Femtosecond mode-locking of an ytterbium-doped fiber laser using self-assembled gold nanorods

The forerunners of ultrafast lasers, passively mode-locked fiber lasers that can produce femtosecond pulses, are widely used in an array of optical and biomedical applications eg, processing of materials, supercontinuum generation, nonlinear optics, laser surgery/imaging, and spectroscopy [1–4]. Passively mode-locking of fiber lasers is realized by using nonlinear optical phenomena such as nonlinear polarization rotation (NPR) and saturable absorption. Self-starting, reliable mode-locking features make saturable absorption superior to NPR in the design of lasers.

Until now, III–V compound semiconductors have been commonly used as base materials for achieving saturable absorption phenomenon. The initiation and maintenance of relevant mode-locking processes were facilitated by well, known, commercially available III–V semiconductor-based saturable absorber mirrors (SESAMs) [5–7]. However, fabrication of SESAMs is complicated and requires expensive facilities. In addition, the operating bandwidths of SESAMs are limited to several tens of nanometers. In order to solve these issues, researchers intensified the search over the last decade for alternative nonlinear optical materials. As a result, carbon-based materials such as carbon nanotubes (CNTs) and graphene were found to be efficient saturable absorption materials used in pulsed fiber lasers [8–19]. Other materials such as topological insulators (TIs) [20–29], gold nanoparticles [30–37], transition metal dichalcogenides (TMDs) [38–47], and black phosphorus (BP) [48–52] were also identified as saturable absorption materials that could operate at various wavelengths.

One important issue in the field of mode-locked fiber lasers is the implementation of stable femtosecond fiber lasers at 1 µm wavelength region. Since a silica fiber with a step-index profile has normal dispersion below a 1.3 µm wavelength, the use of a dispersion-compensating method within the ytterbium (Yb)-doped fiber-based 1 µm laser cavity is essential for the direct generation of femtosecond soliton pulses from the cavity [53]. Other SAs based on CNTs, graphene, TIs, gold nanoparticles, TMDs, or BP have proven useful for mode-locking of fiber lasers at various wavelengths, but those mode-locked fiber lasers were mainly used in the 1.5 µm wavelength region [13–16, 19–22, 24, 26, 32, 38, 39, 45–51]. In recent separate studies, researchers have constructed fiber lasers that can operate near 2 µm [12, 23, 36, 37, 41, 44]. It is noteworthy that only a limited number of investigations have thus far been conducted into femtosecond pulse generation from 1 µm fiber lasers and all of the published reports have been based on CNT-SAs [10–12]. As a matter of fact, there have been quite a few reports on the use of a SA based on other nonlinear optical materials such as graphene, TIs, gold nanoparticles, TMDs, or BP for passive mode-locking of an Yb-doped fiber laser; however, those demonstrations concerned the generation of dissipative soliton pulses, the temporal widths of which ranged from tens of picoseconds to a few nanoseconds [17, 25, 27, 28, 31, 35, 40, 43, 52]. To the best of the authors' knowledge, there has been no report on the direct generation of femtosecond soliton pulses from an Yb-doped fiber-based 1 µm laser cavity using a SA based on graphene, TIs, gold nanoparticles, TMDs, or BP.

In this paper, we demonstrate the use of a SA based on gold nanorods (GNRs) for the generation of femtosecond optical pulses from an Yb-doped fiber-based 1 µm laser ring cavity. GNRs were reported to have efficient, nonlinear saturable absorption properties with rapid recovery times [54–56]. They have already been used as base-saturable absorption materials for the generation of Q-switched or mode-locked pulses in the 1 µm–2 µm wavelength ranges [31, 32, 36]. However, there has been no report on the femtosecond mode-locking capabilities of these materials at 1 µm wavelengths. In this study, a GNRs-based SA was prepared by our recently-proposed, end-to-end self-assembly technique [37], i.e. through hot-air induced rapid drying of GNRs in a deionized (DI) water, that are placed on top of the flat surface of a side-polished fiber, in order to enhance optical absorption in the 1.06 µm wavelength region. Using the prepared SA within a dispersion-managed Yb-doped fiber ring cavity, we showed that we could readily obtain femtosecond soliton pulses. For each pulse, the optical spectrum could readily be fitted with a hyperbolic-secant function and the temporal bandwidth shape of the output pulses resembled a Gaussian curve rather than a hyperbolic secant.

In [31] Kang et al demonstrated the generation of dissipative soliton pulses from an all-normal dispersion fiber ring cavity. The temporal width of the output pulses was ~440 ps. No dispersion compensation scheme was employed. This means that the ultrafast mode-locking capability of GNRs-based saturable absorbers (SAs) in the 1 µm wavelength region was not fully explored in [31] even if the potential of GNRs as a base material for the implementation of a 1 µm mode-locker was investigated. Note that our work was focused on the direct generation of femtosecond soliton pulses from a dispersion managed cavity to verify the ultrafast mode-locking capability of a GNRs-based SA in the 1 µm wavelength region. It should also be noticed that dispersion-managed soliton and dissipative soliton are completely different in terms of their underpinning physical mechanisms [57]. Furthermore, our work also used our proposed GNR preparation method different from [31]. Note that our prepared GNRs were not mixed with sodium carboxymethylcellurose (NaCMC) but were processed in a low-cost DI water solution

We employed our recently proposed, end-to-end self-assembly technique [37], in order to produce a GNRs-based SA that has a strong absorption in the 1.06 µm wavelength region. We used a side-polished fiber platform for the implementation of an all-fiberized SA. This process uses evanescent field interaction between guided modes in the optical fiber and the deposited saturable absorption material. In this experiment, we used commercially available GNRs (NR-10-850, NanoSeedz) with an average aspect ratio of 5.1. The longitudinal plasmonic excitation band peak of the GNRs was 850 nm before the self-assembly process. As reported in [37] a GNRs-based SA was prepared using the following procedures. As shown in figure 1, a small amount of GNRs/DI-water suspension with an aspect ratio of 5.1 was dropped onto the flat surface of a prepared side-polished fiber that had initial insertion- and polarization-dependent losses of 0.3 dB and 0.1 dB, respectively, at 1060 nm. The physical distance between the flat surface and the edge of the prepared side-polished fiber was 2 µm.

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For the linear absorption measurement a spectrophotometer (UV-3600 Plus, Shimadzu) was used. The spectral dip at ~830 nm can be attributed to the change of the built-in light source of the spectrophotometer at the wavelength. The light source change induced a small amount of discontinuity in the measured spectrum. As shown in figure 2(a), the longitudinal plasmonic excitation band peak of the starting material was located at 850 nm. Next, the deposited GNRs/DI-water suspension was dried for 3 min by using hot temperature air flow (100 °C). As shown in figure 2(b) GNRs were observed to self-assemble with rough end-to-end attachments. Compared to the initial GNRs/DI-water suspension, it is clearly evident from figure 2(a), that the longitudinal plasmonic excitation band peak of the self-assembled GNRs shifted to 1040 nm and that, additionally, the absorption bandwidth substantially broadened to provide enhanced optical absorption at a wavelength of 1060 nm. The self-assembled GNRs are believed to have an average aspect ratio of 6.1, according to the previous report by Ye et al [58]. The linear optical absorption measurements were conducted with GNRs/DI-water suspension dropped on slide glasses and then dried. We used a spectrophotometer (UV-3600 Plus, Shimadzu) for absorbance measurements. An absorbance baseline of control glass slides without GNRs served as a background reference. At a wavelength of 1060 nm, the insertion- and polarization-dependent losses of the prepared GNRs-based SA were 3 dB and 3.7 dB, respectively.

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We conducted the nonlinear absorption measurement at a wavelength of 1.56 µm since the measurement setup for the nonlinear absorption curve in the 1 µm wavelength region was not available in our laboratory during this experiment. Note that our prepared self-assembled GNRs had substantial linear absorption at 1.56 µm unlike the GNRs/DI-water suspension, for which no linear absorption was observed at wavelengths beyond at 1.35 µm, as shown in figure 2(a). For this particular measurement we prepared a SA sample using a fiber ferrule-based sandwich structure. As shown in figure 3, the modulation depth and saturation intensity were estimated as ~1.2% and 6.4 MW cm⁻², respectively. The modulation depth of the SA using a side-polished fiber in the 1 µm wavelength region must be larger than 1.2% due to much longer length of mutual interaction between GNRs and the oscillating beam.

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The laser configuration used in this study is shown in figure 4. A 0.7 m-long Yb-doped fiber (SM-YSF-HI, Nufern) with a peak absorption of 250 dB m⁻¹ at 975 nm was used as a gain medium. The Yb-doped fiber was pumped with a 976 nm pump laser diode via a 976/1060 nm wavelength division multiplexer (WDM) coupler. A polarization-independent isolator was used for unidirectional beam propagation, and a polarization controller (PC) was used to optimize the polarization state of the oscillating beam within the cavity. The prepared GNRs-based SA was located after the PC. A bulk-grating pair (1200 line mm⁻¹, Spectrogon) was inserted after the fiber collimator to compensate for the normal dispersion in the fiberized laser cavity. A 10:90 coupler was used, and the output pulses were extracted from the 10% output port. Output pulses were recorded using a 100 GS s⁻¹ high-speed real-time oscilloscope (DSA71604C, Tektronix) combined with a 15 GHz photodetector and an optical-spectrum analyzer.

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First, we checked the efficacy of the prepared, self-assembled GNRs-based SA for the generation of stable mode-locked pulses from the cavity without a bulk-grating pair. Dissipative soliton pulses were readily produced from the laser cavity at a pump power of 126 mW with a properly-adjusted PC. Figures 5(a) and (b) show the respective optical spectrum and oscilloscope trace of the output pulses, respectively. All of the components that were used in the cavity have normal dispersion properties. The optical spectrum of a rectangular shape, which is a typical feature of dissipative soliton pulses, is clearly shown in figure 5(a) and the center wavelength and 3-dB bandwidth were 1068.2 nm and 1.2 nm, respectively. The repetition rate of the output pulses was 18.69 MHz, and average output power was 4.1 mW at a pump power of 126 mW. The temporal width of the output pulses was ~460 ps, which was measured with the real time high-speed oscilloscope, as shown in the inset of figure 5(b). Such a broad pulse width is comparable to previously reported values using 1 µm dissipative soliton fiber lasers with other nonlinear optical materials serving as SAs, e.g. graphene, TIs, TMDs, or BP [17, 31, 35, 40, 43]. To confirm the stability of the output dissipative soliton pulses, we also measured the electrical spectrum of the output pulses. The signal-to-noise (SNR) of the output pulses was ~72 dB, as shown in figure 5(c). Figure 5(d) shows the wide-span view of the measured electrical spectrum over 200 MHz.

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Next, we inserted a bulk-grating pair within the cavity to directly generate femtosecond soliton mode-locked pulses under a dispersion-managed cavity condition. Stable soliton pulses were produced from the laser cavity at a pump power of 108 mW. The average power of the output pulse was 0.94 mW, and the corresponding pulse energy was 53 pJ. Figure 6(a) shows the measured output spectrum of the pulses together with a Sech()² fitting curve. The center wavelength and 3 dB bandwidth were 1063.9 nm and 1.5 nm, respectively. The output pulse repetition rate was 17.94 MHz, as shown in figure 6(b). As shown in figure 6(c), the electrical spectrum showed a SNR of 73 dB, indicating stable mode-locked pulse operation. The wide-span view over a 200 MHz range is also shown in figure 6(d) reconfirming the stable output pulses.

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An autocorrelation measurement was also conducted using a second harmonic generation-based autocorrelator in order to precisely measure the temporal width of output pulses. Figure 7 shows the measured autocorrelation trace with two fitting curves of Gaussian and Sech()². Unlike the case of the output optical spectrum, where a hyperbolic-secant curve is more compatible with the data, the fit of the Gaussian curve is more compatible than that of the hyperbolic-secant curve on the autocorrelation measurement. Even if a Gaussian curve was closer to the measured temporal shape of the output pulses, we used a hyperbolic-secant curve to estimate the output pulse width since its optical spectrum has evidently a hyperbolic-secant shape as shown in figure 6(a). The pulse width was estimated at 840 fs assuming that the output pulses have a hyperbolic-secant shape. Based on the 3 dB optical bandwidth of 1.5 nm and the output pulse width of 840 fs, the estimated time-bandwidth product is 0.334, indicating that the output pulses is slightly chirped.

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We have demonstrated femtosecond mode-locking of an Yb-doped fiber laser using a self-assembled GNRs-based SA. The SA was prepared by using end-to-end self-assembly of the GNRs through hot-air induced rapid drying of the GNRs/DI-water suspension deposited on the flat surface of a side-polished fiber, to enhance optical absorption in the 1.06 µm wavelength region. Using the prepared SA within a dispersion-managed, Yb-doped fiber ring cavity, a mode-locked soliton pulses with a temporal width of 840 fs was readily produced at a wavelength of 1063.9 nm. This is the first experimental demonstration of the effectiveness of a GNRs-based SA for the generation of femtosecond soliton pulses operating in the 1 µm wavelength region.

Our study confirms that our proposed self-assembly technique could be a simple and practical option to obtain a GNRs-based SA with an ultrabroad operating bandwidth for ultrafast laser applications.

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A11000907). This research was supported by the Ministry of Science, ICT and Future Planning (MSIP), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2017-2015-0-00385) supervised by the IITP (Institute for Information & communications Technology Promotion). This work was supported by the Korean Ministry of Trade, Industry and Energy within the project, 'Development of Process & Equipment Technology to Engrave Roll Molds with 10-micron Scale Line Width using a Pulse-width Tunable Ultrafast Laser (10048726)'.