# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
## 3 Application of Directly Grown Graphene
### Ultrafast Optical Switching  
In FWM, new frequency components can be generated either by using two strong pumps (\(\omega_{1}\
eq\omega_{2}\)), which is called the non-degenerate case, or one strong pump (\(\omega_{1}=\omega_{2}\)), which is called the degenerate case. FWM is a nonlinear optical Kerr effect derivative that occurs if two waves propagate through a nonlinear medium simultaneously and generate one or two new waves. However, by attaching an optically nonlinear material to the polished surface of an optical fiber, the newly produced signal, based on the FWM concept, can be enhanced to enable ultrafast optical switching for ultra-high-speed optical communication systems. It has already been demonstrated by previous studies that various nonlinear 2D materials, such as carbon nanotubes and black phosphorus (BP) are suitable for FWM [122; 123]. However, considering all the new features in the electronic and optical realms, such as nonlinear optical switching, FWM with graphene could be a new pathway for the computational design of systems and applications. Figure 7: **(a) Schematics of the mode-locker device made from in-situ grown graphene on SMF end facet; (b) Output characteristics of pulse spectrum in wavelength domain mode-locked with device shown in (a); (c) Pulse duration with the mode-locker device shown in (a); (d) Schematics of mode-locker device made from in-situ grown graphene on the flat face of a D-shaped fiber; (e) Output characteristics of pulse spectrum in wavelength domain mode-locked with device shown in (d); (f Pulse duration with mode-locker device shown in (d); (g) Superiority of ACS-grown graphene over transferred graphene; (h) Intracavity power dependent optical spectra of the pulses; (d) Duration of the mode-locked pulses. (a–f) Reproduced with permission from [82]. Copyright WILY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2015. (g–i) Reproduced with permission from [109]. Copyright WILY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2020. **To investigate the FWM process with in situ grown graphene, two CW lasers were employed for the demonstration: one for launching the pump channel and another for the modulated signal channel (tunable) at \(\lambda_{\text{P}}\) = 1552.4 nm and \(\lambda_{\text{s}}\) = 1559 nm, respectively [83]. The resulting wavelengths of the newly produced optical channels were \(\lambda_{\text{Chg1}}\) = 1545 nm and \(\lambda_{\text{Chg2}}\) = 1566 nm for the aforementioned pump channel and signal channel (see Figure 8a), respectively. The in situ graphene-coated D-shaped fiber had a 2 dB (58.5% nonlinear increment) higher value of ER, as indicated in Figure 8b, than that of the bare D-shaped fiber. To exploit ultrafast optical switching with ACS grown graphene demonstrated the values of pump and signal channel wavelengths of \(\lambda_{\text{P}}\) = 1551.92 nm and \(\lambda_{\text{s}}\) = 1555.46 nm, respectively. Due to the optical nonlinearity derived from ACS grown graphene, the newly generated signal channels were found at 1559.01 nm and 1548.39 nm, respectively, by the ultrafast FWM process. By employing an ACS graphene-coated nonlinear device, 2.23 dB (an increase of 67% of the nonlinear process) nonlinear enhancement at 20 GHz modulation frequency was recorded by calculating the extinction ratio (ER) difference between with and without ACS graphene cases [109]. Figure 8c shows the evolution of the spectral broadening in the produced channels with the separation of the sidebands as the modulation frequency is increased for ACS grown graphene. To evaluate the detuning experiment using an ACS-grown-graphene-enabled optical device, the pump channel was fixed at 1551.92 nm, while the modulated launched signal channel wavelength was tuned up to 11.828 nm with a resolution of 0.2 nm.