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.
Electro-optic modulators which modulates the intensity or phase of the light through the electric signal control, have been extensively investigated for the diverse field applications including optical communication, bio-sensing, and security-monitoring based on lightwave. With recent technological advance of the fabrication of high quality graphene over large area, graphene have been intensively studied as a basic element to build novel photonic and electro-optic devices. However, low optical absorption in ultra-thin layered graphene often limits the performance of the device. Although there have been several attempts to increase graphene-light interaction, realization of efficient and broadband graphene-based electro-optic modulators is still challenging.
In this work, we demonstrate an all-fiber graphene-based electro-optic modulator with a modulation depth of > 25 dB. In order to achieve non-resonant strong interaction with graphene, we employed a side-polished fiber (SPF) with high numerical aperture (NA) as a novel platform that evanescently interacts with graphene. The high NA fiber has about six times smaller mode-field area than that of the standard single-mode fiber, and we found that this can critically enhance the graphene-light interaction without significantly sacrificing the insertion loss. We experimentally fabricate the bi-layer graphene field-effect transistor onto the high-NA SPF, and covered index matched ion-liquid for further increase of the graphene-light interaction and effective gating. As a result, we observed that the fabricated device exhibits the modulation depth of 27.6 dB with low scattering loss at the applied voltage range within 2.5 V, which well agrees with our numerical expectation.