Externally pumped low-loss graphene-based fiber Mach-Zehnder all-optical switches with mW switching powers

We propose and experimentally demonstrate an all-optical switch based on a graphene-coated fiber Mach-Zehnder interferometer, where the phase of the signal light in one arm of the interferometer is changed by the heat generated from external pump light absorption by the graphene coating. The external pumping scheme allows efficient pump absorption with multiple layers of graphene coated on an ordinary fiber or a slightly tapered fiber without introducing significant additional signal loss. Without using any wavelength multiplexer/demultiplexer, the switch can be pumped at any convenient wavelength or even with broadband light. Our experimental device, which is based on a standard 125-μm-diameter single-mode fiber with a 5-mm-long graphene coating, can be switched with a pump power of 5.3 mW at an extinction ratio of 19 dB with no additional signal loss. The switching power is insensitive to the graphene coating's length and can be reduced to 4.8 mW, with the fiber tapered to 40 μm. The measured switching powers agree well with the theoretical values obtained by treating the graphene coating as a uniform sheet of heat source without thickness. The switch's response time decreases with the fiber diameter and inversely with the graphene coating's length. The switch's rise and fall times, based on a 40-μm tapered fiber with a 20-mm-long graphene coating, are 30 ms and 50 ms, respectively.