1. Introduction

In recent years, passively mode-locked fiber lasers have been extensively investigated because of their many advantages over actively mode-locked lasers, such as compact structure, low cost [1–3]. A lot of passive mode-locking techniques have been applied into fiber lasers, in which nonlinear loop mirror based on nonlinear interferometry and nonlinear polarization rotation depended on Kerr nonlinearity have been widely used to obtain high-power, low cost and compact ultrafast lasers [4–6]. Semiconductor saturable absorption mirrors (SESAM) are quite mature but still limited by complex fabrication methods, narrower wavelength operation range and expensiveness [7]. A family of carbon materials including single-wall carbon nanotubes, oxide-graphene, graphene, with advantages of wide operation wavelength range and cost effective, can be applied to mode-locking in fiber lasers [8–12]. Despite of all these, researchers always seek better saturable absorber (SA) to assist mode-locking.

Recently, topological insulators such as Bi₂Se₃, Bi₂Te₃ and Sb₂Te₃ have attracted much attention for their promising applications in fiber lasers [13–17]. Topological insulator is a novel kind of quantum electronic matter which behaves metallic states in surface but insulator states in interior [18–21]. Bi₂Se₃ has a topologically non-trivial energy gap of about 0.2-0.3eV, so it shows saturable absorption effect when the light wavelength is shorter than 4.1μm (0.3eV) [18, 19]. The saturable absorbers of carbon nanotube, graphene and graphene oxide, topological insulators have the advantages of easy fabrication and cheap cost. However, carbon nanotube’s working wavelengths were connected with the diameter of the nanotubes. Graphene and graphene oxide could be used as wideband SAs. The saturable absorbers based on topological insulators have the advantages of low saturation intensity, broad effective bandwidth. As to our result, it was noteworthy that saturable optical intensity (580 MW/cm²@800 nm) was also less than that (2.77 GW/cm²@800 nm) of single-layer graphene [22, 23]. Recently, Chujun Zhao et al. firstly demonstrated a wavelength-tunable erbium-doped picosecond soliton fiber laser with Bi₂Se₃ saturable absorber [24]. A 2 GHz passively harmonic-mode-locked fiber laser by Bi₂Te₃ was reported by Zhi-Chao Luo et al. [25]. This indicated that Bi₂Te₃ have both high nonlinear effect and saturable absorption character [26]. Y. H. Lin et al. obtained 403 fs pulses by nanoscale p-type Bi₂Te₃ particles [27]. To date, the shortest pulse width generated from a mode-locked fiber laser by TI was 128 fs [28]. The above results all operated near 1.5 μm. At 2 μm waveband, a pulse width of 795 fs was achieved at a wavelength of 1935 nm from a thulium/holmium co-doped fiber ring cavity [29]. At 1 μm waveband, Q-switched fiber laser had already been achieved [22]. Cheolhwan Chi et al. realized the dissipative-soliton mode locked fiber laser using a bulk Bi₂Te₃ at 1.06 µm with a pulse width of 230 ps and output power of 0.86 mW [13]. However, mode locked fiber lasers operating near 1µm using Bi₂Se₃ had not been reported.

In this paper, we demonstrated an Yb-doped mode-locked fiber laser based on pure Bi₂Se₃ saturable absorber at 1 μm waveband. This laser implemented an all-fiber structure with all-normal dispersion state. The Bi₂Se₃ was absorbed on the fiber end-facet without film-forming agent, exempt from unnecessary absorption of film-forming agent. The maximum power output of the laser was 33.7 mW and the pulse width was 46 ps.

2. Preparation of Bi₂Se₃ and the Bi₂Se₃ SA

Preparation of Bi₂Se₃ was similar to Ref [30, 31]. The polyol method was utilized to synthesize TI. We selected Bi₂Se₃ nano-sheets solution that had the reaction time of 180 min. In order to remove the impurity, the Bi₂Se₃ solution was washed with isopropyl alcohol (IPA) and centrifuged several times. Dispersion suspensions of Bi₂Se₃ in deionized water solution were prepared by centrifugation and ultrasonication agitation for 1 hour, as shown in Fig. 1(a). The Bi₂Se₃ solution was observed by atomic force microscope (AFM), as shown in Fig. 1(b). The thickness of the TI: Bi₂Se₃ NPs was about 10-15 nm. The thickness of the single-layer Bi₂Se₃ was 0.96 nm [31], so the TI: Bi₂Se₃ we obtained were estimated to be 10-16 layers.

 

Fig. 1 (a) Bi₂Se₃ deionized water solution. (b) The AFM image of Bi₂Se₃ nano-sheets. (c) Photographic image of Bi₂Se₃-SA film. (d) The nonlinear transmittance of the pure Bi₂Se₃.

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Figure 2 shows the procedures of the preparation of pure Bi₂Se₃-SA (BS-SA) film. Firstly, filter paper (FP) (GVWP02500 Millipore) with the pore size of 0.22 μm was immersed in deionized water until it soaked completely, as shown in Fig. 2(a). Then Bi₂Se₃ water solution was dropwise added on the filter paper slowly, as displayed in Fig. 2(b). The pure Bi₂Se₃ remained on the surface of filter paper due to the diameter of Bi₂Se₃ morphologies (1.2 μm) was larger than the pore size of the filter paper. Put the filter paper with Bi₂Se₃ in drying oven until filter paper dried thoroughly, as displayed in Fig. 1(c). Thirdly, shear a small piece from the prepared Bi₂Se₃ filter paper and put it on the face of a fiber end-facet, as shown in Fig. 2(c). At last, put the fiber end-facet with the Bi₂Se₃ filter paper into acetone solution to remove the filter paper (Fig. 2(d)). The filter paper needed washing quite a few times. We would observe the fiber end-facet each time and replace new acetone solution to make sure that the filter paper was removed thoroughly and the surface of Bi₂Se₃ film was smooth. In this way, pure Bi₂Se₃ film was perfectly deposited on the fiber connector and BS-SA was achieved.

 

Fig. 2 Procedures of the preparation of pure Bi₂Se₃-SA film.

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We used open-aperture Z-scan technology to investigate the nonlinear saturable absorption of Bi₂Se₃. The central wavelength of the source (Mira-900F Coherent Inc.) was near 812 nm [18, 22]. The nonlinear transmission was shown in Fig. 1(d). The nonlinear modulation depth and saturable energy density were ~5.2% and ~70 μJ/cm² respectively. The saturable optical intensity of Bi₂Se₃ was 580 MW/cm² at 800 nm. The Bi₂Se₃ non-saturable loss of 56.5% was relative large, primarily due to the scattering and the absorption of residual impurities. Furthermore, Bi₂Se₃ film pasted on the fiber end-facet was thick and the filter paper may have not been washed away by acetone solution completely, which would also lead to the increase of the non-saturable loss.

3. Laser experiment and results

The schematic of all-normal dispersion mode-locked fiber laser was shown in Fig. 3(a). The fiber laser was pumped by a 980 nm laser diode via a 980/1030 nm wavelength division multiplexer (WDM). A 22-cm Yb-doped fiber (YDF, LIEKKI Yb1200-4/125) with peak core absorption 1200 dB/m at 976 nm and the dispersion parameter −43ps/(nm⋅km) at 1031nm was used as the gain medium. One polarization controller (PC) was used to adjust the intra-cavity polarization and stabilize the mode-locked operation. A polarization insensitive isolator (PI-ISO) was introduced to force laser operating unidirectionally. Pigtail (Nufern 1060xp) of PI-ISO with −42ps/(nm⋅km) at 1031nm was 47 cm. A 51-cm polarization maintaining fiber (PMF, Nufern PM980-xp) with beat length of 2.5 mm at 980 nm and dispersion parameter of −43ps/(nm⋅km) at 1031nm was employed as polarization filter. When the light coupled into the PM fiber, birefringence of PM fiber led to the accumulation of a relative phase difference in the fast and slow axes, corresponding to wavelength-dependent rotation of the polarization state. This could induce Sine periodic filter effect. Longer PM fiber would result in a narrower bandwidth [32, 33]. The birefringence and filter bandwidth were calculated to be ~4.12 × 10⁻⁴ and ~2.6 nm respectively. The laser was coupled out using a 30% output coupler (OC). The pigtail of WDM and OC (OFS-980) was 154cm.The rest fibers were Nufern 1060xp and the length was 177cm. The total cavity length was ~4.51 m.

 

Fig. 3 (a) Schematic of the all normal-dispersion fiber laser with a TI-SA. (b) The output power of the fiber laser versus pump power in three laser states: CW output, single pulse output, two-six pulses output.

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Self-started mode-locking occurred when pump power reached 153 mW as shown in Fig. 3(b). Stable mode-locking could be obtained when pump power varied within 153 mW to 270 mW. Figure 4(a) showed the typical spectrum of the mode-locked fiber laser at 230 mW. Intensity modulation of pulses was weak. Here, the central wavelength and 3-dB spectral bandwidth were 1031.7 nm and 2.5 nm, respectively. The spectral bandwidth was close to that of the birefringence fiber. The spectrum with steep edges displayed typical character of the all-normal dispersion fiber lasers [34]. The corresponding oscilloscope trace was depicted in Fig. 4(b).

 

Fig. 4 (a) Spectrum of the fiber laser at pump power of 230 mW. (b) A typical mode-locking pulse train. (c) RF spectrum of the mode-locked laser. Inset: spectrum in 1GHz span. (d) Autocorrelation trace.

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The pulse width was deduced to be 46 ps from the autocorrelation trace (Fig. 4(d)), which was larger than the common dissipative soliton lasers in short cavity length. This was due to the relatively narrower spectrum of 2.5 nm for the existence of the filter effect, corresponding to a Gaussian Fourier-transform-limited pulse width of 625 fs. Furthermore the all-normal dispersion cavity had a large net positive dispersion, which also contributed to the large pulse width. With suitable extra-cavity compression by dispersion-compensation elements, the pulse width could be compressed to less than 1 ps.

The RF spectrum of the laser with a repetition rate of 44.6 MHz was shown in Fig. 4(c). The signal to noise ratio (SNR) of the fundamental frequency was about 58 dB. RF spectrum appeared a small noise, but the difference of major peak and small noise was more than 60 dB and RF spectrum in 1GHz span was also stable, which demonstrated the stability of the mode-locking. The maximum average output power was 33.7 mW at the pump power of 270 mW when the laser operated in single pulse state.

Multi-pulses output can be realized when pump power exceeded 270 mW. When pump power increased from 273 mW to 310 mW, pulse traces can split successively and stably from one-pulse state to four-pulse state, as shown in Figs. 5(a)-(d). When pump power was further increased, intracavity power had approached to Bi₂Se₃-SA film damage threshold. So when the laser operated at five-pulse and six-pulse state, some instability would occur, which is as shown in Figs. 5(e)-(f). We attributed this splitting phenomenon to the highly-nonlinear refractive index of topological insulators [23, 24] and the accumulation of nonlinear phase shift. When the pump power exceeded 335 mW, mode-locking could not be observed. Furthermore, when we adjusted the position of the polarization controller precisely, mode-locking could not be constructed, which indicated that the Bi₂Se₃ film had been damaged. At the same time, in order to verify whether NPR mechanism had effect on this laser, we removed the Bi₂Se₃ film from the surface of the fiber end-facet and mode-locking could not be obtained. So we think Bi₂Se₃ film played a key role in the mode locking mechanism.

 

Fig. 5 (a)-(f) Typical pulse train of multi-pulse state.

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4. Summary

We demonstrated an all-fiber Yb-doped mode-locked laser based on Bi₂Se₃ saturable absorber. The Bi₂Se₃ saturable absorber was prepared by filter paper transfer method. Using this method, we could deposit pure TI on fiber connector and avoid influence of film-forming agent. With the BS-SA, stable mode locking of Yb-doped fiber laser was obtained. The central wavelength, spectral width and pulse width were measured to be 1031.7 nm, 2.5 nm and 46 ps respectively. To the best of our knowledge, it is first time to obtain mode-locking at 1μm wavelength by Bi₂Se₃. Furthermore, the mode locking could sustain in multi-pulse state due to high-nonlinear refractive index of BS-SA, up to six-pulse regime could be obtained.