Graphene-clad microfibre saturable absorber for ultrafast fibre lasers

Abstract

Graphene, whose absorbance is approximately independent of wavelength, allows broadband light-matter interactions with ultrafast responses. The interband optical absorption of graphene can be saturated readily under strong excitation, thereby enabling scientists to exploit the photonic properties of graphene to realize ultrafast lasers. The evanescent field interaction scheme of the propagating light with graphene covered on a D-shaped fibre or microfibre has been employed extensively because of the nonblocking configuration. Obviously, most of the fibre surface is unused in these techniques. Here, we exploit a graphene-clad microfibre (GCM) saturable absorber in a mode-locked fibre laser for the generation of ultrafast pulses. The proposed all-surface technique can guarantee a higher efficiency of light-graphene interactions than the aforementioned techniques. Our GCM-based saturable absorber can generate ultrafast optical pulses within 1.5 μm. This saturable absorber is compatible with current fibre lasers and has many merits such as low saturation intensities, ultrafast recovery times, and wide wavelength ranges. The proposed saturable absorber will pave the way for graphene-based wideband photonics.

Figure 1

Schematic illustration of graphene-based SAs with (a) a graphene film coating on a pinhole (red), (b) a graphene–PVA nanocomposite film integrated between a pair of fibre connector ends, (c) a graphene film coating on the D-shaped fibre and (d) on the microfibre, and (e) a graphene/polymer composite embedded on the microfibre.

Figure 2

Schematic diagram of (a) the GCM SA and (b) the hollow substrate together with GCM. A graphene monolayer is wrapped around a microfibre. The GCM SA is fixed onto a hollow substrate with a height of h ≈ 1.5 mm.

Figure 3. Optical microscope images of a microfibre with the PMMA/graphene.

(a) Top view before cutting. Lateral view (b) before and (c) after cutting. The detailed cutting procedure is illustrated in a video (see video: CutGraphene.swf in the online supplementary information).

Figure 4. Sample characterization.

(a) Diameter of the microfibre along the direction of propagation. x₀ is the position of the microfibre at the minimum diameter. (b) SEM image of the GCM. Scale bar, 10 μm. (c) Raman spectra of the monolayer graphene film on the microfibre. (d) Nonlinear absorption characterization of the GCM SA. The solid curve represents a fit to the experimental data (circles).

Figure 5. Optical properties of GCM.

(a) Power density at the surface of the microfibre and (b) the energy fraction in the air (i.e., P_air/P) for the HE₁₁ mode versus the diameter of the microfibre, D. Inset in (a): the cross-sectional intensity distribution in a 10-μm diameter microfibre (calculation performed using COMSOL).

Figure 6. Laser setup.

EDF, erbium-doped fibre; WDM, wavelength-division multiplexer; PC, polarization controller; SMF, single-mode fibre; PI-ISO, polarization-independent isolator; LD, laser diode; GCM SA, graphene-clad microfibre saturable absorber.

Figure 7. Typical laser characteristics.

(a) Optical spectrum with a spectral resolution of 0.02 nm at pump power P = 27 mW. The FWHM spectral width Δλ is approximately 2.08 nm. (b) Autocorrelation traces of the experimental data (circles) and sech²-shaped fit (solid curve). (c) Fundamental RF spectrum with a resolution of 1 Hz and a span of 100 Hz. Inset: oscilloscope trace with a separation of 529.4 ns, corresponding to 1.888847 MHz of the fundamental cavity frequency, which is independent of the pump power. (d) Wideband RF spectrum up to 500 MHz.