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Abstract
Transition metal oxides (TMOs) have been successfully demonstrated as Q-switchers for pulsed fiber lasers. In this work, the ferroferric-oxide (Fe3O4) nanoparticles are synthesized via chemical co-precipitation. Filmy Fe3O4/polyvinyl alcohol (PVA) is adopted as the Q-switcher in a 1 μm region. The Fe3O4/PVA film has the modulation depth of 7.8% and saturable intensity of 71.32 MW/cm2, respectively. By incorporating the Fe3O4/PVA film into Yb-doped fiber laser (YDF) cavity as a saturable absorber (SA), the stable Q-switching operation is obtained in 77-157 mW pump power range. At the pump power of 147 mW, the Q-switched YDF laser emits stable laser pulses with the maximum single pulse energy of 50.35 nJ and shortest pulse duration of 1.63 μs. The experimental results indicate that the zero-dimensional Fe3O4 nanoparticles have a bright prospect for nonlinear optics applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nonlinear optical materials have recently drawn widely attention from laser optics, photonics integrated circuits and photonics switching because they offer a plenty of novel optical properties, including high carrier mobility, high order harmonic generation, strong photoluminescence, and ultrafast nonlinear optical modulation effects [1–3]. Because of the outstanding physical and chemical properties, low dimensional materials become a new class of nonlinear optical materials that have promising applications in various fields such as nanotechnology, photonic and optoelectronic devices [4–8]. Most importantly, there is significant interest in studying the low dimensional materials’ nonlinear optical modulation properties, which make low dimensional materials adopt as saturable absorbers (SAs) for pulsed lasers applications [9–11]. In 2004, one-dimensional (1D) material carbon nanotubes (CNTs) have been investigated as SAs, which show excellent nonlinear optical properties [12]. However, the nonlinear optical response of CNTs is restricted in their diameters and chirality. With the emergence of graphene, two-dimensional (2D) materials open up new opportunities in optical modulator devices field [13–18]. Based on the zero-gap structure, graphene is suitable as a kind of ultra-wideband SAs. Since the graphene, topological insulators (TIs) and transition metal dichalcogenides (TMDs) have been widely studied. Due to the excellent properties of high third-order nonlinear susceptibility, strong light-material interaction, ultrafast carrier dynamics and broadband absorption, these 2D materials have potential applications in pulsed lasers [19–28]. In addition, the broadband nonlinear saturable absorption characteristics also have been discovered in other representative 2D materials, including black phosphorus [29], antimonene [30], bismuthene [31], perovskites [32], MXenes [33]. While during the preparation process, the defects in these 2D materials are unavoidable, which usually result in poor homogeneity, thus making the pulse instability when used as SAs. As is well known, SA is an effective component to generate stable laser pulses in passively Q-switched or mode-locked fiber lasers [34–37]. Therefore, there is a strong impetus to pursue new and high performance SA materials.
Very recently, another kind of zero-dimensional nanomaterial, TMOs including aluminum oxide (Al2O3), bismuth oxide (Bi2O3), Nickel oxide (NiO), titanium dioxide (TiO2), Zinc oxide (ZnO), Fe2O3 and Fe3O4 attract the researchers’ interest [38–44]. These TMOs have advantages of large third-order optical nonlinearity, recovery time with tens of picoseconds, excellent thermal stability, and easy preparation. Furthermore, the optical nonlinearity of TMOs nanoparticles can be tuned by controlling the size and shape, thus making TMOs have potential applications for nonlinear optics. As a typical kind of TMOs, Fe3O4 nanoparticles have been widely used in the areas of biotechnology, medical, sensors for the outstanding physical properties of high field irreversibility, superparamagnetic, high biocompatibility, and extra anisotropy contributions. In addition, the Fe3O4 nanoparticles have been used as SAs for pulsed lasers at different wavelengths [45,46]. Compared with 2D materials, the methods of fabricating Fe3O4 are more flexible, the diameters can be finely controlled during the preparation process. Besides that, the diameter from nanometer to micrometer affects the nanoparticles bandgap energy gap, which makes the Fe3O4 nanoparticles can be a promising wideband SAs.
In this paper, we demonstrate the saturable absorption property of Fe3O4 nanoparticles at 1 μm region. The Fe3O4 nanoparticles are synthesized via chemical co-precipitation. The filmy SAs are fabricated by mixing Fe3O4 nanoparticles with PVA solution. The Fe3O4/PVA SA possesses the modulation depth of 7.8% and saturable intensity of 71.32 MW/cm2. By using the Fe3O4/PVA SA, passively Q-switched operation is established in YDF laser. In the pump power of 77-157 mW, stable laser pulse trains are emitted. At the pump power of 147 mW, the maximum single pulse energy of 50.35 nJ and shortest pulse duration of 1.63 μs are obtained. The Q-switched laser features a slop efficiency of 4.5%. The experimental results reveal that the zero-dimensional Fe3O4 is a kind of promising optical modulation material for pulsed lasers applications.
2. Q-switcher fabrication
Here, the chemical co-precipitation method is used to synthesize Fe3O4 nanoparticles. This method has the advantages of simplicity and superiority. The FeCl2 and FeCl3 are used as precursors. Firstly, FeCl2 and FeCl3 are dissolved in deionized water under a nitrogen gas flow within 30 minutes. Then, stirring the mixture solution violently 20 minutes at the temperature of 60 °C and the NH4OH is added into the solution. After the stir and ultrasound process, Fe3O4 is obtained as the follow chemical reaction: Fe2+ + 2Fe3+ + 8OH- = Fe3O4 + 4H2O
After the filter and wash process by using deionized water and ethanol, the Fe3O4 nanoparticles are obtained without chloride ions. The morphology and size of Fe3O4 nanoparticles have been observed by the transmission electron microscopy (TEM). As shown in Fig. 1(a), the Fe3O4 nanoparticles show near spherical shape and the average diameter is 15 nm. An X-ray diffractometer (XRD) is used to study the crystalline structure. Figure 1(b) depicts the experimental results. It is noted that there is a series of characteristic peaks appeared in the points of 220, 311, 400, 511 and 440. In the next step, we fabricate the filmy type Fe3O4 Q-switcher. The PVA solution is prepared by blending PVA powder with deionized water. Then, the Fe3O4 nanoparticles dispersion and the PVA solution are blended. Finally, the thin Fe3O4/PVA film is obtained after evaporating the mixture dispersion at 60°C.
The linear optical absorption spectrum of Fe3O4/PVA film in 350-1100 nm waveband is shown in Fig. 2(a). It is obvious that the prepared film has large absorption in 350-600 nm waveband, indicating it is unsuitable used as SA in this waveband. With the wavelength grows, the transmission increases to 75% and then keep unchanged. It is noted that the linear transmission is 75.2% at 1030 nm. The Fe3O4/PVA SA is a key element for the stable pulse generation. Therefore, to investigate the nonlinear optical saturable absorption property of Fe3O4/PVA film, balanced twin-detector measurement system is used. A home-made mode-locked YDF laser is used as the laser source. The parameters are as follow: central wavelength is 1030 nm, pulse duration is 30 ps, and repetition rate is 10.56 MHz. The measured results are fitted by the function:
whereand are the saturable and nonsaturable absorption, Isat is the saturation intensity. In order to determine these coefficients (,, and Isat) of fitting function, the transmission intensity is fitted by 1-, which is shown in Fig. 2b. The nonlinear transmission increases rapidly and tends to keep saturable gradually with the intensity increasing. According to the fitting data, the modulation depth (ΔT), saturable intensity (Isat) and nonsaturable loss are calculated to be 7.8%, 71.32 MW/cm2 and 17.7%, respectively. The insert loss of Fe3O4/PVA is measured to be 1.25 dB.3. Experimental setup
The experimental setup of YDF laser with Fe3O4/PVA is displayed in Fig. 3. The ring laser cavity is used for generating laser pulses. A 20 cm long YDF with absorption coefficient of 1200 dB/m at 980 nm is adopted as the gain medium. The YDF is pumped through a 980 nm laser diode (LD) by a wavelength division multiplexer (WDM). The LD has the maximum power of 650 mW. A polarization independent isolator (PI-ISO) is employed to eusure the laser circulate in one direction only. A polarization controller (PC) is engaged to make the polarization sates adjustable. The Fe3O4/PVA film is inserted into the cavity. A 20/80 optical coupler (OC) is used. A 9 nm bandpass filter is fused into the ring cavity for pulse easy formation. The total length of laser cavity is 8.6 m.
4. Results and discussions
In the experiments, as pump power increases to 77 mW gradually, Q-switched pulse trains are appeared in this YDF laser. The fiber laser can maintain the Q-switching operation state until the pump power reaches to 157 mW. The Q-switched laser characteristics are studied by measuring the laser pulse trains at the pump power of 77 mW, 87 mW, 97 mW, 107 mW, 117 mW, 127 mW, 137 mW, 147 mW and 157 mW respectively. Figure 4(a) shows the pulse trains have uniform intensity. In addition, there is no significant pulse modulation appeared when adjusting the pump power. With the pump power increasing to 147 mW, the Q-switched pulses intensity tend to increase. While the intensity tend to decrease slightly with the pump power increasing to 157 mW. The Q-switched optical spectrum at the pump power of 77 mW, 107 mW, 147 mW and 157 mW are measured, which are displayed in Fig. 4(b). The central wavelength has a little blue shift as the pump power increase. With the pump power increasing from 77 mW to 147 mW, the optical spectrum shows a slightly widening trend. While the optical spectrum of 157 mW narrower than that of other pump power.
Figure 5(a) shows the law of change in repetition rate and pulse width with pump power increasing from 77 mW to 157 mW. The repetition rate increases from 52.78 kHz to 102.23 kHz. As the pump power increase from 77 mW to 147 mW, the pulse duration goes down from 6.01 μs to 1.63 μs with a sharply decline. When the pump power increase to 157 mW, the pulse width goes up to 1.93 μs. Figure 5(b) shows the dependence of average output power and single pulse energy on the pump power. As the pump power increase, the average output power shows a linear increase. The maximum output power of 4.98 mW is obtained at pump power of 157 mW. The maximum single pulse energy of 50.35 nJ is obtained at the pump power of 147 mW. While the pump power reaches to 157 mW, the pulse energy reduces to 48.72 nJ. In this work, the maximum single pulse energy of 50.35 nJ and the shortest pulse duration of 1.63 μs are obtained at the pump power of 147 mW not the 157 mW. It is also noticed that the optical spectrum at 157 mW shows slightly deterioration. This phenomena can be explained by that the filmy Fe3O4 SA is saturable after pump power exceeds 147 mW. Further increasing the pump power slowly, there is some intensity fluctuation appeared in the Q-switching pulse trains. At last, the Q-switching pulses disappeared. When reduce the pump power down to 147 mW, we could observe stable optical pulses again. So we can conclude that the Q-switching operation based on Fe3O4/PVA is repeatable. We compare recent results of Q-switched fiber lasers with TMOs as SAs in Table 1. Currently, the nonlinear optical absorption property of TMOs based SAs are mainly studied at 1.55 μm region. It is noted that Fe2O3 nanoparticles exhibits broadband saturable absorption, which have been used as Q-switcher in 1038/1557/1942 nm. The high pulse energy of 174.9 nJ has been obtained at 1.55 μm region. The relevant research results are relative few at 1 μm region. In this work, we obtain the experimental results of 1.63 μs pulse width and 50.35 nJ single pulse energy, which are comparable to that in previous reported results with TMOs as Q-switchers. The slop efficiency of 4.5% is much larger than that of other results. The output performance comparison in Table 1 also confirm that our experimental results are better in 1 μm region.
Fig. 5 (a) Repetition rate and pulse duration versus different pump power; (b) output power and pulse energy versus different pump power.
Table 1. Performance comparison of Q-switched fiber lasers with TMOs.
5. Conclusion
In conclusion, we fabricate the Fe3O4 nanoparticles by chemical co-precipitation method. Fe3O4/PVA film is further prepared, which shows the modulation depth of 7.8%, saturable intensity of 71.32MW/cm2, and nonsaturable loss of 17.7%. Using the Fe3O4/PVA as Q-switcher in YDF fiber laser, the stable Q-switching operation is realized. The laser pulses generated from Q-switched fiber laser have the maximum single pulse energy of 50.35 nJ and shortest pulse duration of 1.63 μs. The experimental results show that Fe3O4 have potential applications for fiber lasers.
Funding
National Natural Science Foundation of China (No. 61705183); Central University special fund basic research and operating expenses (No. GK201702005); Nature Science Foundation of Shaanxi Province, China (No. 2017JM6091); Fundamental Research Funds for the Central Universities (No. 2017TS011).
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