Graphene-based Optical Waveguide Mode Filters and Mode Converters


Student thesis: Doctoral Thesis

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Award date1 Aug 2018


Graphene, a two-dimensional single layer of carbon atoms, possesses many useful optical properties, such as electrically tunable absorption, exceptionally strong nonlinearity, saturable absorption, efficient luminescence, etc. and promises many interesting applications in the field of photonics. By incorporating graphene films into optical waveguide structures, in particular, efficient guided-wave devices like electrically tunable optical modulators, broadband optical polarizers, and optical sensors can be realized. For the analysis of graphene-incorporated waveguides, three optical models of graphene are available: the interface model, the isotropic model and the anisotropic model. In the interface model, graphene is treated as a conductive interface that has a zero thickness and a surface conductivity. In the isotropic model, which is the most widely used model, graphene is treated as a film that has a finite thickness and an isotropic refractive index. In the anisotropic model, graphene is also treated as a film that has a finite thickness, but its optical property is characterized by a relative permittivity tensor where the conductivity in the normal direction is absent.

In the first part of the study, we analyze the three models of graphene for slab waveguide structures and design experiments to verify the three models. By comparing the measured graphene-induced losses in carefully designed multimode graphene-embedded slab waveguides with the values calculated from the three models, we confirm that the interface model and the anisotropic model give correct results for both the transverse-electric (TE) and transverse-magnetic (TM) modes of the waveguides, while the isotropic model gives correct results only for the TE modes. Our results show that graphene does not significantly affect the phase velocity of the wave propagating in the waveguide, but it can introduce a large absorption loss to the wave whose major electric-field component is parallel to its surface (the TE mode), and little loss to the wave whose major electric-field component is perpendicular to its surface (the TM mode).

In the second part of the study, we explore the polarization-dependent loss property of graphene and propose spatial-mode filters based on embedding graphene films in optical waveguides. The idea is that a graphene film placed inside the core of a waveguide can selectively attenuate specific modes that have major electric-field components parallel to the surface of the graphene film. To demonstrate the effectiveness and the flexibility of the idea, we design and fabricate several mode filters with polymer waveguides, where graphene films of different widths and lengths are placed in different locations of the waveguides. Our mode filters are easy to make and can provide high mode extinction ratios (> 20 dB) that are insensitive to the operation wavelength. Such mode filters could find applications in broadband mode-division-multiplexing transmission systems and other areas that require mode selection or mode stripping.

In the third part of the study, we propose all-optical control of graphene-induced loss with the mode filter demonstrated earlier. According to Pauli blocking effect, when light at a shorter wavelength with sufficiently high power pumps graphene, the generated electron-hole pairs can fill up the low-energy bands of graphene and thus block the absorption of light at a longer wavelength. With this effect, it is possible to change the graphene-induced loss to signal light by controlling the power of pump light launched into a graphene-embedded waveguide. We theoretically analyze the relationship between the graphene-induced loss and the pump power and experimentally achieve a maximum loss modulation of 4.6 dB for 1550-nm signal light with 12.4-dBm 980-nm pump light co-propagating in a 10-mm long graphene-embedded polymer waveguide. Our results open up a new approach to achieving all-optical modulation of modal losses.

In the last part of the study, we propose the use of graphene electrodes for a lithium-niobate (LiNbO3) electro-optic (EO) device with the objective of reducing the driving voltage to the device by exempting the need of placing a buffer layer between the waveguide and the electrodes. As a z-cut LiNbO3 waveguide supports only the TM modes, graphene electrodes can be placed directly on the surface of such a waveguide to maximize the applied electric field experienced by the guided modes without inducing significant modal losses. To demonstrate the idea, we design and fabricate an EO mode converter on a z-cut LiNbO3 substrate, where a long-period grating is generated electrically to achieve conversion between the fundamental mode and the higher-order mode. Our experimental device using graphene electrodes shows a reduction in the driving voltage by almost three times, compared with the one using conventional metal electrodes. With the buffer layer exempted, the device fabrication process is also significantly simplified. The use of graphene electrodes is an effective approach to enhancing the efficiency of EO devices and, at the same time, reducing the fabrication cost of such devices.