a water trap can detect a change in a surrounding temperature of a water level sensor by mounting a separate temperature sensor in the water level sensor and accurately output the water level in the water trap regardless of the change of the surrounding temperature through a water sensor output value correction algorithm based on a detected temperature.

Abstract: Provided is a fiber ring laser capable of obtaining a back-up laser light and standard light having a wavelength continuously tuned in a single mode with equal spacing of 12.5 GHz, 25 GHz, 50 GHz or 100 GHz recommended by ITU-T Recommendation G. 692 & G. 694.1 in a C-band and an L-band using a fiber tunable etalon filter, an air gap etalon filter and a saturable absorber in an optical fiber laser resonator having a serial ring shape using a C-band optical amplifier and an L-band optical amplifier. The fiber ring laser can generate laser light having a wavelength tuned by more than 70 nm, excellent output power flatness and a source spontaneous emission ratio of more than 70 dB in 361 channels with equal spacing of 25 GHz by applying a bias voltage to a voltage-operated piezoelectric element of a tunable etalon filter. A single longitudinal mode operation of the fiber ring laser can be obtained by using a saturable absorber serving as a narrow-bandwidth filter.