1 Introduction

Recently, many reports have experimentally proved that metallic nanoparticles can be applied as saturable absorbers (SA) for Q-switched and mode-locked lasers, because of their large third-order nonlinearity (Liao et al. 1997), broadband absorption (Kim et al. 2010) and ultrafast response time (Elim et al. 2006). Among these, we study mainly the family of gold nanoparticles (GNPs) for their widespread applications in medical diagnostics, biosensing imaging, and near-field super-resolution imaging. These applications are enabled by their outstanding saturable absorption properties, which arise mainly from the local surface plasmon resonance (SPR) (Chen et al. 2013; Jones et al. 2011; Cobley et al. 2011). SPR is an optical phenomenon attributed to the collective oscillation of conduction electrons, resulting in strong plasmonic electromagnetic field at the surface of plasmonic nanoparticles, which brings about strong optical absorption and Surface Enhanced Raman Scattering (SERS) (Kim et al. 2012; Link et al. 2000). Besides, the local SPR band of GNPs can be tuned easily and continuous from visible to near-infrared (NIR) regions by tailoring their size, shape, and internal structure (Kim et al. 2010). All these excellent optical properties show that GNPs can be use as excellent SAs. Jiang et al. successfully used a gold nanocrystals (GNCs) film as SA in 1.5 μm fiber laser (Jiang et al. 2012). In 2013, Kang et al. achieved 4.8 μs Q-switched and 12 ps mode-locked erbium-doped fiber laser using gold nanorods (GNRs), respectively (Kang et al. 2013a, b). In 2014, Fan et al. reported a Q-switched EDF laser based on gold nanosphere (GNS) with the repetition rate of 58.1 kHz and the pulse width of 1.8 μs at 1549.8 nm (Fan et al. 2014). Additionally, Zhang et al. reported a YDF Q-switched fiber laser based on gold nanobipyramids (GNBPs) with a pulse width of 2.6 μs and pulse repetition rate of 66.3 kHz at 1 μm (Zhang et al. 2017). Shortly thereafter, we have achieved an all-fiber passively Q-switched YDFL with an Au-NCs/SiO2 SA at 1 µm (Bai et al. 2018). Compared with those GNPs, Au-NCs/SiO2 have unique optical properties. With the cubic structures of hollow interiors and porous walls, Au-NCs has large absorption cross section (approximately 5 orders of magnitude larger than conventional dyes) (Bai et al. 2018; Chen et al. 2005a, b). And because of this structure, Au-NCs has the hotpots between them as well the hotpots inside them, which enhances the activity of SERS (Zhang et al. 2015). In addition, the exist of SiO2 shells protect effectively the surfaces of Au-NCs, defend them from coagulation and oxidation (Yang and Gao 2005; Duarte et al. 1998; Hu et al. 2015; Schulzendorf et al. 2011; Teng et al. 2012). This improves their photo-thermal stability and increases these optical damage threshold. However, there is still no demonstration about the Au-NCs/SiO2 used as SA in passively mode-locked fiber laser.

In this work, an all-fiber passively mode-locked YDF laser using the Au-NCs/SiO2 SA film was demonstrated for the first time. Firstly, the Au-NCs/SiO2 SA film was fabricated by the seed-mediated growth method and the spin-coating method. Secondly, the optical transmission spectrum from 400 to 1100 nm and the saturable absorption property at 1064 nm were investigated. Lastly, by insetting the Au-NCs/SiO2 SA film, a stable passively mode-locked YDF laser with 1.116 ns duration and a 32.02 MHz repetition rate was obtained at 1063.14 nm. Under the pump power of 342 mW, the maximum average output power of 4.82 mW and single pulse energy of 0.15 nJ were obtained. These results indicate that Au-NCs/SiO2 has excellent saturable absorption properties with the important potential applications in the field of ultra-short pulse lasers.

2 Preparation and characterization of Au-NCs/SiO2 SA

The Au-NCs/SiO2 in our experiment were synthesized by the seed-mediated growth method. The details of the preparation process of the Au-NCs/SiO2 solution can be seen in our previous work (Bai et al. 2018). Figure 1a shows a transmission electron microscopy (TEM) image of the Au-NCs/SiO2 with a scale bar of 50 nm. But the fabrication of Au-NCs/SiO2 SA was different from our last work. This time, we chose to make the Au-NCs/SiO2 SA in the form of PVA film. Firstly, 4.0 g of PVA was dissolved in 100 mL of double distilled water and continuously stirred over a magnetic stirrer at the constant temperature of 80 °C for 2 h in order to obtain the homogeneous PVA solution at ~ 4 wt %. Next, 50 mL of the Au-NCs/SiO2 solution and 100 mL of the PVA solution were pipetted by pipette into a beaker and vigorously stirred for another 2 h. The mixture was then sonicated by using an ultrasonic homogenizer for 24 h to produce well-dispersed colloidal solution. The solution was stood for 24 h to observe whether there was precipitate. Then 0.1 mL of the suspension was sprayed onto a glass substrate by the spin-coating method and evaporated at 30 °C by an oven for 4 h to obtain the Au-NCs/SiO2/PVA film. Finally the dried film was carefully peeled off from the glass substrate and sandwiched between two fiber ferrule connectors. The fiber-compatible SA was successfully prepared, as shown in Fig. 1b. This method can decrease the insertion loss in the all-fiber laser.

Fig. 1
figure 1

a TEM image of the Au-NCs/SiO2. b The integration of the Au-NCs/SiO2-SA film between two fiber patch cords

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By using an ultraviolet–visible (UV) spectrophotometer (U-4100), the absorption spectra of the Au-NCs/SiO2 solution was measured from 400 to 1100 nm, as shown in Fig. 2a. It was obvious that the Au-NCs/SiO2 had two absorption peaks. The weak transverse SPR band was located at the visible region, while the longitudinal SPR absorption peak was located at about 1060 nm. The nonlinear optical absorption of the Au-NCs/SiO2 SA was measured using an open-aperture Z-scan technique (the inset in Fig. 2b was our Z-scan measurement bench). For this, a picosecond fiber laser operating at 1064 nm with the pulse duration of 15 ps and pulse repetition rate of 41 MHz was used as the pump light. As described in Fig. 2b, the modulation depth and saturation intensity are 5.3% and 0.461 MW/cm2, respectively, further indicating that Au-NCs/SiO2 SA can be used as SA to induce mode-locking.

Fig. 2
figure 2

a The absorption spectrum of the Au-NCs/SiO2. Inset: The image of the Au-NCs/SiO2 solution. b The relationship between nonlinear transmission of Au-NCs/SiO2 SA and incident laser intensity at 1.0 μm

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3 Construction of mode-locked YDF laser

The schematic diagram of the mode-locked fiber laser with the fabricated Au-NCs/SiO2-SA was demonstrated in Fig. 3. In order to investigate the nonlinear saturable absorption properties of Au-NCs/SiO2, the prepared Au-NCs/SiO2 SA film was incorporated into the all-fiber laser between the polarization controllers (PC) and the optical coupler (OC). The fiber laser was pumped by the 976 nm LD via a fused 980/1064 nm wavelength division multiplexer (WDM). A 28 cm long YDF (LIEKKI Yb1200-4-125) with an absorption coefficient of 1200 dBm−1 at 976 nm was used as the gain medium. The total cavity length was about 6.2 m and the rest of fibers used were all standard single mode fiber (SMF, SMF-28). Two PCs were used to fine tune the linear cavity birefringence and optimize the pulse stability. The polarization independent isolator (PI-ISO) was incorporated to guarantee the single-direction operation. The filter with the 3 dB bandwidth of 2 nm at 1064 nm was used to suppress the mode competition effect. The parameters of output pulse from the 10% output OC were measured by an optical spectrum analyzer (Yokogawa AQ6370D), digital storage oscilloscope (Tektronix TDS4054B, 500 MHz bandwidth, 2.5 G samples s−1) with a fast positive intrinsic negative (PIN) photo-diode, a radio-frequency (RF) spectrum (R&S FPC1000), and a power meter (MolectronPM3).

Fig. 3
figure 3

Schematic configuration of the all-fiber passively mode-locked Yb-doped fiber laser based on the Au-NCs/SiO2 SA

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4 Experimental results and discussion

During the experiment, we firstly investigated the laser output performance without inserting the Au-NCs/SiO2 SA film, and the laser works in a continuous-wave mode, being insensitive to the polarization state. Then incorporated the Au-NCs/SiO2 SA into the cavity, mode-locking would was obtained when the pump power increased to ~ 223 mW. Figure 4a shows the mode-locked optical spectrum, which exhibits an approximately rectangle with steep rising and falling edges, indicating the dissipative soliton operation. Dissipative soliton generation is a generic feature of nonlinear pulse propagation in all normal dispersion (ANDi) fiber lasers. It is the balance between the mutual interaction among the effects caused by normal cavity dispersion, the laser gain, the loss, the effective cavity gain bandwidth filtering and the fiber nonlinear Kerr effect (Zhao et al. 2006; Chong et al. 2006). The central wavelength and 3 dB spectral width of the mode-locked pulse were 1063.14 nm and 0.2913 nm, respectively.

Fig. 4
figure 4

a Output power of the mode-locked fiber laser as a function of the pump power. b Optical spectrum of the generated pulses

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Figure 4b shows the average output power of the mode-locked fiber laser as a function of the pump power. When the pump power was increased to 161.8 mW, continuous-wave operation was observed. As the pump power is increased, stable passively mode-locked performance is achieved when the pump power was higher than 223.4 mW. For stable passively mode-locked operation, one can clearly see that the output average power linearly increases with the incident pump power up to 4.82 mW. Corresponding to a slope efficiency of 4.03%, which is relatively high.

Figure 5 shows the temporal traces of mode-locked pulses. The pulse repetition rate was 32 MHz, which matched well with the total cavity length of 6.2 m. The pulse duration of a single pulse (\(\Delta \uptau\)) was 1.116 ns. And as mentioned in Fig. 4a, \(\Delta \uplambda = 0.2913 \,{\text{nm}}\) and \(\uplambda_{c} = 1063.14 \,{\text{nm}}\), we can obtained the time-bandwidth product (TBP) through the formula \({\text{TBP}} = \Delta {\text{v}} \cdot \Delta \uptau\), where \(\Delta {\text{v}} = \left( {{\raise0.7ex\hbox{$c$} \!\mathord{\left/ {\vphantom {c \lambda }}\right.\kern-0pt} \!\lower0.7ex\hbox{$\lambda $}}} \right)^{\prime} = \frac{c}{{\lambda^{2} }} \cdot \Delta \lambda\). Thus, the corresponding TBP was about 89.79, which suggested that the optical pulse was higher chirped. In the future, we will try to obtain picosecond and femtosecond mode-locked fiber laser based on Au-NCs/SiO2 by adopting a series of compensating dispersion method.

Fig. 5
figure 5

A typical pulse train of the mode-locked laser at a pulse repetition rate of 32 MHz and a single pulse sharp of 1.116 ns

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In addition, we employed a spectrum analyzer (R&S FPC1000) to measure the spectral quality of the mode locking operation, the radio spectrum (RF) of the pulse train was record and shown in Fig. 6. It is obvious that gives a signal to noise (SNR) of > 50 dB, in a span of 100 kHz with resolution bandwidth (RBW) of 1 kHz, indicating the pulse train of the mode-locked laser with low amplitude fluctuations.

Fig. 6
figure 6

RF spectrum of the mode-locked YDFL for a pump power of 342 mW

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To display our work, we summarize some representative mode-locked fiber lasers based on 2D SAs in near-infrared wavelengths reported recently in Table 1. It can be seen that our work is competitive in all aspects, which indicates the effectiveness of using Au-NCs/SiO2 as SA in mode-locked fiber lasers.

Table 1 Comparison of passively mode-locked Yb-doped lasers based on different 2D SAs
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5 Conclusion

In summary, an all-fiber passively mode-locked YDFL operating at 1063.14 nm with an Au-NCs/SiO2 SA was demonstrated for the first time. The pulse with the width of 1.116 ns and the pulse repetition rate of 32 MHz was obtained when the pump power was higher than 223.4 mW. The maximum average output power of 4.82 mW was obtained under the pump power of 342 mW, corresponding to the optical conversion efficiency of 1.41% and slope efficiency of 4.03%, which are relatively high. Our experiment shows that the Au-NCs/SiO2 is a promising material of SA for mode-locked fiber lasers.