High-Power Limitations of Graphene Nanocoated Optical Taper Saturable Absorbers

Abstract

In the last two decades, graphene has attracted much attention because of its unique properties [1, 2]. Particularly, thanks to its Saturable Absorption (SA) characteristics, graphene is widely used in mode-locked fiber lasers in order to produce ultrashort pulses [2]. Different methods have been proposed to realize a graphene-based SA such as direct deposition onto the fiber end. In our case, we use graphene nanocoated fiber tapers (GNOTs) [3]. These latter are fabricated by reducing a single mode fiber's diameter to approximately 20 μm, over 5 mm waist length. Graphene nano-flakes initially in suspension in water are then deposited on the taper waist thanks to the optical tweezers effect. While graphene has demonstrated very good saturable absorption properties for low power applications, there are very few results concerning its use in high power fiber lasers [3, 4]. In this communication, we point out the deterioration of its nonlinear optical properties under high average optical power. Indeed, when a certain optical power is injected in a GNOT, its SA properties are irreversibly modified. The experimental setup is shown in figure 1-(a). The pulsed laser emits 5 ps pulses at a central wavelength of 1562 nm with a repetition rate of 12 MHz. The average power injected in the GNOT is controlled by the variable attenuator. Figure 1-(b) presents the transmission of a GNOT for increasing incident average powers. For the first measurement, the transmission is the usual one for a standard SA: it is low at low powers and then becomes progressively higher for increasing incident power. After this first measurement, the same GNOT has been used to make a second measurement. Surprisingly, the resulting transmission is completely independent from incident power and remains nearly constant as shown in figure 1-(b). Similar measurements with different GNOTs have been done in order to confirm this observation. All the series of measurements confirm that above a certain value of injected average power (around 150 mW) in the GNOTs, their SA properties are irreversibly modified and don't fit with theoretical predictions. These modifications have been connected with the temperature evolution of a GNOT for different incident powers. The results revealed a very high temperature increase of the GNOT under high incident power. This temperature increase could be responsible of irreversible damages of the SA since it has been demonstrated that above some temperature, the graphene undergoes structural changes [5].