1. Introduction

Q-switched erbium-doped fiber lasers (EDFLs) have attracted great attention due to their various applications in telecommunications, optical fiber sensing, medicine, and long-pulse nonlinear experiments. In comparison with actively Q-switched fiber lasers, passively Q-switched fiber lasers have advantages of compactness and flexibility in design. A key element in a passive Q-switched fiber laser is a saturable absorber (SA). Passively Q-switched EDFL have been intensively investigated by using different kinds of SAs [1–12], such as transition metal ions doped bulk crystals, bleachable dyes, semiconductor SA mirrors (SESAMs). Recently, carbon nanotubes and graphene are generally thought of as the ideal SAs for realizing all-fiber Q-switched fiber lasers [5–11], due to their advantages such as good compatibility with optical fibers, low saturable intensity, sub-picosecond recovery time, and simple manufacturing process.

Very recently, nonlinear optical properties of gold nanoparticles have been widely investigated. Joanna et al. reported the third-order nonlinear optical properties of gold nanorods in water solution [13]. Ji et al. calculated nonlinear absorption in gold nanostructures by using the discrete-dipole approximation model [14]. Lin derived the figure of merit of surface plasmon resonance of gold nanorods [15]. Li et al. reported anisotropic and enhanced nonlinear absorption in a macroscopic film induced by aligned gold nanorods [16]. Lamarre et al. measured the anisotropic nonlinear optical absorption coefficient of gold nanorods in a silica matrix [17]. Elim et al. observed saturable and reverse-saturable absorption at longitudinal surface plasmon resonance in gold nanorods [18]. Due to their large third-order nonlinearity, broadband absorption induced by surface plasmon resonance (SPR) and fast response time of few picoseconds [13–21], gold nanoparticles (GNPs) have been also proposed as SAs for constructing all-fiber Q-switched or mode-locked lasers [22, 23]. Especially, GNPs have large third-order nonlinear coefficient (~10⁻⁶ esu) compared to carbon nanotubes (10⁻⁸ ~10⁻⁹ esu) and graphene (~10⁻⁷ esu) [6, 24]. Since the nonlinear absorption of a SA is proportional to its third-order nonlinear coefficient, it is easy to obtain self-starting Q-switched or mode-locked lasers by using GNPs as SAs. In order to investigate the potential of GNPs for pulsed lasers, several works have been done. Jiang et al. demonstrated that spherical GNPs could be used to realize passively Q-switched fiber laser [12]. In Jiang’s work, GNPs only shows an absorption peak at 523 nm, which originates from transverse SPR absorption. Q-switched was caused by the broadband absorption of aggregated GNPs in the film. However, it is difficult to control the aggregation of GNPs as required. In comparison with GNPs, the structure of gold nanorods (GNRs) is non-spherical symmetric and the resulting SPR is anisotropy. So GNRs have two SPR absorption bands [25]. One is transverse SPR absorption, originating from electron resonance that is perpendicular to the axial of rod. The other one is longitudinal SPR absorption originating from electron resonance that is along the axial of rod. In contrast to the transverse SPR absorption, the longitudinal SPR absorption can be tuned in a wide range by varying the aspect ratio of GNRs [18, 26]. Therefore, Q-switched fiber lasers operating in a wide wavelength region can be constructed by using GNRs as SAs. In a previous paper, we first experimentally demonstrated a passively mode locked erbium-doped fiber laser by using GNRs as a SA [27]. The longitudinal SPR absorption of GNRs plays an important role in constructing mode-locked fiber lasers. Although mode-locking fiber lasers using GNRs have been demonstrated experimentally, Q-switched lasers by using GNRs as SAs have not yet been demonstrated experimentally.

In this paper, we demonstrated an all-fiber passively Q-switched EDFL by using GNRs as a SA. The SA film was fabricated by mixing the GNRs and sodium carboxymethylcellulose (NaCMC) aqueous solution. By inserting the SA film in an EDFL cavity pumped by a 980 nm laser diode, stable passive Q-switching was obtained for a threshold pump power of ~27 mW, and 4.8 μs pulse duration at 1560 nm with a pulse repetition rate of 39.9 kHz were obtained for a pump power of 275 mW.

2. Fabrication and chacterization of gold nanorods (GNRs) saturable absorber

GNRs used in our experiment were prepared by a seed-mediated growth method [25, 28]. The seed solution for GNRs was prepared as reported previously [27]. A 10 mL amount of 0.2 M Hexadecyltrimethyl ammonium bromide (CTAB) was mixed with 10 mL of 0.5 mM HAuCl₄ in a beaker. 1 mL of 0.01 mM freshly NaBH₄ solution was then injected into the HAuCl₄-CTAB solution under vigorous stirring. The color of the solution was changed from dark yellow to brownish yellow, and the stirring was stopped after 10 min. Then the seed solution was kept for 2 h at room temperature and used for the synthesis of the GNRs. The growth solution was prepared by mixing 20 mL of 0.15 M CTAB, 12.5 mL of 0.2 M 5-bromosalicylic acid, 1.5 mL of 4 mM AgNO₃ aqueous solution and 1ml of 0.1mM ascorbic acid in a flask, and the solution was vigorously stirred for 30 s until it became colorless. After mixing of the solution, 1 mL of the seed solution was injected into the growth solution to initiate the growth of the GNRs. The final solution was kept for 48 h and no precipitation was observed. The stable suspension of GNRs in 1.5 wt.% aqueous solution of NaCMC were prepared by ultrasonication. The suspension was kept for 24 h and no precipitation was found. The GNRs-NaCMC film was formed by casting the solution onto a flat substrate, and then followed by a slow drying at room temperature. This fabrication method is very simple and cost-effective. Figure 1(a) shows a transmission electron microscopy (TEM) image of GNRs (the scale bar is 200 nm). GNRs with negligible shape imperfections are obtained with the dimensions of (10 ± 2 nm) × (38 ± 4 nm) and only a small fraction of spherical nanoparticles exists in the as-synthesized GNRs sample. The inset of Fig. 1(a) shows the photograph of the aqueous solution of GNRs. The color of the aqueous solutions is pink. Figure 1(b) shows the aspect ratio distribution of GNRs. 200 of the as-prepared GNR sample were counted and measured to determine the aspect ratio distribution. The aspect ratio varies from 1.5 to 5.5. Nearly 45% of GNRs have an aspect ratio of ~4.5. The wide range aspect ratios may cause broadband absorption. Figure 1(c) shows the AFM image of the film. It can be seen that the surface of the thin film is smooth and crack free (The scan range is 20 × 20 μm). Figure 1(d) shows the absorption spectra of NaCMC film with and without GNRs. They were measured with an ultraviolet (UV)-visible-near infrared (NIR) spectrophotometer (UV-3600 Shimadzu). As can be seen from Fig. 1d, the GNRs-NaCMC film has two absorption peaks at 532 nm and 1145 nm respectively. The 532 nm peak is caused by the transverse SPR absorption of the GNRs and spherical nanoparticles [25]. The other one at 1145 nm is caused by the longitudinal SPR absorption of GNRs. The broadband absorption from 850 nm to 1650 nm was achieved by using GNRs with aspect ratios from 1.5 to 5.5. In contrast, the host polymer of NaCMC only has a low absorption in the whole range from 500 nm to 1750 nm. We also synthesized GNRs with average aspect ratios of 1.5~9.5 by adjusting the addition of CTAB, AgNO₃, and 5-bromosalicylic and measured their absorption spectra. The results showed that the main longitudinal SPR absorption peak could be tuned from 1044 nm to 1455 nm by varying the aspect ratios of GNRs.

 

Fig. 1 (a) TEM image of the GNRs, the inset of Fig. 1(a): Photograph of the aqueous solution of GNRs. (b) Aspect ratio distribution of GNRs. (c) AFM image of GNRs-NaCMC film (the scan range is 20 × 20 μm). (d) Absorption spectra of NaCMC with and without GNRs.

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Figure 2 shows the transmission ratio of GNRs-NaCMC film (the average aspect ratio of GNRs is ~4.5) as a function of pump peak power density. The film was placed between two fiber connectors to form a fiber-integrated device. Then we measured the dependence of the transmission ratio of the device on the pump power density by using a 1560 nm mode-locked laser with a repetition rate of ~50 MHz and a pulse width of ~600 fs. By fitting the data shown in Fig. 2 with the equation α(I) = α_s/(1 + I/I_s) + α_ns (where α(I) is the absorption coefficient, α_s and α_ns are the saturable and nonsaturable absorption components, and I and I_s are input and saturable intensities, respectively), the modulation depth (ΔT), non-bleachable loss (α_ns) and saturation intensity (I_s) were determined to be 16%, 27%, and 19.09 MW/cm², respectively. These results showed that the GNRs-NaCMC film could be used to induce Q-switching.

 

Fig. 2 Transmission ratio of GNRs-NaCMC film as a function of pump peak power density.

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3. Construction of Q-switched EDFL

In order to see whether the GNRs-NaCMC film can be used to induce Q-switching or not, the film was placed between two fiber connectors to form a fiber compatible SA, and then integrated into an EDFL cavity, as shown in Fig. 3. A 980 nm laser diode was used as the pump source. A 20 cm long erbium doped fiber (EDF, its GVD value is 24.543 ps²/km) was used as the gain medium. The pump light was launched into the laser cavity through a 980/1550 nm wavelength-division multiplexing (WDM) coupler. Unidirectional laser operation was guaranteed by adding an isolator in the laser cavity. A 10 dB WDM coupler was used as the output coupler. A GNRs SA was inserted into the laser cavity to induce Q-switched operation. The insertion loss of the SA was about 0.55 dB. The length of the other fibers except for EDFs is 5.56 m (SMF-28, its GVD value is −18 ps²/km). The overall length of the laser cavity was 5.76 m. The total cavity dispersion was about −0.095 ps². The Q-switched laser was output from one port (10%) of the 10 dB WDM coupler. The output lasers were analyzed by using an optical spectrum analyzer, a power meter, and a digital oscilloscope together with a photodetector.

 

Fig. 3 Experimental setup of GNRs-NaCMC SA based Q-switched ring cavity fiber laser.

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4. Experimental results and discussions

In the case of absence of the GNRs absorber in the cavity, the laser always works in a continuous wave (CW) regime, being insensitive to the polarization state. Once the NaCMC film with GNRs is inserted into the cavity, the laser starts to operate in a passive Q-switching regime with 27 mW pump threshold in the counter-direction to pump propagation. Figure 4(a) shows the emission spectrum of the Q-switched EDFL for a pump power of 275 mW. The central wavelength of the Q-switched EDFL is about 1560 nm. Figure 4(b) and 4(c) show the pulse train and single pulse profile of the above Q-switched EDFL, respectively. The interval between two adjacent pulses is about 25.06 μs, corresponding to a repetition rate of 39.9 kHz. The duration of a single pulse is 4.8 μs. The pulse duration can be further reduced by optimizing the structure of the laser cavity (e.g. reducing the length of the laser cavity) [29]. Figure 4(d) shows the pulse duration and the repetition rate as a function of pump power. By varying the pump power from 27 mW to 275 mW, the pulse duration can be tuned from 23 μs to 4.8 μs and the repetition rate varies from 7.1 kHz to 39.9 kHz. It is seen that the pulse width decreases and the repetition rate increases by increasing the pump power, which presents a typical feature of passively Q-switched lasers. The Q-switched fiber laser exhibited an excellent stability at room temperature. The spectrum and pulse duration at 275 mW were repeatedly detected 20 times at an interval of 5 min. No significant wavelength shift or peak power fluctuation was observed.

 

Fig. 4 (a) Emission spectrum, (b) pulse train, and (c) single pulse profile of the Q-switched EDFL for a pump power of 275 mW. (d) Pulse duration and repetition rate as a function of pump power.

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Figure 5 shows the output power of the Q-switched fiber laser as a function of the pump power. With increasing the pump power from 27 to 275 mW, the output power increases almost linearly from 1.5 to 12.5 mW, corresponding to a slop efficiency of ~4.5%. In addition, the Q-switched EDFL became unstable when the pump power was larger than 275 mW. The SA film might be damaged due to the photothermal effect occurred in GNRs [30]. Further work will be done for increasing the damage threshold of it.

 

Fig. 5 Output power of the Q-switched fibre laser as a function of the pump power.

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In addition, in order to investigate the effect of the aspect ratio of GNRs on the threshold of Q-switching, we measured the performances of Q-switched lasers by using GNRs with varied aspect ratios as SAs. The results showed that the GNRs with an average aspect ratio of ~4.5 had the lowest Q-switching threshold of ~27 mW.

5. Conclusion

In conclusion, an all-fiber passively Q-switched erbium-doped fiber laser using GNRs as SA was demonstrated experimentally. GNRs had wideband absorption from 850 nm to 1650 nm, which was caused by the longitudinal SPR absorption of GNRs. Stable Q-switched operation was achieved for a threshold pump power of 27 mW. 4.8 μs pulses at 1560 nm with a repetition rate of 39.9 kHz were obtained for a pump power of 275 mW. Our results show that the GNR film is a new type of SA for Q-switched fiber laser.