1. INTRODUCTION

Fiber lasers possess the inherent features of alignment-free structure, excellent beam quality, and high efficiency ^{[1–4" target="_self" style="display: inline;">–4]}, and thus find many applications in fiber optic communications, materials processing, optical frequency combs, etc. With the assistance of the saturable absorber (SA), Q-switched or mode-locked pulses can be generated in fiber lasers ^{[5–8" target="_self" style="display: inline;">–8]}. Compared with the continuous-wave laser, the Q-switched or mode-locked laser possesses much higher peak powers, which is essential for nonlinear optics and laser machining ^{[911" target="_self" style="display: inline;">–11]}. As a result, there is a strong impetus to develop new types of SAs. In the past several decades, semiconductor SA mirrors ^[10,12] and carbon nanotubes ^[13,14] are the most well-known SAs to generate passively Q-switched laser pulses or mode-locked laser pulses. Such SAs have large modulation depths, while they are designed to operate at certain wavebands. Recently, two-dimensional nanomaterials, ranging from graphene ^{[1517" target="_self" style="display: inline;">–17]}, topological insulators ^{[1820" target="_self" style="display: inline;">–20]}, and transition metal dichalcogenides ^{[21–25" target="_self" style="display: inline;">–25]}, to black phosphorus ^{[2628" target="_self" style="display: inline;">–28]} have been proven to exhibit an ultrafast saturable absorption property. By incorporating such materials with polymer ^[29,30], micro-fiber ^[19], side-polished fiber ^[31,32], or fiber facets ^[33], various types of SAs were fabricated to achieve passively Q-switched operation or mode-locked operation.

Magnetic fluids are stable colloidal suspensions of ferromagnetic particles such as ferroferric-oxide (Fe3O4), ferric oxide (Fe2O3), ferrum (Fe), Co, and Ni ^[34,35]. As a typical ferromagnetic material, Fe3O4 nanoparticles possess outstanding physical properties of high field irreversibility, super-paramagnetism, and extra anisotropy contributions, and thus have been widely applied in sensors, sealing, loudspeakers, inkjet printing, domain observation, biomedical applications, etc. ^{[34,3638" target="_self" style="display: inline;">–38]}. In the branch of optics, Fe3O4 nanoparticles exhibit a large third-order optical nonlinearity with a response time in the tens of picoseconds range ^[39,40]. Recently, several groups have demonstrated that Fe3O4 nanoparticles display a semi-conductive property, and the bandgap can be tuned by controlling their sizes ^[41]. Such nanoparticles possess a large third-order optical nonlinearity, semi-conductive property, and size-dependent bandgap, which satisfies the saturable absorption condition and thus can be used as a Q-switcher in lasers ^[42].

In this paper, we demonstrate that Fe3O4 nanoparticles exhibit the saturable absorption property at 1.55 μm. Two filmy SAs are fabricated by mixing Fe3O4 nanoparticles with a polyvinyl alcohol (PVA) solution or polyimide (PI) solution, and then evaporating the mixture on a glass slide. By using the filmy SA, passively Q-switched operation is established in an erbium-doped fiber (EDF) laser. Compared with a PVA-based SA, the PI-based SA can afford larger powers without damage, which attributes to the higher fusion point of the PI film.

2. EXPERIMENT RESULTS AND DICUSSION

The Fe3O4 nanoparticles used in our experiment are purchased from Ferrotec (MF: Ferrotec EMG 509, 0.8 mg/mL). The average diameter of the nanoparticles is about 10 nm. In this water-based magnetic fluid, anionic is used as a surfactant to prevent agglomeration from Van der Waals attraction. As shown in the inset of Fig. 1(a), the suspension is homogeneous with a rust brown color. The nanoparticles are first characterized by a scanning electron microscope, as shown in Fig. 1(a). The lateral sizes of nanoparticles are measured as 30–70 nm, which is much larger than the heights of 10–15 nm [Fig. 1(b)]. Such difference is attributed to the anionic surfactant coating and random agglomeration of nanoparticles.

Fig. 1. (a) Scanning electron microscope image of Fe3O4 nanoparticles; the inset shows the suspension of the nanoparticles. (b) Atomic force microscope image. (c) Schematic diagram of preparing two filmy Fe3O4 SAs.

下载图片 查看所有图片

Various methods have been proposed to fabricate nanomaterial-based SAs. The first method is depositing the nanomaterials on a tapered fiber or side-polished fiber to realize nonlinear interaction between the evanescent field and the nanomaterial ^[19]. The parameters of such SAs depend on the deposition depth and evanescent field of light, which are difficult to precisely control. The second method is directly depositing the nanomaterials on the fiber facet ^[43]. This type of SA is quite vulnerable because the nanomaterial is exposed to air. The third method is embedding the nanomaterials into polymer to fabricate filmy SAs ^[44]. By cutting the film into tiny pieces (0.5mm×0.5mm), hundreds of SAs with similar performance can be obtained in a single experiment.

Here, we use the filmy polymer method to fabricate two types Fe3O4 SAs. The procedure is described in detail in Fig. 1(c). First, the 5 wt. % aqueous PVA solution is prepared by blending PVA powder with deionized water using a magnetic stirrer at 90°C. The 2 wt. % PI solution is prepared by dilute 4 wt. % liquid PI with N-methyl-pyrrolidone at a volume ratio of 1:1. Second, the Fe3O4 nanoparticle dispersion and the as-prepared PVA solution (or PI solution) are blended at a volume ratio of 1:3 (or 1:4). The compounds are then ultrasonic treated for 10 min at power of 180 W to realize uniform mixing. Third, a thin Fe3O4-PVA film or Fe3O4-PI film is obtained after evaporating the mixture at 50°C and ambient pressure for several hours. The terminal weight proportions s of nanoparticles in the PVA film and PI film are about 5.3% and 10%, respectively.

The thicknesses of Fe3O4-PVA film and Fe3O4-PI film are given as 3–6 μm, as shown in Figs. 2(a) and 2(b). The linear transmission spectra of Fe3O4-PVA film, Fe3O4-PI film, PVA film, and PI film are measured by a spectrometer (Hitachi UV4100). As shown in Figs. 2(c) and 2(d), the transmission coefficient increases from the visible to the near-infrared band, in good agreement with other reports ^[41]. The nonlinear optical properties of the Fe3O4-PVA film and Fe3O4-PI film are further investigated with a balanced twin-detector measurement technique ^[45]. The illumination pulse is generated from a home-made mode-locked fiber laser (wavelength, 1.55 μm; pulse duration, ≈1.3ps; repetition rate, ≈12.5MHz). The nonlinear transmission of the film is measured by comparing the input power and output power with a double-channel powermeter. As illustrated in Figs. 2(e) and 2(f), both films exhibit the saturable absorption property that the transmission (absorption) increases (decreases) with optical intensity. The modulation depths of Fe3O4-PVA film and Fe3O4-PI film are given as 0.8% and 4.1%, respectively. It is worth noting that the concentration of nanoparticles is higher for the PI film, while the linear transmission is almost equivalent for two films. As a result, the different modulation depths of two SAs can be attributed to the higher aggregation rate of nanoparticles in the PVA film than that in the PI film. As a comparison, the transmission spectra of pure PVA film and PI film versus optical power are investigated with the same technique. However, we have not observed any nonlinear response from pure PVA film or PI film, confirming that the saturable absorption property solely originates from Fe3O4 nanoparticles.

Fig. 2. Side profiles of (a) Fe3O4-PVA film and (b) Fe3O4-PI film. Linear absorptions of (c) Fe3O4-PVA film and PVA film and (d) Fe3O4-PI film and PI film. Nonlinear absorption of (e) Fe3O4-PVA film and (f) Fe3O4-PI film in comparison with PVA film and PI film.

下载图片 查看所有图片

An EDF laser is constructed to investigate the performance of as-prepared films, as shown in Fig. 3. The fiber laser is composed of 6 m EDF with 3dB·m−1 absorption at 980 nm (Nufen: EDFC-980-HP), a fiber-fused coupler with output ratio of 5%, a polarization-independent isolator, an SA, 20 m single-mode fiber (SMF), a polarization controller (PC), and a wavelength-division multiplexer. A 980 nm laser diode pumps the EDF with the wavelength division multiplexer (WDM). The PC can adjust the polarization state of the laser, though it is not essential for the Q-switching action. The total length of SMF is 33 m. For EDF and SMF, the dispersion parameters D are −16 and 17ps·(nm·km)−1, respectively.

Fig. 3. Experimental setup of the Q-switched fiber laser. LD, laser diode; PI-ISO, polarization-independent isolator. The SA is based on Fe3O4-PI film or Fe3O4-PVA film.

下载图片 查看所有图片

In the experiment, two films (Fe3O4-PVA film and Fe3O4-PI film) are inserted separately into the fiber laser. By using Fe3O4-PVA film, a continuous-wave laser is first observed when the pump power approaches to 13 mW, as demonstrated by the blue curve in Fig. 4(a). When further increasing the pump power to 23 mW, Q-switched pulses are observed in the fiber laser. The red curve in Fig. 4(a) shows the output spectrum of a typical Q-switched pulse at the pump of 55 mW. The central wavelength and the 3 dB bandwidth of the spectrum are 1557 and 2.3 nm, respectively. A radio frequency spectrum in the inset of Fig. 4(a) confirms the stability of the Q-switched pulses. The repetition rate of the pulses is measured as 16.5 kHz. Figure 4(b) shows the corresponding pulse train. The pulse–pulse interval is given as 62 μs, which agrees with that of the radio frequency spectrum. The inset shows the pulse profile, giving a duration of 7.2 μs.

Fig. 4. (a) Spectra of a continuous-wave laser and a Q-switched laser. Inset: radio frequency spectrum. (b) Pulse train. Inset: pulse profile. (c) Repetition rate, pulse duration, (d) output power, and pulse energy versus pump power. The SA is based on the Fe3O4-PVA film.

下载图片 查看所有图片

We further investigate the evolution of Q-switched pulses as a function of pump power. Figure 4(c) shows the dependence of the pulse repetition rate and pulse duration on the pump power. By increasing the pump power from 25 to 150 mW, the pulse repetition rate increases from 8.5 to 28 kHz, and the pulse duration decreases from 23.5 to 6 μs, which is the typical characteristic of the Q-switched operation. The output power and pulse energy versus pump power are shown in Fig. 4(d). One can observe that the output power increases almost monotonically with the pump power, while the pulse energy increases in the beginning and then decreases when the pump power is higher than 110 mW. Such an abnormal phenomenon can be attributed to the degeneration of SA that caused by laser-induced heat accumulation. The maximum output power and pulse energy reach 1.7 mW and 71 nJ, respectively. When the pump power is larger than 150 mW, the Q-switched operation evolves into amplified spontaneous emission at 1530 nm, and pulses cannot be obtained again by varying the pump or polarization state, indicating that the Fe3O4-PVA SA is destroyed.

By replacing the Fe3O4-PVA SA with Fe3O4-PI SA, Q-switched operation can also be achieved at 25 mW in the same fiber laser. Figure 5 shows the output properties of the Q-switched pulses. For a typical operation state at pump of 45 mW, the central wavelength, 3 dB bandwidth, pulse duration, and repetition rate are 1562 nm, 2.6 nm, 7.8 μs, and 11 kHz, respectively. It is worth noting that the central wavelength of Q-switched operation is 1562 nm when the pump is below 102 mW, while it is 1530 nm when the pump is above 128 mW. Between two pump powers, the laser is unstable and dual-wavelength Q-switched operation is observed. When the pump increases from 25 to 650 mW, the pulse repetition rate increases from 5.5 to 49 kHz and the pulse duration decreases from 47 to 3.5 μs. The maximum output power is 4.6 mW using the 5% output coupler. By utilizing a fiber coupler with the output ratio of 40%, the maximum output power of the Q-switched laser reaches 31 mW.

Fig. 5. (a) Spectrum of the Q-switched laser. Inset: radio frequency spectrum. (b) Pulse train. Inset: pulse profile. (c) Repetition rate, pulse duration, (d) output power, and pulse energy versus pump power. The SA is based on the Fe3O4-PI film.

下载图片 查看所有图片

In contrast with the Fe3O4-PVA SA, the Fe3O4-PI SA can afford much higher power without damage. For example, the Q-switched operation is maintained at 650 mW for several hours when using Fe3O4-PI SA, while it vanishes at 150 mW when using Fe3O4-PVA SA. The long-term stability of the Q-switched pulses is also investigated at pump power of 50 mW, as shown in Fig. 6. For the Fe3O4-PVA SA, the operation state of the Q-switched laser becomes unstable after 2 h at pump power of 50 mW. For the Fe3O4-PI SA, the Q-switched operation is quite stable during the whole measurement process. The results can be understood by noting that PI has a higher fusing point (450°C) than that of PVA (180°C). The insertion loss of the two films is almost equal, and thus the laser-induced heat accumulation is equivalent. Consequently, we conclude that the degradation of Fe3O4-PVA Q-switcher results from the collapse of PVA induced by heat accumulation.

Fig. 6. Long-term stability of the Q-switched pulses at pump power of 50 mW. (a) and (b) are optical spectra based on Fe3O4-PVA SA and Fe3O4-PI SA, respectively. (c) Repetition rate and (d) output power.

下载图片 查看所有图片

Additionally, for the Fe3O4-PVA SA, an unwanted continuous-wave component usually appears on the spectrum of the Q-switched pulse, and robust Q-switching operation can be achieved only by carefully adjusting the PC and the pump power. However, with the Fe3O4-PI film, robust Q-switching operation can always be obtained in the fiber laser. Table 1 summarizes the properties of SAs and the corresponding laser performance. Unlike other two-dimensional nanomaterials, the arrangement and optical property of the Fe3O4 nanoparticles can be controlled with an external magnetic field, which could be used to adjust the optical parameters of the SA.

To confirm whether the Q-switched operation is purely induced by Fe3O4 nanoparticles, we replace Fe3O4-PVA SA and Fe3O4-PI SA with pure PVA film and PI film, respectively. In these cases, Q-switched operation is not observed, even though the PC is tuned over a full range and the pump power is varied from zero to the maximum value hundreds of times. By inserting the Fe3O4 SA into the fiber laser, Q-switched pulses are obtained again. Consequently, one can conclude that the Q-switched operation is purely caused by Fe3O4 nanoparticles rather than the polymer. Moreover, we have prepared other devices by changing the concentration of Fe3O4 nanoparticles and the thickness of films. However, femtosecond mode-locked pulses are not obtained in any case by changing the pump power, polarization state, or the laser configurations, which is attributed to the relatively large recovery time of Fe3O4 nanoparticles ^[40].

Previous work shows that Fe3O4 nanoparticles possess a large third-order optical nonlinearity, semi-conductive property, and size-dependent bandgap ^{[39–42" target="_self" style="display: inline;">–42]}. Based on a two-level model for semi-conductive materials ^[26,46], the nonlinear absorption property can be described as follows: under low incident power, the absorption of such nanoparticles is independent of the laser intensity. However, if the excitation is strong enough, most of the carriers in the valance band are excited to the conduction band and the states in the valence band become depleted, while the finial states in the conduction band are partially occupied. Because of the Pauli blocking effect that two identical electrons cannot fill the same state, further excitation from the valence band is prevented, which leads to saturable absorption in that high-intensity (low-intensity) light experiences small (large) loss.

3. CONCLUSION

We have fabricated Fe3O4-PI film and Fe3O4-PVA film and found that Fe3O4 nanoparticles exhibit a saturable absorption property at 1.55 μm. The modulation depths of the Fe3O4-PVA SA and Fe3O4-PI SA were given as 0.8% and 4.1%, respectively. By increasing the pump power from 25 mW (23 mW) to 150 mW (650 mW), the pulse repetition rate increased from 8.5 kHz (5.5 kHz) to 28 kHz (49 kHz), and the pulse duration decreased from 23.5 μs (47 μs) to 6 μs (3.5 μs) by using the Fe3O4-PVA (Fe3O4-PI) SA. As PI had a higher fusing point than PVA, the Fe3O4-PI SA can afford stronger laser powers without damage than the Fe3O4-PVA SA.