An apparatus for locating a measurand anomaly along an optical waveguide comprising: an optical waveguide; a light source configured to transmit light along the waveguide, wherein the light source comprises a laser which is selectively switchable between a continuous wave mode and a pulsed mode; a plurality of sensors provided along the waveguide, each sensor being configured to reflect a portion of light propagating along the waveguide at a respective sensor wavelength corresponding to a measurand, wherein the plurality of sensors is configured into one or more sets according to their sensor wavelengths, each set comprising a plurality of sensors with respective sensor wavelengths, wherein the sensors are configured such that the sensor wavelength for each sensor in a respective set is equal to within a wavelength range of 0.1 nm when the measurand experienced by each of the sensors in that set is equal; a detector configured to monitor the light reflected by the sensors; and a control system configured to cause the following steps to be performed: (i) transmitting (101, 103) light along the optical waveguide using the light source by operating the laser in the continuous wave mode; (ii) monitoring the light reflected by the sensors so as to obtain a measured spectrum representing a measurand experienced by each of the sensors; (iii) detecting (104, 105) an anomalous signal in the measured spectrum using a spectral analysis technique, the anomalous signal having a characteristic wavelength and originating from an anomalous sensor of the plurality of sensors, the anomalous sensor experiencing the measurand anomaly, and wherein the spectral analysis technique comprises comparing the measured spectrum with a target spectrum so as to detect the anomalous signal in the measured spectrum corresponding to a difference between the measured spectrum and the target spectrum; and (iv) locating (106a-108) the anomalous sensor by operating the laser in the pulsed mode. an optical waveguide; a light source configured to transmit light along the waveguide, wherein the light source comprises a laser which is selectively switchable between a continuous wave mode and a pulsed mode; a plurality of sensors provided along the waveguide, each sensor being configured to reflect a portion of light propagating along the waveguide at a respective sensor wavelength corresponding to a measurand, wherein the plurality of sensors is configured into one or more sets according to their sensor wavelengths, each set comprising a plurality of sensors with respective sensor wavelengths, wherein the sensors are configured such that the sensor wavelength for each sensor in a respective set is equal to within a wavelength range of 0.1 nm when the measurand experienced by each of the sensors in that set is equal; a detector configured to monitor the light reflected by the sensors; and a control system configured to cause the following steps to be performed: (i) transmitting (101, 103) light along the optical waveguide using the light source by operating the laser in the continuous wave mode; (ii) monitoring the light reflected by the sensors so as to obtain a measured spectrum representing a measurand experienced by each of the sensors; (iii) detecting (104, 105) an anomalous signal in the measured spectrum using a spectral analysis technique, the anomalous signal having a characteristic wavelength and originating from an anomalous sensor of the plurality of sensors, the anomalous sensor experiencing the measurand anomaly, and wherein the spectral analysis technique comprises comparing the measured spectrum with a target spectrum so as to detect the anomalous signal in the measured spectrum corresponding to a difference between the measured spectrum and the target spectrum; and (iv) locating (106a-108) the anomalous sensor by operating the laser in the pulsed mode. (i) transmitting (101, 103) light along the optical waveguide using the light source by operating the laser in the continuous wave mode; (ii) monitoring the light reflected by the sensors so as to obtain a measured spectrum representing a measurand experienced by each of the sensors; (iii) detecting (104, 105) an anomalous signal in the measured spectrum using a spectral analysis technique, the anomalous signal having a characteristic wavelength and originating from an anomalous sensor of the plurality of sensors, the anomalous sensor experiencing the measurand anomaly, and wherein the spectral analysis technique comprises comparing the measured spectrum with a target spectrum so as to detect the anomalous signal in the measured spectrum corresponding to a difference between the measured spectrum and the target spectrum; and (iv) locating (106a-108) the anomalous sensor by operating the laser in the pulsed mode.