Analyzing modal contents of arbitrary beams from optical fiber provides useful information for high-power fiber laser and mode-division-multiplexed optical communication. Numerical modal decomposition can be applied to diverse applications universally without complicated setup, but often leads to a false solution which is called as a local minimum. We propose several ways to solve this issue and compare the results to find the best way for global optimization based on stochastic parallel gradient descent algorithm.
Graphene has received great attention over the past decade because of its extraordinary optical, electrical, and mechanical properties. The outstanding thermal properties can be another advantage of graphene, which enabled various applications such as transparent flexible heaters, photo-thermal therapy, and thermo-optic modulator based on graphene. Graphene-based thermo-optic modulators have been recently investigated based on several platforms including tapered fibers and a ring resonator fabricated with silicon waveguides, or micro-fibers to increase graphene-light interaction. But the device exhibiting high extinction ratio and low insertion loss at broad spectral range has not been reported yet.
In this work, we propose a highly efficient all-optical, fiber-optic modulator assisted by photo-thermal effect in monolayer graphene. We use a side-polished fiber (SPF) covered with monolayer graphene, where the guided light experienced strong absorption at graphene layer in the presence of matched index over-cladding. Photo-excited electrons generated by strong optical absorption are then converted to thermal energies via ohmic heating around the graphene sheet, which subsequently changes the refractive index of the over-cladding material possessing large thermo-optic coefficient. This leads to variation of mode-field distribution of guided light at the SPF, resulting in significant absorption change at graphene layer. As a result, the transmitted optical power in our device could be efficiently controlled. Experimental results showed an optical output power variation of ~ 30 dB at 1550 nm in our device with relatively low insertion loss when we adjusted the control beam power by 100 mW at the wavelength of 980 nm.