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Passively Q-Switched Erbium-Doped Fiber Laser via Evanescent Field Interaction with Few-Layer Molybdenum Ditelluride
We demonstrate an all-fiberized passively Q-switched erbium-doped fiber laser (EDFL) via evanescent field interaction between molybdenum ditelluride saturable absorber (SA) and guided mode of the D-shaped fiber. By integrating the few-layer molybdenum ditelluride prepared by CVD method onto the side-polished fiber, the SA can be realized by the strong interaction between the evanescent field of the waveguide and the nonlinear optical material. The proposed passively Q-switched EDFL could deliver output pulses at 1566 nm wavelength with pulse width of 5.03 μs, a repetition rate of 13.9 kHz, a pulse energy of 150.6 nJ, and an output power of 2.1 mW when pumped by a 980 nm laser diode of 180 mW.
Passively Q-switched fiber lasers have been extensively used in optical communication, optical sensing, industrial processing, etc., for the high beam quality, high stability, high energy, and the compact setups [1–5]. The saturable absorber (SA) is indispensable for the passively Q-switched fiber laser for the quality factor Q of the optical cavity can be modulated by the intensity-dependent nonlinearity of SAs [1, 2]. Inspired by the requirements of the compact, cost-effective, stable Q-switched fiber laser, the researchers from optics, optical material, and some other research community have paid much attention on the fields. As we know, the semiconductor saturable absorber mirrors (SESAMs) have been commercialized for the precise control of the absorption wavelength, saturation threshold, modulation depth, and relaxation time. However, SESAMs suffer from the limited bandwidth and complicated preparation procedures . With the emergence of the low-dimensional materials [6–15], especially the two-dimensional (2D) nanomaterials [3, 4], their unique characteristics, such as the ultrafast recovery time, high damage threshold, and broadband nonlinear optical response, have provided an ideal platform to improve the laser performance. With the remarkable progress of graphene, other 2D materials, such as transition metal dichalcogenides (TMDs) [8–11], topological insulators [12, 13], and black phosphorus [16, 17], have also attracted growing research attention resulting from their excellent physical/chemical properties. However, graphene possesses a relatively low modulation depth, and topological insulators are limited to complex preparation processes, and black phosphorus is prone to oxidation and humidity. In contrast to them, TMDs have outstanding layer-dependent semiconducting and nonlinear optical properties.
TMDs, characterized by the chemical formula MX2, where M is a transition metal (commonly Mo, W) and X is a group VI element (S, Se, and Te), are a family of highly anisotropic layered semiconductor materials. For the TMD monolayer, it is composed of two hexagonal planes of X atoms and an intermediate hexagonal plane of M atoms coordinated by covalent bonds with the X atoms in a trigonal prismatic arrangement [8, 18]. With the technical evolution, the nonlinear optical performance of TMDs, such as MoS2, WS2, MoSe2, and WSe2, has been investigated. Compared to other TMDs, telluride material has not attracted adequate attentions. In addition, it is found that MoTe2, in contrast to MoS2 and MoSe2, has smaller bandgap, which implied it can exhibit broadband absorption. Most recently, Mao et al. reported the nonlinear optical properties of few-layer MoTe2 fabricated by a liquid exfoliation method and achieved soliton mode-locking operations in an EDFL . Wang et. al. realized the high energy soliton pulse output with magnetron-sputtering-deposition-grown MoTe2 saturable absorber . Liu et. al. have implemented a MoTe2-based passively Q-switched EDFL via magnetron sputtering technique . Yan et. al. have also investigated the nonlinear optical performance of MoTe2 around 3 μm in a solid-state laser system . However, the material quality and laser energy based on MoTe2 still need improvement.
In this contribution, we have prepared the few-layer MoTe2 via the chemical vapor deposition method and fabricated an all-fiberized SA by transferring the MoTe2 onto the D-shaped fiber. With the help of the strong interaction between evanescent field and the nonlinear optical material, the all-fiberized Q-switched fiber laser can be realized, and the high energy pulsed fiber laser can be delivered.
2. Materials Preparation and Characterizations
The MoTe2 has been prepared via the chemical vapor deposition method. The quartz boat containing 5 mg Te powders is placed upstream of the tube with a distance of 14 cm from the heating center, while the second boat containing 0.5 mg MoO3 powders is placed on the heating center of the tube. The substrate is placed above the second boat and faced up. The mixed gas of H2/Ar (95% Ar) is flowing with 100 sccm for 30 min as the cleaning gas before heating and with 30-100 sccm as the carrier gas during the heating process. The pressure is in atmospheric environment. The furnace is heated to 720°C in 20 min and then maintained at given temperature for 30 min before it is naturally cooled. Then the few-layer MoTe2 can be prepared.
The Raman spectra of prepared MoTe2 show the few-layer nature of the material, which agree well with the earlier finding . As shown in the inset of Figure 1, the scanning electron microscope (SEM) images suggest the 2D nature of the MoTe2. With the grown MoTe2, we have characterized its nonlinear absorption performance via the Z-scan technique with a 1560 nm mode-locked fiber laser whose repetition rate is ~20 MHz and pulse width is ~1.5 ps. Figure 2(a) shows the typical open-aperture curve of the MoTe2 sample, which manifests that the material can exhibit nonlinear saturable absorption characteristics. In addition, we have given the relationship between incident intensity and the nonlinear transmittance, as shown in Figure 2(b), by fitting the experimental results with the following equation :where α() is the absorption coefficient, is the saturable loss, is nonsaturable absorbance, and I and are input and the saturation intensities, respectively. The fitted modulation depth and saturable intensity are 3% and 6 MW/cm2, respectively.
3. Experimental Results and Discussions
To validate the nonlinear optical response of the MoTe2, we have fabricated a fiberized SA by integrating the MoTe2 onto the D-shaped fiber. Moreover, we followed the methods of Du et al.  to design and build an EDFL with a ring cavity configuration modulated by the MoTe2 SA, as shown in Figure 3. In the experimental setup, the overall cavity length is about 8.6 m, which consists of a piece of high concentration erbium doped fiber (LIEKKI Er80-8/125) with length about 0.8 m, which is pumped by a commercial 980 nm laser diode (LD), a polarization independent isolator (PI-ISO) to guarantee the unidirectional laser operation, an intracavity polarization controller (PC) to tune the cavity birefringence, and some other single mode fiber (SMF-28) optical components. We adopted a 980/1550 WDM coupler to introduce the 980 nm pump laser into the fiber ring cavity. Different from the work with transmission-type SA , we introduced a MoTe2 SA via evanescent field interaction between guided wave and the MoTe2, which can make the whole laser setup compact in an all-fiberized way. In addition, the evanescent field interaction can improve the interaction length and alleviate the material damage. To monitor the laser performance, an optical spectrum analyser (Ando AQ-6317B) and a real-time oscilloscope with a bandwidth of 4 GHz (Agilent DSO9404A) together with a photodetector (MC PD-12D) have been employed to simultaneously monitor the optical spectrum and the temporal evolution of the output pulse train. In addition, the output power of the Q-switched fiber laser was measured with a power meter (Ando, AQ2140).
In this experiment, the self-started Q-switching operation can be obtained by increasing the pump power up to around 60 mW. Figure 4 summarizes the typical operation characteristics of the Q-switching state at a pump power of 180 mW. Figure 4(a) shows the optical spectrum centered at 1566 nm delivered from the Q-switched fiber laser. Figure 4(b) shows the typical Q-switched pulse train with a repetition rate of 13.9 kHz. It can be seen that no peak intensity modulation had been observed on the pulse train, illustrating the high stability of Q-switching operation. The inset of Figure 4(b) shows the profile of the Q-switched pulse with a symmetric Gaussian-like shape with a full width at half maximum (FWHM) of 5.03 μs.
We have also investigated the power-dependent pulse duration, repetition rate, output power, and the pulse energy of the Q-switched fiber laser. Figure 5(a) presents the evolution curves of the repetition rate and pulse duration with the pump power. It shows typical features of the Q-switching operation. By increasing the pump power, the repetition rate will linearly increase from 4.9 kHz to 13.9 kHz, while the pulse duration varied in the range of 32.0 μs to 5.03 μs. Figure 5(b) shows the relationship between the average output power and the pulse energy with the pump power. The average output power increased almost linearly with the pump power. Moreover, at the maximum pump power of 180 mW, the maximum obtained pulse energy is up to 150.6 nJ, which is higher than or comparable to that of the Q-switch pulses obtained in the Er-doped fiber lasers with other SAs, such as CNTs, graphene, and TI, and higher than that of the reported results based on MoTe2 SA [17–19]. The improved pulse energy can result from the effective interaction between the evanescent field of the fiber and the few-layer MoTe2, which can provide a good platform to alleviate the damage threshold and tune the linear or nonlinear interaction with an all-fiberized configuration. Although the microfiber can have strong evanescent field to interact with the material [19, 20], the waveguide will be easy to introduce large loss and strong nonlinearity for the small core size. However, the D-shaped fiber can provide a robust platform for the evanescent field interactions between light field propagating along the D-shaped fiber and the few-layer MoTe2.
We have successfully demonstrated an all-fiberized passively Q-switched EDFL by using an evanescent field interaction based MoTe2. The MoTe2 has been prepared via chemical vapor deposition method and then integrated with D-shaped fiber to act as the nonlinear optical modulators. Maximum output pulse energy of 150.6 nJ has been obtained at a repetition rate of 13.9 kHz from the laser cavity under the input pump power of 180 mW at 980 nm. The experimental results can not only broaden the nonlinear application of MoTe2, but provide an inroad for other TMDs nonlinear materials and their hybrid systems.
The experimental data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interests regarding the publication of this paper.
The authors would like to acknowledge Dr. Donglin Lu from Xiangtan University for providing the MoTe2 sample for the experiment.
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Copyright © 2018 Man Wu and Ying Li. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.