1 Introduction

Over the past few years, many works have been reported on Q-switched erbium-doped fiber lasers (EDFLs), which operates in the 1.5 µm region. Research on Q-switched EDFL has grown tremendously due to its potential applications in medicine, sensing, and material processing [1,2,3]. Q-switched fiber lasers are normally achieved through passive techniques, where a saturable absorber (SA) device is integrated inside the laser cavity. This technique is more attractive than the conventional active approaches as it does not require a modulator device, is lower in cost, and is simpler. Up to date, many types of SAs such as semiconductor saturable absorber mirrors (SESAMs) [1], single-walled carbon nanotubes [2], graphene [3], topological insulators (TIs) [4, 5], black phosphorus (BP) [6], and transition metal dichalcogenides (TMDs) [7,8,9]) have been employed for obtaining Q-switching pulses in various EDFL cavities. However, SESAMs have a narrow operation bandwidth, relatively high cost due to a complex preparation process and low damage threshold [1]. These limitations have restricted the development of SESAM-based Q-switched fiber lasers. SWCNTs are advantageous due to the easier material preparation [2], but its absorption efficiency, which determines the operation bandwidth, is dependent on its tube sizes. Graphene has a wider absorption range and has been demonstrated for pulsed laser generation. However, optoelectronics applications of graphene have been limited due to its zero-bandgap structure. Inspired by the success of graphene, other two-dimensional materials such as TIs, BP, and TMDs have also been extensively investigated in recent years as SAs. But efforts in exploring other new SA materials as well as new designs for generating Q-switched pulses trains have continued.

Metal-based nanostructure materials have recently received attention due to their optical properties, which are excellent in terms of response time, third-order nonlinearity, and surface plasmon resonance (SPR) absorption [10,11,12]. Nowadays, metal nanostructures such as gold nanoparticle-based SAs have also been reported for mode-locking and Q-switching applications [13, 14]. The characteristic of broadband SPR absorption allows lasers to operate at mode-locking or Q-switching states. Those findings show that metal nanoparticles are promising to be used as SAs. Wavelength tunability is an important criterion that needs to be considered for applications in communications and sensing. A tunable Q-switched EDFL can be achieved using a digital micro-mirror array [15] or bandpass filters or fiber Bragg grating (FBG). Previously, Popa et al. [16] demonstrated a simple wideband-tunable Q-switched fiber laser with a 32 nm tuning range by exploiting a graphene saturable absorber. In another work, Dong et al. [17] demonstrated a tunable nanotube-based Q-switched EDFL using an FBG as a tunable filter.

In this paper, a tunable passively Q-switched EDFL is demonstrated using a silver nanolayer deposited onto a polyvinyl alcohol (PVA) film as an SA. The film is sandwiched between two fiber ferrules with the aid of index matching gel. Self-started Q-switching pulse train operation can be tuned from 1535 to 1565 nm by adjusting a tunable filter. This is comparable to the 32 nm range reported for graphene [16] and a much larger than the 5 nm range obtained for single-wall carbon nanotubes [17]. Our results indicate that SNP SAs have a significant potential for use in realizing stable pulse trains with a high pulse energy.

2 SA preparation and laser configuration

In this experiment, pure PVA film was firstly prepared by dissolving 1 g of PVA powder (40,000 MW, Sigma-Aldrich) into 120 ml of distilled water. The mixture was then stirred with the aid of heat at 145 °C to completely dissolve the powder inside the PVA solution. Five milliliters of the prepared PVA solution was then slowly poured and spread out into a petri dish container and left to dry in ambient condition for 3 days to obtain a 50-μm thin-layer film. Then, a thin-layer silver film was deposited onto the surface of the prepared PVA film using an electron beam machine (KENOSISTEC E-beam). During the deposition process, pure silver (Ag) pallets were heated inside the thermal evaporation chamber. From the heating process, the Ag nanoparticles were evaporated thermally and deposited onto the surface of the PVA film. PVA is a synthetic polymer which acts as the host material to hold the Ag nanoparticles together. The thickness of the Ag nanoparticles deposited onto the PVA was around 16 nm. Figure 1 illustrates the image of the PVA film deposited with the SNP layer. The surface morphology of the SNP layer was also characterized using a focused ion beam scanning electron microscope (FiB-SEM) at 20 k magnification, and the image is shown in the inset figure. It is shown that the high density of SNP was homogenously distributed onto the PVA film without any aggregation.

Fig. 1
figure 1

PVA film with deposited SNP layer. Inset shows the surface image obtained using FiB-SEM equipment at a magnification of 20 kX

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The linear absorption and nonlinear saturable absorption of the SNP film were also investigated. Figure 2(a) shows the linear absorption spectrum of the SNP film, which was obtained by transmitting a white light source through the film. It is observed that the absorption loss was about 1 dB at 1550 nm region. To confirm the saturable absorption ability of the fabricated SNP film, we employed the amplified mode-locked fiber laser to measure nonlinear optical response based on a balanced twin-detector measurement technique. In the measurement, we used a homemade mode-locked fiber laser operating at 1560 nm with a 1.0 MHz repetition rate and 3 ps pulse width as an input pump source. As the absorption power is recorded as a function of launched photon intensity on the SNP film by varying the input laser power as shown in Fig. 2(b), it is observed that the absorption decreases with optical intensity, which verifies the saturable absorption property of the film. The SA film has a modulation depth of 2%, saturable intensity of 0.7 MW/cm2, and nonsaturable absorption of 98%.

Fig. 2
figure 2

(a) Linear and (b) nonlinear absorption profile of the fabricated SNP film

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The schematic of the setup using the SNP-based passively Q-switched EDFL is shown in Fig. 3. The laser setup is designed based on a ring cavity scheme, where a laser diode operating at 980 nm is employed as the pump source. The pump laser is injected into the ring resonator through a 980/1550 nm wavelength division multiplexer (WDM). The gain medium is a 2.4-m-long erbium-doped fiber (EDF) with a peak absorption of 23 dB/m at 980 nm. The polarization insensitive isolator is inserted in the ring cavity to keep a unidirectional operation of the laser. An inline tunable bandpass filter (TBF) is inserted in the cavity to tune the operating wavelength of the laser. The TBF has a transmission bandwidth of 1 nm. The SNP-based SA is placed between the TBF and isolator to function as a Q-switcher. An 80/20 optical fiber coupler is deployed to monitor the laser output through its 20% port. The proposed laser has a total cavity length of 7.5 m. The output laser is measured and recorded by a 1.2 GHz photodetector, a 350 MHz digital oscilloscope, a power meter (PM100DS122C), an optical spectrum analyzer (AQ6317), and a 7.8 GHz spectrum analyzer.

Fig. 3
figure 3

Setup of the SNP-based Q-switched EDFL

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3 Results and discussion

A stable Q-switching pulses train was first generated at the threshold pump power of 14.9 mW. The laser operation was sustained up to the pump power of 37.8 mW. Figure 4 shows the output spectra for the Q-switched EDFL, which was obtained by keeping the pump power at 37.8 mW and tuning the bandpass filter wavelength from 1535 to 1564 nm. It is seen that the Q-switched pulses are wavelength-tunable, where the center wavelength of the laser could be continuously tuned from 1535 to 1565 nm by varying the passband of the tunable filter. This is attributed to the wide saturable absorption bandwidth of the SNP SA, which produces a steady train of pulses within this wavelength region. This wavelength location could be precisely adjusted and controlled by tuning the bandpass filter. As the erbium gain varies with the operating wavelength, the laser’s output power slightly changes with the pump. It is also worthy to note that the tuning range of the laser is restricted by the gain bandwidth of the EDF rather than the SA. Therefore, with an active fiber supporting broader bandwidth, we expect that the laser operation can be tuned within a wider wavelength range.

Fig. 4
figure 4

Output spectra of the SNP-based Q-switched laser at various operating wavelengths

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Figure 5 depicts the output power of the Q-switched EDFL against the pump power, which was tuned for seven different operating wavelengths. With increasing the laser diode power from 14.9 to 37.8 mW, the output power almost linearly increases for all the operating wavelengths. For instance, at 1560 nm operation, the output power rises from 0.20 to 0.78 mW, corresponding to a slope efficiency of about 2.5%. The efficiency is the highest at the 1560 nm operation due to the gain medium, which provides the highest gain at around 1560 nm region. It is also observed that the Q-switched laser operation becomes unstable when the pump power exceeds 37.8 mW. As the SNPs’ film is removed from the cavity, the EDFL always operates in a continuous-wave mode. This indicates that the SA film was responsible for the Q-switched operation.

Fig. 5
figure 5

Output power against input pump power for various operating wavelengths

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Figure 6(a) shows the typical Q-switching pulses trains operating at 1560 nm as the pump power is fixed at 14.9 mW. It shows a stable pulse train with 66.2 μs pulse spacing, corresponding to a 15.1 kHz repetition rate. The pulse duration is measured to be around 19.5 μs. Figure 6(b) shows the radio frequency (RF) spectrum of the corresponding output laser, showing a high signal-to-noise ratio of ~ 55 dB. This indicates that pulses train stability was excellent and comparable to other Q-switched fiber lasers [4,5,6,7,8,9,10]. There are 19 harmonics within the span of 300 kHz.

Fig. 6
figure 6

Temporal characteristic of the tunable Q-switched laser operating at 1560 nm. (a) Typical oscilloscope trace. (b) RF spectrum

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The operation of the laser was also investigated at various input pump powers. It is noted that the pulse properties in Q-switched lasers with CW laser pumping depend on nonlinear dynamics in the active medium and SA. This leads to the dependence of both repetition rate and pulse width upon pump power as depicted in Fig. 7. In the experiment, the operating wavelength was fixed at 1560 nm. By varying the pump power from 14.9 to 37.8 mW, the repetition rate can be tuned from 15.1 to 28.4 kHz and pulse width varies from 19.45 to 11.85 μs. In a typical fiber laser, more gain is available at higher pump power. Thus, the threshold energy which is required to saturate the SA is accomplished earlier with higher pump power, which results in an increase in pulse repetition rate and pulse width reduction. The change in both repetition rate and pulse width causes the peak power to jump from 6.8 to 23.2 mW as shown in Fig. 8. Figure 8 presents the peak power and pulse energy of the Q-switched EDFL operating at 1560 nm. The increase in pump power from 14.9 to 37.8 mW also increases the pulse energy from 13.3 to 27.5 nJ. Since the cavity employed was long, a long pulse duration was expected due to a longer cavity lifetime. The pulse duration can be further improved by reducing the cavity length and optimizing the structure of the laser cavity. The stability of the proposed Q-switched laser was also very excellent at room temperature. The output spectrum and oscilloscope trace at 37.8 mW were also repeatedly detected at an interval of 10 min for 20 times. We observed no peak power variation or significant wavelength shift in the laser output.

Fig. 7
figure 7

Repetition rate and pulse width at various pump powers for the tunable laser operating at 1560 nm

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Fig. 8
figure 8

Pulse energy and peak power at various pump powers for the tunable laser operating at 1560 nm

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4 Conclusion

A tunable fiber laser was successfully demonstrated using SNPs’ PVA film as a Q-switcher in a ring EDFL. A broadband tunability from 1535 to 1565 nm was obtained with the highest slope efficiency of 25.3% at 1560 nm due to the wideband operation of the SNPs. At 1560 nm operating wavelength, the repetition rate can be raised from 15.1 to 28.4 kHz, while the pulse width can be reduced from 19.45 to 11.85 μs with an increasing input pump power from 14.9 to 37.8 mW. The maximum pulse energy is observed at 27.5 nJ. The Q-switched laser with such widely tunable operation is suitable for various applications including sensing, biomedical diagnostics, and metrology.