Nanosecond passively Q-switched fibre laser using a NiS2 based saturable absorber

Q-switched pulse laser generation is successfully demonstrated in both Erbiumdoped fibre laser (EDFL) and Thulium-doped fibre laser (TDFL) cavities by employing Nickel disulfide (NiS2) as a saturable absorber (SA). Q-switched pulse laser operation at 1.55 μm and 2.0 μm is obtained at low pump power levels of 37 mW and 48 mW, respectively. For the EDFL, stable passively Q-switched laser output at a wavelength of 1561.86 nm is achieved, with a minimum pulse duration of 237 ns and a repetition rate of 243.9 kHz. For the TDFL, the centre wavelength of the laser output is 1915.5 nm, with a minimum pulse duration of 505 ns and a repetition rate of 214.68 kHz. NiS2 is used as SA for Q-switched laser generation over a broadband wavelength for the first time. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Among the numerous SA materials, TMDs show strong chemical bonds in-plane but display weak Van der Waals interaction between layers [19]. For few-layer TMDs, the atomic structure is characterized by a sandwiched-like structure [20]. The chemical formula of TMDs is written as MX 2 . M (represents transition metal atom such as Mo, W, Ni) is a layer sandwiched between two layers of X (chalcogen atoms such as S, Se) [20]. To date, more than 40 types of TMDs have been discovered depending on the combination of the chalcogen and transition metal atoms [20]. Among the numerous TMDs, WS 2 and MoS 2 have been mostly investigated encompassing a wide range of optical functions, such as two-photon absorption [21] and saturable absorption [6]. TMDs have also been used as an SA to achieve passive Q-switched and mode-locking fibre laser incorporating side-polished fibres [22], microfibres [6], polyvinyl alcohol-based films [23], etc.
Nickel disulfide (NiS 2 ), another new type of TMD, has attracted significant attention in recent years owing to its superior electrical [24][25][26] and optoelectronic properties [27,28]. NiS 2 is a semiconductor, similar to WS 2 and MoS 2 [20]. Compared with typical large bandgap TMDs (above 1 eV), the bandgap of NiS 2 is 0.3 eV [29], which forms the basis for its application at long wavelengths (about 4.13 μm). However, the nonlinear optical properties of NiS 2 and its application in fibre laser have seldom been reported. Thus, it is a very valuable work to investigate the applications of NiS 2 in fibre laser, such as the Q-switched pulse laser in a wideband region using the NiS 2 -based SA In this paper, the NiS 2 material was synthesized using a chemical method and used as an SA to generate passive Q-switched pulses in both an Erbium-doped fibre laser (EDFL) and a Thulium-doped fibre laser (TDFL). The NiS 2 exhibits excellent nonlinear optical properties and this is further proved by the achieved Q-switched pulsed laser output. The experimental results clearly indicate that NiS 2 has excellent potential for use in passive Q-switched fibre lasers, especially for broad waveband operation.

Synthesis and optical measurements of NiS 2
The NiS 2 was synthesized entirely using a chemical method. Initially, a 30 mL mixture of Nickel Nitrate and thiourea was poured into a beaker with a molar ratio of 5:1 and stirred at 25 °C for 2 h. Then the mixture was transferred into an autoclave for hydrothermal treatment at 200 °C for 5 h. The precipitates were separated using centrifugation, washed using distilled water and ethanol, and dried at 40 °C for 10 h under vacuum. In order to fabricate the NiS 2 -PVA film, the NiS 2 powder was poured into another beaker with the PVA solution and then slowly stirred at 130 °C for 2.5 h. The resulting suspension was then poured into a petri dish and dried at 25 °C for 10 h. Finally, the NiS 2 -PVA film was carefully peeled from the petri dish.
The nonlinear absorption (NLA) properties of NiS 2 were investigated using an openaperture Z-scan method [30]. The excitation laser pulses were generated using a femtosecond Ti:sapphire laser system (center wavelength: 800 nm, pulse duration: 200 fs, repetition rate: 2 kHz). The nonlinear transmission ability was measured using a balanced twin-detector technique [6]. A femtosecond fibre laser (center wavelength: 1550 nm, Pulse width: 300 fs, Repetition rate: 15 MHz) and a picosecond fibre laser (centre wavelength, 1.91 μm; pulse duration, ~3.6 ps; repetition rate, ~22 MHz) were used as the pump sources.

Results and discussion
The image of NiS 2 -PVA film is shown in Fig. 1(a). The film is nearly colorless and transparent, and is highlighted by the green circle in 1 (a). The sample was characterized using Raman spectrometry in order to confirm its chemical composition. The Raman spectrum was obtained and the result is shown in Fig. 1(b). Two weak peaks were observed at 274.0 cm −1 and 284.8 cm −1 which correspond to the S-S pair vibrational modes (T g and E g ). The two strong peaks at 479.7 cm −1 and 489.8 cm −1 correspond to stretching modes of the S-S pair (A g and T g ). The Raman spectrum agrees well with earlier findings [31,32].
The NLA curve is shown in Fig. 2(a). It is clear that NiS 2 exhibits a typical saturable absorption effect. In addition, the nonlinear transmission of the NiS 2 -PVA film SA is also investigated. Figure 2(b) and Fig. 2(c) show the nonlinear transmission curves at the wavelength of 1.55 μm and 1.9 μm, respectively. The blue points in Fig. 2(b) and Fig. 2(c) represent the experimental data, which confirms the trend of nonlinear absorption. The red solid curves in Fig. 2(b) and Fig. 2(c) represent the fitting lines of the experimental data based on a simplified two-level saturable absorption model [33], which clearly indicates the presence of a typical saturable absorption effect. The trend of the nonlinear transmission curves indicates that the transmission increases with the increase of pulse intensity until it reaches satura saturable inte respectively. saturable inte respectively. T to its small b suited for app infrared wave   Due to the excellent optical properties of the NiS 2 -PVA thin film, the film as a SA was deployed inside the pre-designed cavity to generate Q-switched laser output pulses. The proposed fibre lasers based on an Er-doped fibre laser (EDFL) and Tm-doped fibre laser (TDFL) have the same configuration, as shown in Fig. 2(d). The pump source provides source lasing to pump the gain medium, a short section of rare earth doped (Er-doped or Tm-doped) fibre was optically coupled via a wavelength division multiplexer (WDM). The generated photons then propagate into the polarization independent isolator (PI-ISO) which is pigtailed with the gain medium. In the designed laser cavity, the PI-ISO not only maintained lasing in a unidirectional operation, but also reduced Brillouin back-scattering, which could potentially disturb the stability of the pulsed operation [34]. The SA device was spliced between the 90%-port of the optical coupler (OC) and the WDM to complete the ring cavity. The 10% port of the OC was connected to an optical spectral analyzer (OSA, YOKOGAWA, AQ-6370C) to measure the output spectrum of the laser. The time resolved output signal of the pulsed laser was also measured using a digital storage Oscilloscope (Tektronix MDO4054-6, 6 GHz) and a photodetector (Kemai, PDA, 10 GHz). An optical power meter (Newport 1918-R) was used to measure the output power.
In the case of the Er-doped fibre laser (EDFL), the 4.5 m Er-doped fibre (Likkie-8/125) was pumped using a 976 nm laser diode (MCPL-980-SM), the total length of the cavity being ~24 m. When the pump power was increased to 35 mW, continuous wave (CW) lasing was observed. There was no evidence of any pulse-like behavior in the time-based waveforms observed on the oscilloscope. As the power was further increased to 37 mW, passive Qswitched operation was initiated and recorded. The Q-switched threshold is at a low level mainly due to the fact that no polarization controller (PC) was used in the designed laser configuration, which uses fibre bending to introduce significant losses in the cavity. In addition, the absence of the PC in the cavity ensured that no nonlinear polarization evolution (NPE) effect occurred, which can potentially limit the output pulse energy [35]. The whole evolution of the Q-switched pulse operation is shown in Fig. 3. The output spectrum and typical pulse train waveforms with a pump power of 200 mW are shown in Figs. 3(a) and 3(b), respectively. The centre wavelength of the Q-switched laser output is 1561.86 nm. The temporal pulse separation (between two adjacent pulses) is 4.1 μs corresponding to 243.9 kHz repetition rate. The narrowest pulse duration of the Q-switched operation shown in Fig. 3(c) was 237 ns, which can be further reduced by using the shorter laser cavity [36]. The repetition rate and pulse width versus the pumping power are shown in Fig. 3(d). In the NiS 2 -based EDFL, the pump power was varied from 37 mW to 200 mW, the repetition rate increased from 195.3 kHz to 243.9 kHz with the pulse width in the opposite state: decreased from 822 ns to 237 ns. The evolution of the repetition rate and pulse width indicates that the NiS 2 -based EDFL exhibits typical passive Q-switched behavior, respectively [37]. When the pumping power was adjusted over 200 mW, the pulse train would be in unstable state. It is mainly due to the overbalanced of NiS 2 -PVA SA in high power.   In order originated fro size. In both gradually and pulse could n both EDFL an

Conclusio
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Funding
National