A four-core optical fiber is used to investigate one-dimensional heat transfer measurements. Heat pulses from a Nd:YAG laser of 600 ms duration with a repetition rate of the order of 10 s are delivered onto one of the fiber cores. This results in an optical path length difference between the guiding cores due to the change in the refractive index and physical length of the targeted fiber core. As a result of this process, a phase shift of 1.30 rad is measured with a digital camera for 140 mW pulses in reflection scheme. The heat diffusion length in the selected fiber core is determined to be 2.8 mm, which contains 33.2 kJ/m2s heat, causing a temperature rise of 4.30 K.