After receiving his doctorate, Mitra left for France in 1920 to continue his studies at the University of Paris. There he earned a second doctorate under Charles Fabry in 1923, for a thesis on the determination of wavelength standards in the 2,000–2,300 Å region of copper. He subsequently worked briefly under Marie Curie at the Curie Institute. Made aware of continual developments in the new science of radio communications, Mitra went to the University of Nancy and joined the laboratory of Camille Gutton. Under Gutton, Mitra conducted research on radio valve circuitry before returning to India in late 1923.