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We experimentally demonstrate the use of gold nanorods (GNRs)/PVA composite-deposited side-polished fiber as an efficient saturable absorber for use in an all-fiberized, high-energy Q-switched fiber laser. The modulation depth of the prepared saturable absorber was of ~7.5% at wavelength of 1549.8 nm. Stable Q-switched pulses with a single pulse energy of ~2.56 μJ were readily generated from a fiber Bragg grating-based Fabry-Pérot cavity at a pump power of ~229 mW. The pump efficiency was estimated at ~19.2%, and the impact of the variation in the cavity length with respect to the output pulse characteristics was investigated with fixed pump power. The output performance of the proposed all-fiberized laser was compared to that of Q-switched, 1.5-μm fiber lasers incorporating other types of nonlinear optical nanomaterials that had been previously proposed.
© 2015 Optical Society of America
1. Introduction
In recent years, saturable absorbers based on novel nonlinear optical materials have been extensively investigated as a result of the intense scientific and technical interest in passively mode-locked or Q-switched lasers. A saturable absorber is a passive modulating device that operates according to Pauli’s blocking principle, which describes a change in absorption that is induced by the optical intensity [1, 2]. In principle, a saturable absorption phenomenon can be observed in materials that have an energy band structure with ground and excited states. For decades, semiconductors with separate valence and conduction bands have been commonly used as base materials for saturable absorbers [3–5].
In 2002, carbon nanotubes (CNTs) were proposed as an alternative due to their semiconducting energy band structure and possibility for mass production at a low cost [6, 7]. In 2009, graphene, which is another carbon-based nanomaterial, was subsequently found to have an efficient saturable absorption property [1]. Recently, topological insulators (TI) [8–21] and molybdenum disulfide (MoS2) have also been proposed as efficient saturable absorption materials [22–31].
On the other hand, Elim et al. demonstrated nonlinear saturable absorption at longitudinal surface plasmon resonance (SPR) in gold nanorods, and since then, gold nanoparticles have been of high technical interest in the field of ultrafast photonics [32]. Gold nanomaterials mainly possess three different morphologies: nanospheres [33], gold nanorods [35–43], and gold nanocrystals [34]. These are known to have both a large third-order Kerr nonlinearity and fast recovery time [38, 39, 44]. In 2012, Jiang et al. were the first to implement a pulsed laser with the use of gold nanoparticles as saturable absorption materials. Specifically, they used gold nanocrystals to demonstrate passive Q-switching of a 1.55-μm fiber laser [34]. Since then, quite a few experiments have been carried out to demonstrate the use of gold nano-particles as saturable absorbers to generate pulsed output from laser cavities [33–43].
In this study, we investigated the use of gold nanorods as saturable absorbers to generate high-energy pulses from an all-fiberized laser cavity. Specifically, we prepared a saturable absorber based on gold nanorods (GNRs)/Polyvinyl alcohol (PVA) composite to demonstrate its use within an all-fiberized Fabry-Pérot cavity to generate micro-joule-level Q-switched pulses. To the best of our knowledge, the output pulse energy of passively-Q-switched fiber lasers incorporating gold nanoparticles had been previously limited to the submicro joule levels [33–36]. We also investigated the impact that the variation in the cavity length had on the characteristics of the output pulse under a fixed pump power.
The saturable absorber was prepared on the basis of the evanescent field interaction of a side-polished fiber platform with a GNRs/PVA composite film that is deposited to increase the damage threshold [47]. The maximum single-pulse energy was at ~2.56 μJ for a pulse width of ~7.1 μs at a pump power of ~229 mW. A pump power efficiency of ~19.2% was achieved, and the output performance of our all-fiberized laser was compared to that of Q-switched, 1.5-μm fiber lasers incorporating gold nanoparticle-based saturable absorbers that had been previously demonstrated. The performance was also compared to that of fiber lasers using saturable absorbers based on other types of nanomaterials, such as CNTs, graphene, and topological insulators.
2. Preparation of a saturable absorber based on a GNRs/PVA composite
The GNRs/PVA composite was prepared from commercially available GNRs. The GNRs had a diameter of 10 nm (NR-10-850, Nano seedz) and were dispersed in deionized water. 3 ml of GNR solution were mixed with 100 mg of aqueous PVA. Figure 1(a) shows a TEM image of the GNRs/PVA composite that was prepared, placed on a glass substrate, and dried in a petri-dish at room temperature for three hours. The inset in Fig. 1(a) shows a photograph of the GNRs/PVA solution that was used. The length of the GNRs was measured to be approximately 55 nm, and the average aspect ratio was thus estimated to be ~5.5.
Fig. 1 (a) TEM image of the proposed GNRs/PVA composite film and (b) Optical absorption spectrum of the proposed GNRs/PVA composite film together with that of a PVA film.
Figure 1(b) shows the optical absorption of the GNRs/PVA composite that were placed on a glass substrate together with that of the PVA film. A broadband absorption can be clearly observed over a wide spectral range from 400 to 2200 nm.
In order to implement an all-fiberized saturable absorber based on the prepared GNRs/PVA composite, we chose to use a side-polished fiber platform, on which the composite was to be deposited. The side-polished fiber was prepared by mechanically polishing one side of the standard single mode fiber fixed onto a V-grooved quartz block. The distance between the core and the upper, polished side of the fiber was set to ~10 μm. The insertion loss and polarization-dependent loss (PDL) of the prepared side-polished fiber were measured to be ~0.5 dB and ~0.08 dB, respectively. The GNRs/PVA composite solution was then deposited on the flat side of the side-polished fiber, and the fiber was then dried at room temperature. Figure 2 shows a schematic and a photograph of the GNRs/PVA composite-deposited side-polished fiber. After deposition, the insertion loss and the PDL of the side-polished fiber were observed to substantially increase to up to ~1.7 dB and ~7 dB, respectively. The damage threshold of the prepared GNRs/PVA-based SA was measured with a 1550 nm continuous wave (CW) amplified laser beam of 1 W power into it, but no damage of the device was observed within the optical power level. The damage threshold must be larger than 1 W. However, it was impossible to measure the precise value due to limited availability of a high power laser in our laboratory.
Fig. 2 (a) Cross section and side-view schematics of the proposed GNRs/PVA-deposited side-polished fiber and (b) a real photograph.
Next, we measured the nonlinear transmission of the proposed GNRs/PVA-deposited side-polished fiber as a function of the incident peak power by using the all-fiberized method demonstrated in [48]. A mode-locked fiber laser with a pulse width of ~600 fs at ~1562 nm was used for the measurement. The average output power of the mode-locked laser was varied from 0.4 mW to 3.8 mW at a repetition rate of 23.7 MHz. We measured the nonlinear transmission according to both transverse electric (TE) and transverse magnetic (TM) mode input beams due to non-negligible PDL of the GNRs/PVA-deposited side-polished fiber. The measured results are shown in Fig. 3. The measured data was fitted by following the well-known fitting equation [21]
where T is the transmission, ΔT is the modulation depth, I is the incident pulse power, Isat is the saturation power, and Tns is the nonsaturable loss.Fig. 3 Measured nonlinear transmission of the GNRs/PVA saturable absorber for the (a) TE- and (b) TM-mode input beams.
The modulation depth and the saturation power for the TE input were estimated to be ~7.5% and ~35 W, respectively, whereas the values for the TM input were ~1.2% and ~98 W, each. Although the GNRs/PVA-deposited side-polished fiber had a relatively lower modulation depth than that exhibited by other groups [33–39], ~7.5% was nevertheless high enough to produce stable Q-switched pulses from the all-fiberized cavity used in this experiment. Note that our prepared GNRs/PVA-based saturable absorber had ~2 mm interaction length between the oscillating beam and deposited GNRs/PVA film, which is much longer than that of saturable absorber based on a thin nanoparticle film sandwiched between two fiber ferrules. We think that such a low saturation power level of ~35 W can be attributed to the long interaction length. Note that the saturation power level of ~35 W is almost comparable to the value of ~29 W in [49], in which the saturable absorber was based on a side-polished fiber platform like ours. Also, it is comparable to the level (~44 W) of our recently demonstrated Bi2Te3-based saturable absorber based on a side-polished fiber platform [16].
3. Experimental configuration of our all-fiberized Q-switched laser
The schematic for our experiment with the passively Q-switched fiber laser is shown Fig. 4(a). For this experiment, a simple Fabry-Pérot cavity was adopted with two mirrors defined by a fiber Bragg grating (FBG) and a cleaved optical fiber facet (4% reflection between air and silica). A 3 m-long EDF (Liekki Er20-4/125, nLight Corporation) was used as gain medium, and its peak absorption at 1530 nm was measured to be ~20 dB/m. The EDF was pumped with a 980-nm laser diode (LD) that was fed into the cavity through a 980/1550 nm wavelength division multiplexer (WDM). A polarization controller (PC) was used to optimize the Q-switching operation of the cavity, and the FBG that was used had a reflectivity of ~99% at ~1549 nm and a 3 dB bandwidth of ~0.4 nm, as shown Fig. 4(b). The GNRs/PVA-deposited side-polished fiber was inserted between the PC and the FBG, and the laser output was extracted from the cleaved fiber facet. All of the components within the cavity were fusion-spliced with a total cavity length of ~48 m.
Fig. 4 (a) Schematic of the laser used in the experiment. (b) Measured transmission spectrum of the FBG used.
An optical spectral analyzer, an oscilloscope, a radio frequency (RF) analyzer, and an optical powermeter were used to monitor the output characteristics of the laser. As we increased the pump power, the laser began to operate in a continuous-wave (CW) mode and then transitioned into a Q-switching mode at a pump power of ~66 mW. The maximum pump power launched into the cavity was ~229 mW. No mode-locking phenomenon was observed despite the adjustment of pump power and PC due to both the narrow FBG bandwidth of only ~0.3 nm and the high photon loss within the cavity. Note that the Q-switching phenomenon always occurred at pump powers larger than ~66 mW without a PC inserted into the cavity. We used the PC within the cavity to optimize the evanescent field interaction between the oscillating beam and the GNRs/PVA film as well as to adjust the cavity propagation loss to a proper level since the GNRs/PVA-deposited side-polished fiber exhibited a significant PDL of ~7 dB.
4. Characterization of the laser
Figure 5(a) shows the oscilloscope traces of the output pulses at various pump powers. A close-up of a single pulse at the maximum pump power of ~229 mW is shown in Fig. 5(b). The observed pulse duration was of ~7.1 μs at ~17.1 kHz.
Fig. 5 Oscilloscope trace of the Q-switched output pulses at various pump powers. (b) Close-up view of an output pulse at a pump power of ~229 mW.
Figure 6(a) shows the optical spectrum measured for the Q-switched output pulses at the maximum pump power of ~229 mW. The center wavelength was ~1549.8 nm, and the 3-dB and 30-dB bandwidth were measured at ~0.04 and ~0.4 nm, respectively. The electrical spectrum of the output pulses was measured with a resolution bandwidth (RBW) of 10 Hz, as shown in Fig. 6(b). The signal-to-noise ratio was measured to be ~49 dB, which indicates that the all-fiberized laser produced stable Q-switched pulses.
Fig. 6 Measured (a) optical spectrum and (b) electrical spectrum of the output pulses at a pump power of ~229 mW.
As we increased the pump power, we then measured the variation in the characteristics of the output pulse, such as the repetition rate, pulse width, average optical power, and single pulse energy, and the results are summarized in Fig. 7. Figure 7(a) shows the repetition rate and the pulse width of the Q-switched pulses against the applied pump power. The repetition rate of the Q-switched pulses increased almost linearly from ~7.8 to ~17.1 kHz, as is commonly observed in passively Q-switched lasers.
Fig. 7 Measured (a) pulse width and repetition rate. Measured (b) average output power and single-pulse energy as a function of a pump power.
The pulse width decreased from ~22.8 to 7.1 µs with the increase in pump power due to a strong pump-induced gain compression effect, as reported in [50]. Both the average output power and the pulse energy of the laser increased as the pump power were increased, as shown in Fig. 7(b). The maximum average output power at a pump power of ~229 mW was ~44.1 mW, and the pump conversion efficiency was estimated at ~19.2%. The maximum pulse energy was measured to be ~2.56 μJ at pump power of ~229 mW, which to date is the largest single-pulse energy reported for saturable absorbers based on gold nanoparticles [33–36]. Due to the limitation of the maximum pump power available, the experiment was conducted within the pump power range of 0 ~229 mW. We think that further increase of the pump power beyond 229 mW would result in increase of both the average output power and pulse energy.
Next, we investigated the impact that the variation in the cavity length had on the output pulse characteristics under a fixed pump power of ~229 mW, and the the results are summarized in Fig. 8. We shortened the cavity length from 48 to 8 m in 8-m steps. As the cavity length decreased, the pulse width of the output pulses decreased from ~7.1 μs to 3.2 μs while the repetition rate increased from ~17.1 kHz to ~37.1 kHz, as expected. The decrease in the pulse width can be attributed to a shortening of the photon life time due to the decrease in the cavity length [51]. Figure 8(b) also shows the variation in the output pulse energy as a function of the cavity length. The pulse energy decreased from 2.56 μJ to 1.18 μJ as the cavity length decreased. The maximum cavity length, which can produce stable Q-switched pulses, was found to be ~48 m. We found that a cavity length longer than ~48 m induced a temporal instability in the output pulses.
Fig. 8 Measured (a) pulse width, repetition rate and (b) single- pulse energy as a function of cavity length at a fixed pump power of ~229 mW.
The output performance of the passively Q-switched fiber laser proposed in this study was compared to that of other passively Q-switched 1.5 μm fiber lasers incorporating gold nanoparticle-based saturable absorbers that had been previously proposed. It was also compared to that of fiber lasers using CNTs [52], graphene [53], Bi2Se2 [15], and Bi2Te3 [21], and the results are summarized in Table 1. It is obvious that our fiber laser had the largest single pulse energy and the highest pump conversion efficiency among the eight lasers tested, even if the pulse width was the largest and modulation depth of our saturable absorber was relatively small.
Table 1. Output performance comparison of the proposed fiber laser against those with other nanomaterials
5. Conclusion
We have experimentally demonstrated a high-energy, pulsed, 1.55 μm all-fiberized laser using a GNRs/PVA-deposited side-polished fiber as a saturable absorber. Using the GNRs/PVA composite saturable absorber was introduced in the erbium-doped fiber Fabry-Pérot cavity, and stable Q-switched pulses were readily obtained with a maximum average ouput power of ~44.1 mW and a single pulse energy of ~2.56 μJ.
We believe that this experimental demonstration indicates that the GNRs/PVA composite has potential as a highly efficient saturable absorption material for implementation in high- energy Q-switched lasers. Mode-locking operation could be achieved using a completely different cavity configuration with a modified side-polished fiber structure. Further mode-locking experiments are thus currently under progress and the results will be presented elsewhere.
Acknowledgments
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1A2A2A11000907), Republic of Korea.
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