Q-switching as a versatile technique that can produce high power and energy short pulses has been widely used in remote sensing, range finding, laser processing, and optical communications [1–3]. For the 3 μm mid-infrared spectral region, Q-switched pulses have many specific applications, including laser surgery, material processing, and infrared countermeasures. Recent fast development of mid-IR fiber lasers also stimulates the development of compact and robust Q-switched laser sources in this band [4]. Generally, Q-switching can be divided into active and passive schemes. Compared to the former one that needs a complex externally controlled modulator, the latter one, based on a low-cost and simple saturable absorber (SA), is more attractive. Until now, SAs based on III-V-compound semiconductors are most widely used because of their mature fabrication processes and ability to customize their properties. The well-known semiconductor saturable absorber mirror (SESAM) is a typical example of those [5–7]. Although their success in Q-switching and mode locking has been seen at ∼3μm recently [8,9], their fundamental limitations, which include a narrow operation bandwidth and a complicated fabrication process, motivate researchers to find new materials to overcome these drawbacks.

In the last two decades, broadband and cost-effective, low-dimensional materials were intensively studied as a potential replacement for traditional SAs [10,11]. In 2003, the carbon-nanotubes-based one-dimensional (1D) SA was first developed by Set et al. [12]. Since then, it has been widely adopted for pulse generation at 1–2 µm [13,14] and recently was used in ∼3μm fiber laser [15]. Gold nanorods (GNRs), another popular 1D material, have generated interest due to their broadband absorption induced by their distinct surface plasmon resonance and used in pulsed fiber lasers at 1–3 µm [16–18]. However, their nonlinear optical response must be tailored by carefully controlling diameters and chirality. The emergence of graphene that has an ultrabroadband feature as a result of its zero-gap structure [19], started the era of 2D materials [20,21]. In 2009, Bao et al. first demonstrated a mode-locked Er3+-doped fiber laser at ∼1.5μm using a graphene SA [22]. After that, other 2D materials, such as topological insulators (TIs) [23–25], black phosphorus (BP) [26–28], and transition metal dichalcogenides (TMDs) [29–31], began to attract attention because of their high third-order nonlinearity, strong light–material interaction, ultrafast carrier dynamics and broadband absorption. Wei et al. reported the first passively Q-switched Er3+-doped ZBLAN fiber laser at ∼3μm based on graphene [32]. Our group then showed the feasibility of TI as a Q-switcher at ∼3μm in a Ho3+-doped ZBLAN fiber laser [25]. BP was also used for Q-switching and mode locking in ∼2.9μmHo3+- and ∼3.5μmEr3+-doped ZBLAN fiber lasers, respectively [27,28]. However, these 2D materials have their own defects, which limited their applications to some extent. For example, graphene has a low modulation depth due to the low absorption efficiency (2.3% per layer), making it unsuitable for short Q-switching. TIs have a complex preparation process since they are made up of two different elements. BP is quite unstable due to its ease of oxidation under ambient conditions. The bandgaps of TMDs are so naturally large that they are unavailable for the mid-infrared, although this situation can be changed by introducing a series of defects, which then complicates the preparation. Accordingly, there still remains a strong motivation to pursue other high-performance saturable absorption materials.

Recently, zero-dimensional (0D) forms of the materials such as aluminum oxide (Al2O3), bismuth oxide (Bi2O3), nickel oxide (NiO), titanium dioxide (TiO2), zinc oxide (ZnO), and Fe3O4 began to attract attention [33–38]. Similar to 1D and 2D materials, they possess large third-order optical nonlinearity, ultrashort recovery time, a broad operation band, excellent thermal stability, and easy preparation. More importantly, they can be prepared more uniformly because of their smaller sizes and more regular shapes, which gives more flexibility to tailor their properties and higher pulse stability. Among them, Fe3O4 nanoparticles have been widely researched. As a transition metal oxide and semiconductor, they have a large third-order nonlinear optical susceptibility of χ(3)=4.0×10−10esu [39] and a recovery time of 18–30 ps on a slow time scale [40], both of which are important for high-energy Q-switching. The narrow bandgap of 0.3 eV (∼4.2μm) [41] can be tuned by selecting the diameter of the nanoparticles, which is achieved by adjusting the pH and ionic strength of the precipitation medium during preparation [42,43]. Thus, they can potentially be used as a broadband SA from the visible to the mid-infrared. So far, Fe3O4 nanoparticles have been used in Q-switched and mode-locked lasers at ∼1μm [44–46], ∼1.5μm [38,47,48–51], and ∼2μm [52–54]. In 2016, Fe3O4 nanoparticles were used as an SA in a passively Q-switched Er3+-doped fiber laser at ∼1.5μm by Bai et al. [38]. Wang et al. applied the material in Q-switching at ∼1μm in a Nd:YVO4 laser at 1064.34 nm [44]. Koo et al. extended it to the 2 μm band and demonstrated passively Q-switched Tm3+/Ho3+ codoped and Tm3+-doped fiber lasers at 1.89 μm [52] and 1.94 μm [54], respectively. Multiwavelength operation at ∼1μm [45] and ∼1.5μm [48], and dark soliton generation at ∼1.5μm [49] were also reported. However, until now there have been no demonstrations at longer wavelengths. Currently, ∼2.8μmEr3+- [55–57] and ∼2.9μmHo3+- [15,25,27,31] doped ZBLAN fiber lasers are the most widely used mid-infrared platforms for pulse generation based on low-dimensional materials. As an alternative, Dy3+-doped ZBLAN fiber can provide a broader emission band from 2.6 to 3.4 μm [58,59], which fully covers the emission bands of both Er3+ and Ho3+ ions, and is thus being regarded as a promising mid-infrared laser platform. Recently, Majewski et al. presented a continuous wave (CW) Dy3+-doped ZBLAN fiber laser pumped at 1.7 μm tunable from 2.8 to 3.4 μm [59]. Fortin et al. reported an in-band pumped high power CW Dy3+-doped ZBLAN fiber at 3.24 μm with a record output power of 10.1 W [60]. Using this platform, picosecond and femtosecond mode-locking based on frequency shifted feedback (FSF) [61] and nonlinear polarization rotation (NPR) [62] was reported. Very recently, we built up a tunable passively Q-switched Dy3+-doped ZBLAN fiber tunable from 2.71 to 3.08 μm using PbS nanoparticles [63]. Meanwhile Woodward et al. used BP as the SA to demonstrate a passively Q-switched Dy3+-doped ZBLAN fiber laser at 3.04 μm and a tunable actively Q-switched version from 2.97 to 3.23 μm [64].

In this paper, we demonstrate for the first time the use of Fe3O4 nanoparticles to Q-switch a tunable Dy3+-doped ZBLAN fiber laser around 3 μm, showing its potential as a broadband saturable absorption material that can work in the mid-IR region. The morphology and nonlinear optical response at 2.87 μm of the SA are characterized. Using this SA, stable Q-switched pulses tunable from 2.81 to 3.03 μm are produced. The output characteristics with the varied pump power and wavelength are also investigated in detail.

The Fe3O4 nanoparticles (FNOPs) are dispersed as a stable colloidal suspension in an alcohol carrier. Figure 1(a) shows its atomic structure seen from the (−100) crystal plane. Fe3O4, in which there are two kinds of iron ions and one oxygen ion, belongs to a cubic system. In the image, yellow, green, and red circles correspond to positive trivalent iron ions, positive divalent iron ions, and negative divalent oxygen ions, respectively. The transmission electron microscopy (TEM) images of the FNOPs sample with different scales are shown in Figs. 1(b) and 1(c), respectively. The low-magnification TEM image as displayed in Fig. 1(b) reveals that the lateral size of the FNOPs is about 15 nm. The high-resolution TEM image shows a clear crystalline lattice spacing 0.42 nm. Figure 1(d) displays the image of X-ray diffractometer (XRD), and we point the three strongest peaks that are used to identify the phase of material corresponding to crystal diffraction faces of the samples [(220), (400), (422)], which indicate the ultrafine nature and small crystallite size of the particles.

Figure 2(a) shows the experimental setup to measure nonlinear absorption. We first built a SESAM passively mode-locked Ho3+/Pr3+-codoped fluoride fiber laser with a similar structure to that in Ref. [8]. This homemade laser source has a center wavelength of 2870 nm, a pulse duration of approximately 20 ps, and a repetition rate of 17.4 MHz. A beam splitter with a measured transmittance/reflectance ratio of 55:45 was placed at an angle of 45° with respect to the optical path. In the transmitted path, the laser was focused on the previously prepared Fe3O4-coated CaF2 substrate with a 20 mm focal length CaF2 lens. Detector 1 was placed after it to record the power P1. The reflected path has the same structure, but the CaF2 substrate was uncoated. Detector 2 was placed after it to record the power P2 as the reference. To find the focal point of the CaF2 lens, the Fe3O4-coated CaF2 substrate was moved with a 1D translation stage until the transmitted power P1 reached a maximum. The transmittance of the Fe3O4 sample could be calculated by 45P1/55P2.

We obtain the parameters of the sample by fitting the data with T(I)=1−ΔT·exp(−I/Isat)−Tns,(1)where T(I) is the intensity-dependent transmittance, ΔT is the modulation depth, Isat is the saturation intensity, and Tns is the nonsaturation loss. The measured nonlinear transmission of the SA fitted based on Eq. (1) is as shown in Fig. 2(b). According to the best fit, ΔT, Isat, and Tns were found to have the values of 11.9%, 14.1MW/cm2, and 49.8%, respectively. The modulation depth is lower than most recent mid-infrared SAs made from low-dimensional materials (e.g., TI [36], BP [57], TMDs [31]), but can be tailored by controlling the material thickness. The saturation intensity is also relatively low, which is helpful for initializing pulses.

Figure 3 shows the experimental setup of our tunable passively Q-switched Dy3+-doped ZBLAN fiber laser using Fe3O4 nanoparticles SA. The pump source is a homemade CW Yb3+-doped fiber laser at 1.1 μm with a single-mode SMF-28e fiber pigtail. It can yield a maximum output power of 10 W. The gain medium is a piece of commercial, single-cladding Dy3+-doped ZBLAN fiber with a core diameter of 15 μm, a cladding diameter of 125 μm, and a cutoff wavelength of 2.55 μm. It has a dopant concentration of 1000 ppm (parts per million) and a length of 1.25 m. The 1.1 μm pump laser was launched into the core of the gain fiber using a pair of aspheric lenses (L1, L2, f=8mm) (Thorlabs, C240TME-C) with a measured launch efficiency of 88%. DM1 specifically designed with a HT at ∼1.1μm and a HR at ∼3μm was butted against the perpendicularly cleaved fiber pump end as one cavity feedback. The other fiber end was cleaved at an angle of 8° to avoid the parasitic lasing. From that end, the laser was collimated using an anti-reflection (AR) coated objective lens (L3, f=12mm) (Innovation Photonics, LFO-5-12, 0.13 NA). Following that, DM2 with a reflectance of ∼50% at ∼3μm was placed at an angle of 45° with respect to the laser as the output coupler. The reflected beam from this DM was projected onto a plane ruled grating (Thorlabs, 450 lines per mm, blaze wavelength λB=3.1μm, blaze angle θB=32°) in a Littman configuration, where the first-order diffracted beam was reflected by the Fe3O4 nanoparticles coated Au mirror after focusing using another AR-coated objective lens (Innovation Photonics, LFO-5-6, 0.25 NA). Two commercial bandpass filters (FB3500-500, Thorlabs) were used to purify the output beam. In this scheme, the wavelength could be tuned by rotating the grating. In this process, the distance between L4 and the Au mirror, and the position of L3 were kept aligned to maximize the output due to the chromatic aberrations from L4 and L3. The laser powers were recorded using high-resolution thermal power sensors (Thorlabs, S405C). The temporal pulses were captured using an InAs detector (Judson J12D) with a response time of 2 ns connected with a 500 MHz bandwidth digital oscilloscope. The optical spectrum was measured using a monochromator with a minimum resolution of 0.2 nm (Teledyne Princeton Instruments Acton SP2300). Note that the laser powers mentioned in this paper are all the corrected values according to the transmittance of the used filters.

First, the position of the grating was adjusted to maximize the output power of the Q-switched pulses at the launched pump power of 2.89 W and then fixed to study the influence of pump power on the laser output. When the launched pump power was increased to 1.62 W, stable Q-switched pulses were first generated, as shown in Fig. 4(a), with a repetition rate of 49.02 kHz and a pulse width of 2.46 μs. Such a high laser threshold was mainly due to the strong re-absorption of the ground state in the section close to the unpumped fiber end, and also to the high insertion loss of the SA. This stable Q-switching regime could be maintained until the launched pump power reached 2.89 W. The corresponding temporal behavior was recorded, as shown in Fig. 4(b), with an increased repetition rate of 123.0 kHz and a reduced pulse width of 1.25 μs. A further increase of the pump power caused the Q-switched pulses to become unstable and then disappear due to the damage of the SA. At the allowable maximum launched pump power of 2.89 W, the optical and RF spectra were also measured, as shown in Figs. 4(c) and 4(d). The center wavelength was 2932.3 nm, corresponding to the largest gain region of the present cavity. The FWHM of 0.3 nm, which was significantly narrower than in the free-running case [63], resulted from the wavelength selectivity of the grating. The signal-to-noise ratio (SNR) of ∼35dB was typical for stable passive Q-switching [65,66].

Figure 5 shows the evolution of the output Q-switched pulses as a function of the launched pump power. Figure 5(a) shows that the pulse width decreases from 2.46 to 1.25 μs and the repetition rate increases from 49.02 to 123.0 kHz when increasing the launched pump power from 1.62 to 2.89 W. These results are in agreement with the standard passive Q-switching theory [67,68]. Figure 5(b) shows the variation of the output power and pulse energy over the same pump range. At a launched pump power of 1619.4 mW, the Q-switched output power was 7.82 mW, corresponding to a peak power of 0.065 W. Thus, the laser peak intensity on the Au mirror was estimated to be 96.6MW/cm2, considering the insertion loss of the objective lens and the diffraction efficiency of the grating, which is larger than the SA saturation intensity. At the maximum launched pump power of 2.89 W, a maximum output power of 111.0 mW with a pulse energy of 0.90 μJ was obtained. The output power fluctuation was measured and calculated to be less than ±2% within 30 min, which indicated its good long-term stability. If the laser was restarted after being off for a long time, however, we needed to slightly align the pump fiber end to make the laser recover to the previous output level.

Then the wavelength tuning of the Q-switched laser was performed at a launched pump power of 2.89 W by rotating the ruled plane grating. A wavelength tuning range of 2812.4–3031.6 under Q-switching was obtained, as displayed in Fig. 6(a). The output power and pulse energy evolutions with wavelength matched the gain spectrum of the Dy3+-doped ZBLAN fiber [25]. Figure 6(b) shows the repetition rate and pulse width as a function of wavelength. It is seen that the repetition rate first increases and then decreases with the wavelength while the pulse width has an opposite trend. At a wavelength of around 2841.6 nm, the largest repetition rate of 161.3 kHz and shortest pulse width of 790 ns were achieved. These observations are also in agreement with the previously reported tunable passively Q-switched fiber lasers in this band [18,25]. After finishing measurements, the laser was placed under ambient condition for one week, during which the laser could always work stably. It indicated good stability of the used Fe3O4 nanoparticles.

In conclusion, we have demonstrated for what we believe is the first time the use of Fe3O4 nanoparticles as an efficient broadband mid-IR Q-switcher in a tunable Dy3+-doped ZBLAN fiber laser around 3 μm. Stable Q-switched pulses were obtained with a tunable wavelength range of 2812.4–3031.6. At a wavelength of 2932.3 nm, a maximum output power of 111.0 mW was obtained with a pulse energy of 0.90 μJ. The corresponding pulse width and repetition rate were 1.25 μs and 123.0 kHz, respectively. This kind of μs pulse laser source after power scaling can find direct applications in the field of laser surgery since its wavelength highly overlaps with the strong absorption region of water [69–72]. The results indicate that Fe3O4 nanoparticles are a promising long-term stable SA material under ambient conditions for the mid-infrared region.