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Abstract
We demonstrated an ultrafast erbium-doped fiber laser (EDFL) based on ferroferric-oxide (Fe3O4) nanoparticles as a saturable absorber (SA). The investigated SA was based on magnetic fluid deposited on the end face of a fiber ferrule connector. When the SA was inserted into an EDFL cavity, a stable 2.93 ps mode-locked pulse can be achieved by adjusting the intra-cavity polarization controller. The pulse had a central wavelength of 1572.39 nm and a 3 dB bandwidth of 1.39 nm. We also obtained Q-switched mode-locked pulses at 1593.4 nm. The repetition frequency and the temporal width of the Q-switched pulse envelope varied with the pump power. When the pump power reached 225 mW, the maximum average output power and the pulse envelope energy were up to 4.51 mW and 235.5 nJ. To the best of our knowledge, this is the first time that mode-locked and Q-switched mode-locked pulses have been obtained in a fiber laser based on Fe3O4 nanoparticles.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers have attracted much attention due to the characteristics of compact and alignment-free structure, high beam quality and cost-effective design, which are widely used in laser material micro-processing, optical communications, biochemistry, and metrology. Incorporating saturable absorber (SA) materials in fiber laser cavities is one of the most important methods to achieve Q-switched or mode-locked operation for generating ultrafast pulses [1–27]. Therefore, it is important to develop new SA materials. Semiconductor saturation absorber mirrors (SESAMs) [1], graphene [2–4] and carbon nanotubes [5–9] have been widely used in mode-locked fiber lasers and Q-switched fiber lasers. In the past decade, some new two-dimensional (2D) materials, such as black phosphorus [10,11], topological insulators [12–14], and transition-metal dichalcogenides [15,16], have also been well-known as SAs, which are usually integrated into optical fibers, such as tapered fibers [8], D-shaped fibers [9], hollow-core fibers [17], or sandwiched between two fiber ferrule connectors [18] to realize ultrafast pulse output. Recently, kinds of zero-dimensional transition metal oxide nanomaterials, such as TiO2, ZnO, Al2O3, ITO, MnO2, NiO and Fe3O4, have also been demonstrated nonlinear saturable absorption properties [19–31]. Similar to the carbon nanotubes and 2D materials, the zero-dimensional nanomaterials also possess high third-order optical nonlinearity, ultrafast response time and broadband absorption. But due to their smaller lateral sizes and more regular shapes, the transition metal oxide nanomaterials based SAs can be prepared more uniformly, which makes it possible to tailor their properties more flexibly to obtain desired pulse lasers with high stability [29–31].
Among them, Fe3O4 is a typical magnetic material with various magneto-optical properties including field-dependent optical transmission, birefringence effect, and magnetic field sensitive refractive index [32–35]. Due to the remarkable magneto-optical properties, magnetic fluid with Fe3O4 nanoparticles dispersed is widely used in magneto-optical devices such as optical modulators or optical switches, magnetic field sensors, microwave devices, and biomedical applications [36–41]. Furthermore, Fe3O4 nanoparticles possess nonlinear photonic properties including saturable absorption, two-photon absorption, nonlinear scattering and optical limiting [42–44]. The saturable absorption effect of this material is mainly attributed to electron intra-band transitions and Pauli blocking, which means that under a strong optical field, the final states are filled and further absorption is blocked, thus achieving saturable absorption [45]. Besides, Fe3O4 nanomaterials also exhibit high nonlinear optical absorption, large third-order optical nonlinearity and ultrafast response time [46,47]. More importantly, different from other types of SAs, the band gap of Fe3O4 nanoparticles can be adjusted by controlling the diameter of the nanoparticles during preparation [48,49], which indicates that they can exhibit light absorption in broadband wavelength range from visible to mid-infrared. In 2016, Bai et al., realized a passive Q-switched erbium-doped fiber laser based on Fe3O4 nanoparticles for the first time [26]. In 2017, Mao et al., studied the nonlinear saturable absorption characteristics of Fe3O4 nanoparticles mixed with polyvinyl alcohol solution and polyimide solution and achieved Q-switched pulse output in an erbium-doped fiber laser [27]. Subsequently, Chen et al., built single-wavelength and multi-wavelength Q-switched fiber lasers based on Fe3O4 nanoparticles [28]. In 2019, Li et al., synthesized Fe3O4 nanoparticles via chemical co-precipitation and achieved a Q-switched Yb-doped fiber laser based on Fe3O4-polyvinyl alcohol film SA [30].
In this paper, we propose to use Fe3O4 nanoparticles as the SA by simply depositing magnetic fluid on the end face of a fiber ferrule connector. We achieved mode-locked and Q-switched mode-locked operations in an erbium-doped fiber laser based on Fe3O4 nanoparticles for the first time. Under the mode-locked regime, a 2.93 ps mode-locked pulse was obtained with good stability. By reconnecting the fiber cavity and adjusting the intra-cavity polarization controller, the Q-switched mode-locked operation was also achieved at 1593.4 nm, in which the amplitude of stable mode-locked pulses was modulated by a pulse envelope with typical Q-switched characteristics.
2. Fe3O4 nanoparticles SA preparation and characterization
The Fe3O4 nanoparticles we used in the experiment were obtained from a water-based magnetic fluid (Ferrotec EMG 700, 5.8% vol.). The magnetic fluid was a kind of black-brown fluid with Fe3O4 nanoparticles dispersed in water carrier, in which anionic surfactant was used to prevent from agglomeration. The average diameter of the Fe3O4 nanoparticles was ∼10 nm. Figure 1 presents the detailed characterization of the Fe3O4 nanoparticles we used. The scanning electron microscope (SEM) image of the Fe3O4 nanoparticles is shown in Fig. 1(a). It can be seen that the lateral sizes of the nanoparticles, varying from about 80 nm to 250 nm, are much larger than the average diameter of the original Fe3O4 nanoparticles due to the random agglomeration of nanoparticles. Figure 1(b) shows the energy dispersive spectroscopy (EDS) spectrum of the magnetic fluid. The intensity peaks of Fe and O can be clearly observed, which match well with Fe3O4 nanoparticles. Other elements shown in the spectrum are introduced by the measuring devices such as the substrates used in the experiment.
Fig. 1. Characterization of the Fe3O4 nanoparticles. (a) The SEM image of the Fe3O4 nanoparticles; (b) The EDS spectrum of the Fe3O4 nanoparticles; (c) Photograph of the Fe3O4 nanoparticles deposited fiber ferrule connector.
To fabricate the Fe3O4-nanoparticles-based SA, we prepared a clean optical fiber ferrule connector and then placed the end face as close as possible to a small drop of the magnetic fluid on a silica substrate. Due to the surface tension of fluid, the magnetic fluid was automatically transferred to the end face of the fiber ferrule connector. During the preparation process, we optimized the Fe3O4 nanoparticles film for mode-locking by changing the drop size, the transferring time and the immersion depth of the fiber connector end face in the magnetic fluid drop. After air drying for 20 minutes at room temperature, a thin Fe3O4-nanoparticles-film was formed on the end face of the fiber ferrule, as shown in Fig. 1(c). Then the fiber ferrule connector with deposited Fe3O4 nanoparticles layer was connected with another clean fiber connector through a fiber adapter, and a Fe3O4-nanoparticles-based SA was finally obtained.
We also investigated the optical absorption properties of this Fe3O4-nanoparticles-based SA, which are shown in Fig. 2. The linear transmission spectrum of the SA shown in Fig. 2(a) was measured using an 800-1600 nm low-intensity broadband light source. It is shown that the transmission of this material rises with the increase of the wavelength and reaches about 65% at 1572 nm, which is adequate for materials achieving mode-locking. The nonlinear optical absorption characteristic of the SA was further studied by using a balanced twin-detector measurement system [50]. The probe light source was a homemade passively mode-locked fiber laser with a pulse duration of ∼1 ps at 1570 nm and a fundamental frequency of 13.4 MHz. The nonlinear transmission curve of the SA is illustrated in Fig. 2(b). The curve in Fig. 2(b) is fitted by the following formula [51],
where T is the transmission, ${\alpha _0}$ is the modulation depth, ${\alpha _{ns}}$ is the non-saturable loss, I is the incident peak intensity and ${I_{sat}}$ is the saturation intensity. It can be observed from Fig. 2(b) that the Fe3O4 nanoparticles SA has typical saturable absorption characteristics and the modulation depth is about 7%, which is sufficient for mode-locking or Q-switching. The saturation intensity and the non-saturable loss are measured to be 0.141 GW/cm2 and 26.68%, respectively.Fig. 2. Optical absorption properties of the Fe3O4-nanoparticles-based SA. (a) The transmission spectrum and (b) the nonlinear transmission curve of the Fe3O4 nanoparticles SA.
3. Experimental setup
To further investigate the performance of the Fe3O4 nanoparticles SA, an ultrafast erbium-doped fiber laser was constructed as shown in Fig. 3. A 980 nm laser diode (LD) with a maximum pump power of 250 mW pumped the gain fiber through a 980/1550 wavelength-division-multiplexer (WDM). A piece of 5 m erbium-doped fiber (EDF) with 35 dB/m absorption coefficient at 1531 nm was served as the gain medium. A polarization controller (PC) was employed to adjust the polarization state of the light in the cavity. A polarization-independent isolator (PI-ISO) was inserted to ensure the unidirectional light propagation. The light was extracted from the cavity for monitoring through an output coupler (OC) with an output ratio of 20%. The total length of the cavity was ∼73 m, consisted of 5 m EDF, 60 m single-mode fiber (SMF) and ∼8 m SMF pigtail fiber. The net dispersion in the laser cavity was about -1.641 ps2. The output spectrum, the frequency spectrum and the pulse profile were measured by an optical spectrum analyzer (OSA), a radio-frequency (RF) spectrum analyzer and an autocorrelator, respectively. A 500 MHz oscilloscope together with a 5 GHz photo-detector (PD) was used to monitor the pulse train from the laser.
4. Experimental results and discussion
4.1 Mode-locked operation
Mode-locked operation is related to various factors in the fiber cavity, such as the nonlinear effect of the fiber, dispersion, gain and loss in the cavity, and the optical properties of the SA [52]. Besides, by adjusting the intra-cavity PC, a disturbance will be introduced to the fiber cavity, which induces noise fluctuation spikes and promotes the establishment of mode-locked operation [53]. Moreover, squeezing and twisting the fiber by rotating the PC can also change the cavity birefringence and make it in a proper state to achieve stable mode-locked pulse output. The pump power also should be high enough to provide adequate gain and nonlinearities in the cavity. Under a high pump power level, weak longitudinal modes are absorbed by the SA material, while modes with high intensity can pass through the SA to stimulate other modes with phase locking, resulting in the formation of passive mode-locked operation [54]. In our experiment, we adjusted the pump power and rotated the PC without Fe3O4 nanoparticles SA in the cavity; there was no Q-switching or mode-locking achieved. By inserting the SA into the cavity, the mode-locked operation was realized when the pump power increased to 100 mW with the PC being properly adjusted. The characteristics of the mode-locked fiber laser output are shown in Fig. 4. Figure 4(a) presents the optical spectra of the laser at different pump powers. The central wavelengths are located at about 1572.39 nm with a 3 dB bandwidth of 1.39 nm. At the low pump power, there are only Kelly sidebands on both sides of the spectrum, which confirms that the mode-locked fiber laser was working in the soliton regime [55]. As the pump power increases, a new set of sidebands appears around the Kelly sidebands, which may result from the soliton modulation instability in fiber lasers [56]. Figure 4(b) shows the oscilloscope trace of the pulse train. It can be seen that the pulse interval is about 356 ns, which is consistent with the cavity length of 73 m. The autocorrelation trace of the pulse together with a sech2 fitting curve is shown in Fig. 4(c). The pulse duration is measured to be 2.93 ps. The corresponding time bandwidth product is about 0.494, which is larger than the theoretical transform limited value of 0.315, indicating that the output pulse was chirped slightly. Figure 4(d) shows the RF spectrum of the laser measured in a 1.25 MHz span with a resolution of 3 kHz. The RF spectrum recorded in a wide span of 100 MHz is presented in the inset of Fig. 4(d). The fundamental repetition rate is 2.806 MHz matched well with the cavity length. The signal-to-noise ratio (SNR) reaches 57 dB, indicating good stability of the mode-locked operation.
Fig. 4. Characteristics of the mode-locked fiber laser. (a) Optical spectra at different pump powers; (b) Oscilloscope trace of the pulse train; (c) Autocorrelation trace; (d) RF spectrum; (e) Output power versus pump power; (f) The long-time stability.
The laser output power rises monotonically as the pump power increases, as shown in Fig. 4(e). The laser conversion efficiency is ∼2.67%. The maximum average output power is obtained to be 6.78 mW at the pump power of 225 mW, corresponding to the maximum pulse energy of 2.41 nJ. The long-time output spectrum is demonstrated in Fig. 4(f). During the two-hour lasting time, the mode-locked operation keeps stable and the output spectrum remains almost unchanged.
4.2 Q-switched mode-locked operation
Besides the mode-locked operation, the Q-switched mode-locked operation can also be realized by reconnecting the fiber laser components and adjusting the PC in the cavity. As the pump power was up to 105 mW, the Q-switched mode-locked pulses were obtained. Rebuilding the fiber cavity and adjusting the intra-cavity PC can lead to large insertion loss and complex cavity loss variations, which imposed periodic attenuation on the mode-locked pulse train. Time for accumulating cavity energy was needed to saturate the absorber [57–59]. In this case, the Q-switched mode-locked operation was obtained. Q-switched mode-locked operation can also be interpreted as a transition state of mode-locked and Q-switched regime. Its main characteristic is that the amplitude of the stable mode-locked pulse train is modulated by a periodic Q-switched envelope. Due to the higher peak power compared to continuous mode-locked lasers at the same average output power level [59–61], Q-switched mode-locked lasers are attractive in some applications, such as precision structure fabrication, nonlinear frequency conversion and medical equipment [62,63]. Figure 5 shows the characteristics of the Q-switched mode-locked state. With the increase of the pump power, the time required for accumulating enough cavity energy was shortened, and thus the whole process of energy accumulation and release became faster, which was manifested as the increase of the pulse envelope repetition frequency. The oscilloscope traces of the pulse trains at different pump powers are shown in Fig. 5(a). It can be seen that as the pump power rises, the repetition frequency of the pulse envelope gradually increases while the pulse envelope width decreases, which is the typical characteristic of Q-switched operation. Figure 5(b) and the inset illustrate the zoom-in detail images of a single pulse envelope at the pump power of 150 mW. We can observe that the amplitude of the mode-locked pulses with the fundamental repetition rate of 2.81 MHz is modulated, and the full width at half maximum (FWHM) of the Gaussian-like pulse envelope is 16.43 µs. The optical spectra are shown in Fig. 5(c). The central wavelengths are at around 1593.4 nm and the 3 dB bandwidth grows from 0.85 nm to 2.34 nm as the pump power increases.
Fig. 5. Properties of the Q-switched mode-locked fiber laser. (a) The pulse trains at different pump powers; (b) Details of a single Q-switched mode-locked pulse envelope; (c) Optical spectra at different pump powers.
To further investigate the Q-switched mode-locked operation, we also studied the RF spectra measured in different spans. The Q-switched repetition frequency is 19.1 kHz under the pump power of 225 mW, as shown in Fig. 6(a), and the SNR is up to 40 dB. By increasing the frequency span to 100 MHz, the fundamental and the harmonic mode-locked frequencies are clearly shown in the inset of Fig. 6(a), indicating that the laser was mode locked well. Figure 6(b) shows the RF spectrum located at the fundamental mode-locked frequency of 2.81 MHz. The SNR of the center peak is about 57 dB. One can also clearly see multiple frequency sidebands with a 19.1 kHz interval around the peak frequency of the mode-locked pulse. It confirms that the amplitude of the 2.81 MHz mode-locked pulse train is modulated at a frequency of 19.1 kHz [61], which is consistent with the experimental results shown in Fig. 5(a).
Fig. 6. RF spectra under the Q-switched mode-locked operation. (a) RF spectrum under the pump power of 225 mW. Inset: RF spectrum in a span of 100 MHz; (b) Details of the RF spectrum around the fundamental mode-locked frequency.
We also studied the variation of the Q-switched mode-locked pulses when the pump power increased. Figure 7(a) shows the repetition frequency and the temporal width of the pulse envelope as a function of the pump power. It can be seen that when the pump power rises from 105 mW to 225 mW, the repetition frequency increases from 12.5 kHz to 19.1 kHz, and the pulse envelope width decreases from 20.4 µs to 14.3 µs. The average output power and the calculated pulse envelope energy with different pump powers are shown in Fig. 7(b). We can observe that the maximum output power is up to 4.51 mW and the pulse envelope energy is about 235.5 nJ at the pump power of 225 mW. The energy conversion efficiency is calculated to be ∼1.25%.
Fig. 7. Variation of the Q-switched mode-locked pulses versus pump power. (a) The repetition frequency and the temporal width of the pulse envelope versus pump power; (b) The average output power and the pulse envelope energy versus pump power.
5. Conclusion
In conclusion, we have demonstrated the mode-locked and Q-switched mode-locked erbium-doped fiber laser based on Fe3O4 nanoparticles SA for the first time. The SA we used was based on magnetic fluid deposited on the end face of the fiber ferrule connector. The modulation depth of the SA was about 7%. The mode-locked pulse duration was 2.93 ps with the fundamental repetition rate of 2.806 MHz. The central wavelength and the 3 dB bandwidth of the spectrum were 1572.39 nm and 1.39 nm, respectively. The mode-locked pulse output also showed good stability with the SNR of 57 dB. By reconnecting the fiber laser cavity and adjusting the intra-cavity PC, we also obtained the Q-switched mode-locked pulses at 1593.4 nm. It is found that the amplitude of the mode-locked pulses with the repetition rate of 2.81 MHz was modulated by a Q-switched pulse envelope. As the pump power increased from 105 mW to 225 mW, the repetition frequency of the pulse envelope rose from 12.5 kHz to 19.1 kHz, and the temporal width of the pulse envelope decreased from 20.4 µs to 14.3 µs. This experiment demonstrated the promising potential of Fe3O4 nanoparticles as a kind of SA for mode-locked fiber lasers and other nonlinear optical modulators devices. Moreover, the magnetic sensitive properties of this material may make it more flexible to achieve different mode-locked fiber lasers output in future researches.
Funding
Key Technologies Research and Development Program (2018YFE0117400); National Natural Science Foundation of China (61775074).
Disclosures
The authors declare no conflicts of interest.
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