We describe a calorimeter for laser power measurements of the conduction type with twin fiber-optic temperature sensor. The sensor employs two fiber-optic polarization insensitive loop interferometers with twisted sections of high-birefringence fiber. The temperature dependent birefringence of these sections is monitored by a white light interferometric technique with signal processing in the spectral domain. One interferometer performs the measurements of the absorber's temperature that depends on the power of incident laser radiation, whereas the other is for monitoring the temperature inside the probe head. A thermal model of the calorimeter has been analyzed. It has been shown that it is possible to reduce the calorimeter's response time, if the laser power is measured using the difference ΔT= T 1− bT 2, where T 1 and T 2 are the readings of the twin temperature sensor, and the factor b depends on the thermal conduction path between the two interferometers. Experimental results are presented for optical power measurements between 0 and 3 W with a resolution of 0.3 mW and a response time of a few seconds.