This thesis investigates the constancy of light speed with respect to the light age using an active fiber-based Mach-Zehnder interferometer (MZI) illuminated by sunlight. The original sunlight in one arm, which has the age of 8.3 minutes since it was emitted from the sun, interferes with the young light generated by stimulated emission from an optical amplifier in the other arm, which is coherent to the sunlight. The speed difference between the sunlight and the newly regenerated light is obtained by measuring the phase variations as the interference signal travels along a span of single-mode fiber. System error is eliminated by the self-calibration which is achieved by alternatively switching other two local amplified spontaneous emission light sources and the sunlight as the input to the MZI, and then comparing the measured phase differences of these three sources. The relative accuracy of light speed measurement is . Consider the 8.3 minutes age of the sunlight, for a possible variation of light speed of c with light age in optical fiber, we obtainĢc/c is less than 1.7 / Master of Science