1. Introduction

Passively Q-switched fiber lasers (PQFLs) have been widely applied in material processing, remote sensing, ranging finding and optical communications [1–3]. Particularly, fiber lasers operating in the 2 μm region is highly desirable in some specific applications such as medicine diagnostics and remote sensing. In order to obtain PQFL, different methods have been proposed to achieve the saturable absorption effect, such as nonlinear polarization rotation (NPR) [4] and saturable absorbers (SAs) [5,6]. At present, semiconductor saturable absorber mirrors (SESAMs) [7] are considered to be one of the most mature and commercial SAs, but SESAMs are expensive for fabrication and have narrow operation bandwidth which might limit the broadband operation. In the past decade, other SAs such as carbon nanotubes (CNTs) [8], graphene [9], and topological insulators (TIs) [10] have been investigated. Comparing to SESAMs, these three materials have advantages of broadband operation, easy-fabrication and low-cost. However, the operation wavelength of CNTs is determined by the nanotube diameter and chirality, restricting its lasing at specific wavelengths and broadband tenability. Although graphene is an intrinsic ultra-broadband SA, it has a small absorption of only 2.3% at 1550 nm and a relatively low modulation depth (typically <1% per layer [11]). Another kind of two dimensional (2D) materials, TIs, have insulating bulky states with indirect band-gap of 0.35 eV [12] and gapless surface states. Since the first demonstration of the nonlinear optical property of the TI: Bi₂Se₃ [13], the TIs have been widely researched in laser photonics. But TIs have the drawback of complicated preparation process (compound with two different elements), which severely limited their applications in optoelectronic devices [14]. Therefore, researchers are still making efforts to seek for new SAs that possess ideal characteristics of broadband saturable absorption, high damage threshold, large modulation depth, low saturable optical intensity and low cost.

Recently, black phosphorus (BP) as a new kind of 2D material has been attracting great interest. Different from the above-mentioned 2D materials, BP has a layer-dependent direct band-gap which changes from 0.35 eV (bulk) to 2 eV (monolayer layer) [15], which can fill up the space between the zero gap graphene and large gap TMDCs, making BP an ideal 2D material for near and mid-infrared optoelectronics. Note that BP comprises only one element, hence it could be easily peeled off by mechanical exfoliation. Most importantly, the unique anisotropic nature [16] of BP makes it attractive in the applications for optoelectronic and electronic devices. Very recently, Lu et al. measured the nonlinear absorption properties of BP at 400, 800, 1563 and 1930 nm using the open-aperture Z-scan technique. The saturation intensity and normalized modulation depth of the BP nanoplatelets (NPs) dispersions in IPA (isopropyl alcohol) were experimentally found to be 455.3 ± 55 GW/cm² and 27.6% at 400 nm, 334.6 ± 43 GW/cm² and 12.4% at 800 nm if under femtosecond laser excitation, and that of BP-PMMA (polymethyl-methacrylate) flakes were 18.54 MW/cm² and 19.5% at 1563 nm, and 4.56 MW/cm² and 16.1% at 1930 nm if under pico-second laser excitation, respectively [17]. These results indicated the broadband saturable absorption response of BP at such wavelengths. Meanwhile, Chen and Sotor et al. presented the 1.5 and 2 μm pulsed fiber lasers using the BP-SA [14, 18]. However, so far, no nanosecond Q-switched fiber laser at 2 μm wavelength based on BP-SA has been reported, the broadband saturable absorption of BP is not fully exploited.

In this contribution, we demonstrated for the first time a nanosecond BP passively Q-switched thulium/holmium-doped fiber laser (THDFL) with the shortest pulse duration of 731 ns, and the repetition rate varying from 69.4 to 113.3 kHz. The high quality BP-SA with a modulation depth of ~24% was obtained through the optical deposition method. The Q-switched thulium/holmium–doped fiber laser operating in the 2 μm wavelength further demonstrated the broadband nonlinear saturable absorption potential of BP for stable pulse generation.

2. Preparation and characteristics of BP nanoplatelets

The BP nanoplatelets were fabricated through a facile solution-based method. Firstly, 500 mg of BP powder was added into 500 mL N-methyl-2-pyrrolidone (NMP) solution in a glass vial and then ground for 1 h. After sealed carefully, the mixture was sonicated in an ice-bath for 8 h at the power of 100 W. After that, the as prepared BP solution was ultra-sonicated for 90 minutes and settled for 36 hours with a centrifugation at a speed of 6000 rpm for 20 minutes to remove the large size sedimentations. The prepared BP NPs were characterized by scanning electron microscope (SEM) and atomic force microscopy (AFM), as shown in Fig. 1(a) and Fig. 1(b). The height profile [Fig. 1(c)] shows that the average thickness of the BP NPs is ~23nm. Considering that the thickness of the single layer BP is ~0.6 nm [19], it is estimated that the BP NPs are ~38 layers. The Raman spectrum of the BP NPs is illustrated in Fig. 1(d). Both two inplane vibrational modes A_g² (at 466 cm⁻¹) and B_2g (at 438.5 cm⁻¹) are clearly exhibited, along with one out-of-plane vibration mode A_g¹ (at 361.5 cm⁻¹), which is in agreement with the earlier findings [20,21].

 

Fig. 1 (a) The SEM image and (b) the AFM image of the BP NPs, (c) the height profile, (d) corresponding Raman.

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In the following, the as prepared BP NPs were deposited onto the fiber end-facet by the optical deposition method. A continuous wave (CW) laser at 1936 nm was injected into the BP NPs solution through a 1 m single mode fiber (SMF) and a standard FC/PC fiber ferrule. The corresponding experimental setup is described in the insert of Fig. 2(a). After 1-min light illumination under a pump power of 50 mW, BP NPs was gradually absorbed and deposited onto the fiber end-facet due to optical trapping force and heat convection effects [22]. After being dried for 1 hour, the BP NPs based SA was fabricated after connecting with another clean and dry FC/PC fiber ferrule. Figure 2(a) illustrates the microscope image of the fiber end-facet (containing the outer cladding with a diameter of ~125 μm and inner core with a diameter of ~10 μm) after deposition. It is clearly shown that the fiber core area was covered by the BP NPs.

 

Fig. 2 (a) Optical image of the fiber end-facet after deposition. Inset: Schematic diagram of optical deposition for BP NPs, (b) nonlinear transmission curve of the BP-SA, (c) the transmission spectrum of the BP NPs on quartz substrate.

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The nonlinear transmission property of the BP-SA was measured by a balanced twin-detector measurement system. The measurement system is similar to Ref [23]. The laser source used in this measurement is a home-made thulium-doped fiber laser (central wavelength: 1932 nm; repetition rate: 33 MHz; pulse width: 3 ps). By gradually increasing the input power, the optical transmittance with respect to different input optical power was recorded, as shown in Fig. 2(b). After fitting with formula: $T(I)=1-\Delta T*\exp (-I/I_{sat})-T_{ns}$ ($T(I)$: transmission, $\Delta T$: modulation depth, $I$: input intensity, $I_{sat}$: saturation power intensity, $T_{ns}$: non-saturable absorbance) [14], the corresponding saturable absorption parameters were obtained. The modulation depth, non-saturable losses and saturable optical intensity (I_sat) of the BP-SA were 24%, 5% and 1.1 MW/cm², respectively. When we changed the polarization of the incident laser, there was no evident polarization dependent loss in the BP-SA. Also, we measured the optical absorption properties of the BP NPs using an optical spectrometer. The measured transmission spectrum from 900 to 2400 nm is described in Fig. 2(c), which shows that the optical transmittance is about 73% over the wavelength range. This result indicates the potential of the BP NPs as a broadband optical material.

3. Experiments and discussions

The scheme of fiber ring cavity incorporating the BP-SA fabricated above is shown in Fig. 3. A section of 1.5 m single mode thulium/holmium-doped fiber (THDF) with absorption coefficient of ~10 dB/m at 1550 nm acted as the gain medium and a 9 m single-mode fiber as the other fiber component. Hence the total length of the laser cavity was ~10.5 m. The THDF was pumped by a 1550 nm laser through a 1550/2000 nm wavelength-division multiplexer (WDM). A polarization controller (PC) was inserted to control the polarization state inside the laser cavity. Meanwhile, a polarization-independent isolator (PI-ISO) was placed inside the laser cavity to ensure the unidirectional propagation of intra-cavity light. A 10/90 optical coupler (OC) was used to extract 10% lasing signal. The laser output was simultaneously monitored by an optical spectrum analyzer with a resolution of 0.1 nm, a high-speed photo-detector together with a 1.5 GHz digital oscilloscope or a radio-frequency (RF) spectrum analyzer.

 

Fig. 3 The experimental setup of the BP Q-switched THDFL.

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In order to confirm the significance of the BP-SA to the passive Q-switching, at first, we purposely did not include the BP-SA in the laser cavity, and the THDFL started the continuous-wave (CW) lasing at the pump power of 210 mW. In contrast, once the BP-SA with the insertion loss of ~29% was spliced into the THDFL cavity, stable Q-switched pulse trains were obtained with the threshold of 260 mW. At a fixed cavity polarization state, through continuously increasing the pump power, the pulse repetition increased while the pulse train still remained a uniform intensity distribution without obvious fluctuation, as shown in Fig. 4. Meanwhile, the pulse width became narrow gradually. These phenomena are the typical features of passive Q-switching [24], [25]. This system did not start the mode-locking operation which is possibly limited by 1) the large cavity loss of ~10 dB, and 2) the relatively large modulation depth of the BP-SA (~24%).

 

Fig. 4 The various pulse trains obtained under different pump powers.

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Figure 5 describes the typical characteristics of the Q-switched pulses at a pump power of 293 mW. The stable Q-switched pulse train has a repetition rate of 79.8 kHz, corresponding to a time interval between adjacent pulses of 12.5 μs. The insert shows the pulse train in a long time scale, from which a uniform intensity distribution without modulation was observed. Figure 5(b) describes the zoom-in of a single pulse, from which the pulse duration is inferred to be about 1.21 μs and the pulse has a symmetric intensity profile. The optical spectrum of the Q-switched pulses is shown in Fig. 5(c). It has a central wavelength of 1912 nm and the 3-dB spectral bandwidth is 0.8 nm. To investigate the laser stability, the corresponding RF spectrum with a resolution bandwidth (RBW) of 15 Hz was measured. As can be seen in Fig. 5(d), the signal-to-noise (SNR) of the Q-switched THDFL is over 32.8 dB, indicating that the Q-switched pulses operated in a relatively stable regime.

 

Fig. 5 (a) A typical Q-switched pulse train (insert is the large time scale), (b) single pulse profile, (c) the corresponding optical spectrum of the Q-switched THDFL obtained at a pump power of 293 mW, (d) the ratio-frequency optical spectrum at the fundamental frequency.

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The relations between the repetition rates of the Q-switched pulses, pulse duration, average output energy with respect to different incident pump power were summarized in Fig. 6. As can be seen in Fig. 6(a), the initial pulse width is 1.42 μs near the threshold power. Then, the pulse width sharply decreased with slightly increasing the pump power, but became much slower once the pump power was over 383 mW. The obtained minimum pulse width of 731 ns was shorter than the reported passively Q-switched fiber lasers with graphene, MoS₂ and CNTs based SAs [26–28], in which the pulse width were 1.4, 1.76 and 7.155 μs, respectively. The pulse duration could be further narrowed by optimizing the parameters, including 1) shorting the cavity length and 2) improving the modulation depth of the BP-SA [29]. Meanwhile, the pulse repetition rate increased almost linearly from 69.4 to 113 kHz with the pump power increasing from 261.5 to 600.5 mW. When the pump power was over 800 mW, the Q-switched pulse train became unstable and strong fluctuation appeared, and finally the Q-switched pulses disappeared. However, when decreased the pump power the stable Q-switched pulses could be obtained again, indicating that the BP-SA had not been damaged.

 

Fig. 6 (a) The pulse duration and the pulse repetition rate as a function of the pump power, (b) the average output power and the single-pulse energy of the Q-switched THDFL as a function of the pump power.

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The average output power and single pulse energy versus incident pump power were shown in Fig. 6(b). It is clearly shown that the output power increases linearly with respect to the pump power. At the pump power of 600.5 mW, the maximum average output power was 71.7 mW. The corresponding maximum single pulse energy was 632.4 nJ, compared to those Q-switched fiber lasers using graphene and TIs as passive Q-switchers, in which the maximum single pulse energy were 85 and 11.54 nJ, the single pulse energy in our experiment was significant improved [26,30]. The higher pulse energy could be enabled by using high-gain fiber (e.g. double-clad fiber) with high-performance BP-SA, and optimizing the cavity designs (e.g. output coupling ratio, cavity loss) [29].

Considering the BP material used in the experiment is not completely unreactive under ambient conditions with oxygen and water molecules [31], the experimental invesitigations were completed in three days to minimize the quality degradation of the BP SA. Meanwhile, when the BP SA was exposed in the ambient conditions for more than one week, it would be very diffucult for the THDFL to generate Q-switched pulses, indicating that the optical property of the BP material had been changed. In order to overcome this proplem, it is expected that the introduction of BP with other strutures, such as polyvinyl alcohol (PVA) film (which can protect the oxidation from the air/water) and optical wave-guide structures (by injecting the BP NPs into photonic crystal fibers) and so on.

4. Conclusion

In summary, a nanosecond passively Q-switched thulium/holmium-doped fiber laser by the black phosphorus nanoplatelets based saturable absorber was experimentally demonstrated. This saturable absorber device was fabricated through the optical deposition method. Using the balanced twin-detector measurement technique, the broadband saturable absorption response of this saturable absorber was characterized with the modulation depth of ~24%. Stable passively Q-switched pulses were achieved with single pulse energy up to 632.4 nJ, narrowest pulse width of 731 ns, and repetition rate varying from 69.4 to 113.3 kHz. These results clearly evidence that black phosphorus possess the desired optical properties for Q-switched fiber laser at 2 μm.