Introduction
The mode-locked pulse fiber lasers have many important applications in nonlinear optics, fiber optic communication, optical time domain clock, laser processing, and optical measurement [1,2]. Popular methods to implement a mode-locked operation include active [3,4] and passive mode-locking [5–8]. The appearance of new optical saturable absorbers promotes the development of passively mode-locked lasers. Compared with traditional mode-locking methods, new materials expand research on pulsed lasers due to their characteristic wavelength independence, high heat dissipation, and high laser damage threshold [9–1]. Semiconductor saturable absorber mirrors [13], carbon nanotubes [14,15], graphene [16–18], bismuthene [19], topological insulators [20,21], transition metal sulfides [22–27], and black phosphorus [28] have been reported and demonstrated for the applications of mode-locking. When carbon nanotubes are used as saturable absorbers, their sizes extremely affect the absorption wavelength and increase the loss of unsaturable absorption [15]. Graphene has a weaker modulation of light for its weak absorption [17]. The tapper insulator cannot achieve higher power laser output as it has a low damage threshold [29,30]. Fe3O4 has obvious nonlinear sensitivity as a transition metal sulfide [31–33]. As its response time is within tens of picoseconds, it can be applied to many large nonlinear optical responses [34,35]. In previous reports, the band gap of Fe3O4 was observed, which was considered as semiconductor and its band gap energy changed with the particle size [36]. The imaginary part of the third-order nonlinear magnetic susceptibility enables nonlinear absorption, which allows the Fe3O4 nanoparticles to act as a saturable absorber of the fiber laser to generate pulses. Simultaneously, the real part of the third-order nonlinear polarizability of the nanoparticle achieves the optical Kerr effect. The nanoparticles of Fe3O4 can be used as nonlinear medium [34–36]. Mode-locked fiber lasers based on Fe3O4 nanoparticles have been demonstrated. Li et al. reported a 1558 nm mode-locked fiber laser based on Fe3O4 nanoparticles as a saturated absorber [37]. Bai et al. used ferroferric-oxide (Fe3O4) nanoparticles (FONPs) as the saturable absorber (SA) to realize the Q-switch operation in an erbium-doped fiber laser (EDFL), where the minimum pulse duration was approximately 3.2 µs [33]. Yang et al. reported the use of Fe3O4 nanoparticles for Q-switched a tunable mid-infrared (Mid-IR) Dy3+-doped ZBLAN fiber laser around 3 mm [38]. Recently, Liu et al. reported the realization of nanosecond pulses in an EDFL using FONPs as SA. Nevertheless, the SAs with Fe3O4 nanoparticles have been focused primarily on Q-switched fiber lasers [39].
In this work, we demonstrated the mode-locked pulse output of an EDFL based on Fe3O4. The Fe3O4 nanoparticles are deposited on the end plane of the fiber patch cord by liquid deposition. The light absorption between the transmitted light field and the Fe3O4 nanoparticles in the single-mode fiber (SMF) achieves the saturation absorption effect. The laser polarization state was adjusted each time and the laser pulse was self-started at a pump power of 70 mW, where the repetition frequency was 7.69 MHz. The simple experimental setup is beneficial to engineering and industrialization. The experimental results show that Fe3O4 can be used as a nonlinear optical modulation material like other optical materials, which has far-reaching influence on the design and development of pulse lasers.
Experimental results
The Fe3O4 nanoparticles have high third-order optical nonlinearity, a large nonlinear optical response, and fast response time. The recovery time for Fe3O4 nanoparticles was assessed at 18–30 ps [36] and classified as a semiconductor material (with a band gap of ~ 0.3 eV), which can be modulated by tuning the nanoparticle diameter [40]. To prepare Fe3O4 nanoparticles, we first took some Fe3O4 powders and acetone into a reagent bottle and placed them in an ultrasonic machine of 40 kHz (KQ-400 KDE) for 4 h. After one hour, the solution and the supernatant were taken out from the machine. Then, we get the SA based on the Fe3O4 nanoparticle membrane, which is a new 2D material. Figure 1 shows the characteristics of the material after magnifying the SA by scanning electron microscope (SEM). It shows the image of the magnification of 24000. We observe that Fe3O4 nanoparticles are nearly spherical and have a diameter of about 200 nm.
The nonlinear absorption characteristics of Fe3O4 used in the experiment were measured by the double-balanced detection method [5]. Figure 2 shows the measured transmission curve. We have also observed that the modulation depth (MD) is about 30%. The seed source femtosecond laser used in the measurement process has a center wavelength of 1560.3 nm, a repetition frequency of 12.34 MHz, and pulse duration of 644 fs.
In this experiment, we used a SMF patch cord. The deposited solution formed a thin film of liquid on the FC/PC fiber end. The dispersion completely evaporated within 5 to 10 min. The solute Fe3O4 nanoparticles were attached to the fiber end. After repeating this process for a few times, some layers of Fe3O4 nanolayers eventually were formed on the end of the FC/PC patch cord after connecting two fiber taps by a flange. We integrated it with the laser cavity.
Figure 3 shows the principal structure of the fiber laser used in the experiment. The ring laser cavity includes an specific length of gain fiber, a 980/1550 wavelength division multiplexer (WDM), a polarization-independent fiber isolator (ISO), a fiber output coupler (OC), a squeezer polarization controller, and a standard single-mode FC/PC optical fiber patch cord, whose FC/PC end was deposited with Fe3O4 nanoparticles as a SA. In this experiment, a tabletop laser with the wavelength of 980 nm at center was used as pump source, whose power could be adjusted from 0 to 255 mW.
The erbium-doped fiber (EDF; NUFERN, EDFC-980-HP, USA) was used as a gain fiber with a length of 3.3 m, and the absorption at 980 nm is about 3 dB/m. We inserted a FC/PC optical fiber patch cord with the end deposited with Fe3O4 nanoparticles into the cavity by a flange. The polarization-independent optical isolator ensures the one-way transmission of light in the laser cavity. A polarizer controls the different polarization states of the light in the cavity. The output coupler with output ratio of 10:90 connects measuring equipment, whose branch of 10% is used as the output, and branch of 90% is connected back to the fiber ring. The total length of the laser cavity was measured to be ~23.5 m, containing ~3.3 m long EDF and ~20.2 m long SMF, which consists of pigtails of optical devices and other fibers, except EDF, in the cavity. At 1550 nm, the dispersion parameters D are -16 ps/(nm·km) for EDF and 17 ps/(nm·km) for SMF. The total net dispersion of the cavity is ~0.38 ps2. So, it is possible to achieve a soliton mode-locked operation in the fiber laser. During the experiment, we detected the output spectrum of the laser by a spectrum analyzer (YOKOGAWA, AQ6375, Japan). The pulse train is observed by a photodetector (THORLABS, DET08CFC, 2 GHz, USA) and a digital oscilloscope (RIGOL, DS4054, 1 GHz, China). The radio-frequency spectrum is observed by a spectrum analyzer (AGILENT, E4447A, USA) and the output power is recorded by a hand-held optical power meter (JOINWIT, JW3208, China).
Self-start mode-locked pulses could be observed at a pump power of 70 mW. The pulse train tends to stabilize and the output power increases with the pump power. The slope of the curve is ~3.16% as shown in Fig. 4(a). The pulse disappears when the power decrease to 65 mW and the continuous wave light outputs due to the hysteresis effect [41].
Results and discussion
Figure 5(a) shows an output spectrum with a center wavelength of 1561 nm and the spectral width of 0.69 nm at the pump power of 190 mW. The outline of the spectral image is M-shaped and the shape of its peak appears as concave. The whole spectrum exhibits an axisymmetric characteristic, extending to both sides from the M-shaped pit point and the peak of the spectral curve undulating corresponding sideband. The appearance of the spectrum with an M-shaped profile is a typical characteristic of dark solitons operation in the cavity corresponding to the waveform below the baseline as shown in Fig. 5(b) [42,43]. These sidebands represent the properties of the optical pulse output and are often called Kelly-sidebands. The entire spectrum curve also presented the feature of dissipative soliton resonance. Figure 5(b) shows the evolution of the spectrum with an increase in pump power. If the pump power increases, the gain will increase as rectangular pulse will be formed. The wavelength of the long wave spectrum will increase, while the wavelength of shorter spectrum will gradually decrease [44]. The pulse characteristic of the rectangular square wave is that it drops approximately 20 ns after reaching its saturation point. The spectral width varies from ~0.69 to 1.05 nm and the leftmost spectral peak is clearly enhanced due to the Kelly effects with the pump power ranging from 190 to 240 mW. However, the overall outline of the spectrum remains unchanged. We observed the single rectangular pulse duration of about 14 ns at pump power of 190 mW as shown in Fig. 5(c). The formation of this type of pulse is related to the dissipative soliton resonance in the cavity [45]. The reason behind the formation of rectangular wave without flat top is that the dynamic saturation effect of the gain in the laser cavity causes the pulses to receive different gains at the front and rear edges [46,47].
Fig.5 (a) Output spectrum at the pump power of 190 mW. (b) Evolution of spectrum with pump power from 190 to 240 mW. (c) Single pulse at the pump power of 190 mW. (d) Evolution of single pulse with pump power from 190 to 240 mW. (e) RF-spectrum of single frequency at the pump power of 190 mW ((e) is a magnified interception of (f)). (f) RF of frequency at the pump of 190 mW. (g) Evolutions of frequency and output power with pump power |
Figure 5(d) shows the evolution of the width of the pulse with the increase in pump power. The quality of rectangular pulse will be improved with a flat top in the future work. The pulse duration increases from 14 to 20 ns with the increase in pump power from 190 to 240 mW, which is similar to the previously reported results [48–50]. This phenomenon also shows a feature of dissipative soliton resonance. We can observe that the pulse duration becomes wider according to the changing trend of spectra. We can see that when the maximum pump power is 240 mW, the rectangular pulse remains unaffected. Here, we demonstrate that Fe3O4, as a SA, shows good compatibility with ultrafast fiber laser. Figure 5(e) shows the radio-frequency (RF) spectrum of the pulse laser with a center frequency of 7.69 MHz, a scan width of 1 MHz, and a resolution of 150 Hz at the pump power of 190 mW. Figure 5(f) shows that the frequency interval between two peaks is same as round trip frequency of the laser cavity with pump power of 190 mW, which shows that the resonant cavity achieves mode-locking. The bandwidth and resolution measured in the experiment are different. So, there are some noises at the base [51]. Figure 5(g) shows evolutions of repetition rate of the pulse laser with pump power. The repetition rate of pulses does not change with change in pump power. Autocorrelation trace cannot be measured because the pulse width is at a nanosecond scale. In the process of pulse width broadening, the rectangular pulses capture the weak pulse generated by the continuous wave, which leads to the soliton phenomenon [52]. Moreover, the mode-locked pulse is realized by Fe3O4 SA, the magnetic nanomaterial deposited on the end of patch cord is wiped off by alcohol. When the optical path was connected again at the pump laser power of 190 mW, no pulse was obtained. Then, polarization controller is adjusted and the power is changed from the highest to the lowest, but no pulse output appears. This confirms that Fe3O4 enables pulse mode-locking and it is a potential optical material that will play an important role in the field of laser mode-locking in the future.
Conclusions
We have observed passively mode-locked pulses in the EDF fiber laser based on Fe3O4 nanoparticles as the saturable absorber, which is deposited on the end of the fiber patch cord. The operation of the laser pulse with a repetition frequency of 7.69 MHz is achieved. When the pump power is 190 mW, a rectangular pulse is obtained. The change of pulse width is characterized by dissipative soliton resonance. The pulse width increases with the increase of pump power, but the intensity remains unaffected. The rectangular square wave is realized for the first time based on Fe3O4 nanoparticles. The phenomenon for dissipative soliton resonance will provide a new basis to the future research of soliton [53]. We can expect the potential applications of Fe3O4 in nonlinear optics and ultrafast photonics.