1 Introduction

Nanosecond pulses are attractive for their potential use in generating high pulse energy laser sources for various industrial and scientific applications such as micromachining, metrology, biomedicine and telecommunications [1,2,3,4]. Nanosecond pulse fiber lasers typically exhibit low repetition rates of around few kHz, which is convenient for reaching microjoule energies through external amplification. Conventionally, nanosecond pulses lasers were achieved by active modulation approach utilizing electro-optic and acousto-optic modulators, which is both bulky and expensive. Thus, there has been substantial work on the realization of nanosecond pulse lasers via passive techniques such as quality-switching (Q-switching) and mode locking. Passively, Q-switched lasers mainly use crystal-based saturable absorber, such as Cr:YAG, to allow for Q-switched and/or mode-locked operation. However, a stable pulse train is difficult to achieve by this approach due to its complexity, particularly due to the alignment needed during the pulsed generation. Alternatively, stable nanosecond pulses can be obtained by extension of the cavity length in a passively mode-locked fiber laser by using nonlinear polarization rotation (NPR) technique [5, 6] or semiconductor saturable absorber mirror (SESAM) [7]. However, NPR-based mode-locked fiber laser has a relatively low environmental stability and reliability since it requires the adjustment of the polarization state of the oscillating light in the cavity. SESAMs have recently become readily available, but it is still quite expensive for purposes of large scale of production.

Recently, two-dimensional (2D) nanomaterials such as graphene, black phosphorus, transition metal dichalcogenides (TMDs), topological insulators (TI) have attracted a great deal of interest for application in saturable absorber (SA) [8,9,10]. Most of these works were focused on obtaining Q-switching pulses with microsecond pulses as well as mode-locking pulses with femtosecond or picosecond pulses. There are relatively a small number of reports on the generation of nanosecond pulse. Previously, the mode-locked nanosecond EDFL using SA’s such as graphene [11] and zinc oxide [12] has been reported with 6 ns and 400 ns pulsed width, respectively. Moreover, MoS2 saturable absorber has been reported on producing the nanosecond pulse laser with pulsed width of 660 ns with pulse energy of 152 nJ at center wavelength of 1549 nm [13]. On the other hand, many researchers explored on metals nanoparticles as SAs in more recent years for constructing Q-switched and mode-locked fiber lasers due to the broadband absorption induced by the surface plasmon resonance and fast time responses in the picosecond timescale. This has been demonstrated by [14,15,16] using Yb, Er and Tm fiber laser at 1030 nm, 1550 nm and 1950 nm with a gold-based SA by controlling the Sp peak. Noble metal nanoparticles were first proposed for SAs in generating nanosecond pulsed (Q-switched and nanosecond mode-locked) due to their intrinsic, large third-order nonlinearity [17, 18] and broadband absorption induced by surface plasmon resonance with a timescale of a few picoseconds, comparable to other reported materials [19]. Moreover, the gold nanoparticle is a noble metal and thus has close lying bands which allow the electron to move freely. The energy has occupied the electron on the conduction before it excited to surface plasmon resonance band when light beam hits the gold particles [20]. Recently, metal nanomaterial was used to demonstrate Q-switched by Muhammad et al. by using copper nanoparticle as saturable absorber with pulse width reduces from 10.19 to 4.28 μs [21]. Ahmad et al. reported Q-switched generated by gold nanoparticle thermal deposition method which achieved maximum pulsed pulse width pulse energy of 4.25 µs [22].

In this paper, we report the gold nanoparticles used to generate stable nanosecond pulses from an EDFL in a long ring cavity. The surface plasmon resonance (SPR) of gold nanoparticles enables the generation of mode-locked laser operating at 1561 nm with constant repetition rate of 1 MHz and the maximum pulse energy of 6.8 nJ at the pump power of 245 mW.

2 Fabrication and nonlinear characterization of rose gold film

Gold nanoparticles were synthesized using poly (sodium 4-styrenesulfonate) (PSSS), gold (III) chloride trihydrate (~ 50% Au basis) (HAuCl4), tri–sodium citrate (Na3C–H5O7, TSC) and sodium borohydrate (NaBH4) solutions, prepared using deionized water (with resistivity of 18 MΩ). All chemicals and solvent were used as received without further purification. The gold nanoparticles were prepared using a NaBH4 reduction method. At first, 50 mL TSC, 3 mL PSSS and 3 mL of NaBH4 were added into a beaker containing 1000 mL deionized water while stirring at 450 rpm. Then, 50 mL HAuCl4 (5 mM, in water) was added dropwise (~ 2 mL/min) to the mixture with continuous stirring followed by the addition of excess amount of TSC (20 mL). The reaction was allowed continue for 5 min before it was centrifuged for cleaning purpose.

The solution was then mixed with PVA solution to fabricate a SA film. The PVA solution was prepared by dissolving PVA powder (40,000 MW, Sigma-Aldrich) into 80 ml of DI water before being stirred slowly for about 2 h at 145 °C until the powder completely dissolves. The resulting suspension was then poured into the Petri dish and left dry at room temperature. After 2 days, the thin film was slowly peeled from the Petri dish. The gold nanoparticles PVA film was then cut into a small piece to attach into an FC/PC fiber ferrule as shown in Fig. 1. The inset shows the resulting gold nanoparticles solution. The ferrule was then connected with another ferrule via a fiber adaptor after depositing a small amount of index matching gel onto the fiber end to construct an all-fiber SA device. Figure 2 shows the TEM image for the rose gold nanoparticle solution, which indicates the size of particles is around 10 nm. Inset of Fig. 2 shows the SEM image for the film, which indicates gold nanoparticles were homogeneously distributed inside the PVA film.

Fig. 1
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The small piece of rose gold nanoparticle PVA film onto fiber ferrule. Inset the rose gold nanoparticle solution

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Fig. 2
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The TEM image of the rose gold nanoparticle solution. Inset shows the SEM image of the rose gold thin film

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The nonlinear optical properties of the gold nanoparticles PVA film were also investigated with the balanced twin-detector measurement technique. The illumination pulse is generated from a home-made mode-locked fiber laser (wavelength, 1.55 μm; pulse duration, ≈ 1.3 ps; repetition rate, ≈ 12.5 MHz). The nonlinear transmission of the film is measured by comparing the input power and output power with a double-channel power meter. As illustrated in Fig. 3, the fabricated film exhibits the saturable absorption property that the transmission (absorption) increases (decreases) with optical intensity. The modulation depth, saturable intensity and non-saturable absorption of the film are obtained as 13.6%, 0.5 MW/cm2 and 47.2%, respectively. It is worth noting that we have not observed any nonlinear response from pure PVA film, confirming that the saturable absorption property solely originates from the gold nanoparticles. The insertion loss of the rose gold nanoparticle film is 0.13 dB at 1560 nm wavelength.

Fig. 3
figure 3

The nonlinear optical properties of gold nanoparticle film by balance twin-detector measurement technique

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3 Experimental setup

The prepared SA device was inserted into an erbium-doped fiber laser (EDFL) ring cavity, schematically shown in Fig. 4. The ring cavity consists of a 20-cm-long EDF with a high concentration of erbium ion (12,500 wt ppm), an isolator, a 193-m-long standard single mode fiber (SMF), the SA device described above, a 10-dB output coupler and a wavelength division multiplexer (WDM). The EDF was pumped by a 980 nm laser diode via the WDM. The EDF has a peak absorption of 100 dB/m at 980 nm and numerical aperture of 2.1. The laser was coupled out from the cavity through 10% point. The isolator was used to force the unidirectional operation in the fiber ring cavity. The total cavity length is approximately 182.2 m. The 193 m SMF was added into the cavity to tailor the dispersion characteristic and nonlinearity of the cavity and allow nanosecond pulse generation. The laser output spectrum was observed using an optical spectrum analyzer (Ando AQ-6370C). The temporal performance was detected via 1.2-GHz InGaAs photodetector, and the pulse train was observed by a 500-MHz digital oscilloscope (Tektronix, TDS3054B).

Fig. 4
figure 4

The schematic diagram of the mode-locked laser with rose gold film saturable absorber

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4 Results and discussion

In this experiment, continuous wave (CW) laser operation began at an input pump power of 100 mW. The relatively high threshold of CW operation could be attributed to large inserting loss and long cavity. Self-started mode-locking operation occurred at a pump power of 157 mW. At this pump level, inducing photon absorption leads to saturable absorption capability. The mode-locking operation was maintained and stable until the pump power reached 245 mW and operated the fundamental frequency of 1 MHz. The mode-locked laser optical spectrum at the threshold pump power of 157 mW is shown in Fig. 5, operating at a central wavelength of 1561 nm with 3 dB spectral bandwidth of 0.3 nm. Small Kelly sidebands were also observed, showing that the mode-locked laser produced soliton pulses and that the entire cavity is operating in the anomalous dispersion regime. The ring cavity of mode-locked consists of EDF and SMF-28 which has group velocity dispersion (GVD) of 27.6 ps2/km and − 21.7 ps2/km, respectively. The mode-locked operated in anomalous dispersion about − 4.18 ps2. Balancing the dispersion and nonlinear effects in the cavity is important to allow for the generation of solitons in the pulse laser [23]. Figure 6 (a) shows the oscilloscope trace of the mode-locked pulse train. The time interval between the pulses is about 1 µs and is consistent with a cavity length of 182.2 m. The repetition rate of the soliton pulses is constant at 1 MHz despite the increase in the pump power from 157 to 245 mW. This operation mode is very stable with respect to any environmental perturbations and fluctuations of the pump power. Figure 6b shows an enlarged figure of the oscilloscope trace and shows a pulse width of 436 ns. We found that no bunched pulses were observed in the pulse internal structure; thus, the observed nanosecond pulse was a clean and stable single pulse.

Fig. 5
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Output spectrum of the mode-locked EDFL at pump power of 157 mW

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Fig. 6
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Temporal performance of the mode-locked EDFL (a) typical oscilloscope trace and (b) enlarged pulses train

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The stable nanosecond output pulses were confirmed with the radio frequency spectrum as indicated in Fig. 7. It provides signal-to-noise ratio (SNR) as high as 65 dB for the fundamental frequency. The peak of the fundamental frequency decreases moderately until 18th harmonic, which indicates a narrow pulse width. It is observed that the intensity fluctuated from 6th harmonic toward 18th harmonic due to the interaction between the harmonics. There is no presence of mode-locked pulses when the SA was removed. The relationship between the output average power and pulse energy with respect to incident pump power is shown in Fig. 8. The output power increases monotonously with the pump power with a slope efficiency of 3.2%. The efficiency is quite low due to the use of an ultra-short gain medium length and long cavity length. The average output power increases from 4.15 to 6.94 mW as the pump power rises from 157 to 245 mW. The single pulse energy also increases linearly with the pump power. A maximum pulse energy of 6.87 nJ was obtained at pump power of 245 mW. The experimental results verify the mode-locking ability of the newly developed gold nanoparticles-based SA. This shows that the gold nanoparticles could be used in optoelectronics device and communication device in the C-band region.

Fig. 7
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The radio frequency (RF) spectrum of the nanosecond pulsed

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Fig. 8
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The relationship between pump power with output power and pulse energy

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5 Conclusion

In conclusion, we have experimentally demonstrated the generation of nanosecond pulses in an EDFL mode-locked by a gold nanoparticles-based SA. The gold nanoparticles were prepared by a simple NaBH4 reduction method and embedded into PVA film to fabricate SA device. The fabricated film shows a modulation depth of 13.6% and a saturable optical intensity of 0.5 MW/cm2. Stable mode-locked nanosecond pulses at 1561 nm with spectral width of 0.3 nm, pulse duration of 436 ns and a fundamental repetition rate of 1 MHz have been obtained. The average output power was 6.94 mW, corresponding to single pulse energy of 6.8 nJ at pump power of 245 mW. The experimental results suggest that multilayer gold nanoparticles are a promising material for ultrafast laser applications operating at C-band region.