An apparatus for sensing a parameter beneath a surface of the earth includes an optical fiber disposed beneath a surface of the earth and comprising at least one fiber Bragg grating sensor configured to sense the parameter and a wide-band light source disposed beneath the surface of the earth and configured to emit light in a wide band of wavelengths to illuminate the at least one fiber Bragg grating. An optical interrogator is disposed beneath the surface of the earth and configured to receive light reflected by the at least one fiber Bragg grating sensor and to transform a shift in wavelength of the reflected light into a variation of light intensity. A photo-sensor is configured to measure intensity of light received from the optical interrogator. Electronics are coupled to the photo-sensor and configured to measure a voltage representing the light intensity at the photo-sensor to sense the parameter.