Fig. 1. (a) Schematic of the configuration and operation principle of the graphene-coated TFBG integrated with a pair of electrodes. (b) Experimental arrangement for measuring the transmission spectrum and photoelectric conversion performance of the TFBG device. BBS, broadband light source; PC, polarization controller; EDFA, Erbium-doped fiber amplifier; OSA, optical spectrum analyzer.

Fig. 2. (a) Preparation process of the graphene-coated TFBG device and determination of orientations of gold-coating and grating planes. (b) Diffraction fringes at a three-dimensional space, showing the weak and strong evanescent fields at two orthogonal directions, respectively. (c) Optical microscopic images of the device under bright-field, there is the junction of uncoated and coated region between the two electrodes. (d) Raman spectrum of the transferred graphene layer, the G mode is at 1593.19 cm⁻¹.

Fig. 3. (a) Polarized transmission spectra of the TFBG before and after graphene coating. (b, c) Simulated electric field distributions of the selected P-polarized and S-polarized cladding modes of the graphene-coated TFBG. (d) Electric field intensity of P-polarized and S-polarized modes along the fiber radial. The inset shows the intensity distributions of the 25th-order cladding modes near the boundary between fiber and graphene layer. (e) Enlarged transmission spectra before and after graphene coating.

Fig. 4. Photoelectric response of the graphene-coated TFBG device. (a, b) Dependence of photocurrent on the power absorbed by graphene with P-polarized light pump under applied bias voltages of 0.1, 0.2 and 0.3 V, and (c) photoelectric responsivity of the device changing with the incident power at a bias voltage of 0.3 V. The inset of (b) shows the linear power-dependence of photocurrent in the low-power case. The inset of (c) shows the operation mechanism of electron-hole pair excitation in graphene in weak light (left) and saturated absorption (right). (d–f) Power-dependence and responsivity of the photocurrent for the case of S-polarized light pump.

Fig. 5. (a) Wavelength dependence (blue solid-line) of the photocurrent, which is well-matched with the transmission (red dashed-line) of TFBG scanned with a TL and measured with a commercial photodetector. (b) Temporal response of TFBG-based photodetector. (c) Temporal responses of the photodetector at different applied voltages with a fixed light power.

Fig. 6. (a) Transmission spectrum of the graphene-coated TFBG. (b) Spectral shift of one (Dip A) of the cladding modes with the increase of current. (c) Wavelength shift versus the square of current. (d) Measured transmission spectra of the TFBG without (red) and with (blue) the electrical injection of 2.85 mA, and the spectrum of a switching signal light. (e) Temporal response of the “electric-optical” switching effect. (f) Enlarged temporal response over a period with a rise/fall time of 148 ms/56 ms.