Ultrafast pulses operating at 2-μm region have received enormous attention due to their important applications such as materials processing, micromachining, and medicine.^[1,2] It has been demonstrated that the ultrafast pulses could be generated efficiently by inserting a saturable absorber (SA) in a passively mode-locked fiber laser. Up to now, Various SAs were utilized to generate ultrafast pulses at different wavelengths.^[3–30] Among these SAs, topological insulators (TIs) are particularly interesting due to the extraordinary charge and spin properties on the edge or surface modes of TIs. Unlike extensively studied graphene, TIs have the characteristics with a small band gap in the bulk state and a gapless metallic state in the surface.^[31,32] The combination of the small bandgap bulk (0.2 eV ∼ 0.3 eV) and the gapless surface enables TIs to possess an ultra-broad bandwidth of saturable absorption operation,^[15] which can cover the mid-infrared spectral region. Sb₂Te₃, a typical TIs, has been successfully served as a SA in ultrafast fiber lasers^[31] due to its superior saturable absorption feature and giant third order nonlinear optical property. However, the studies for its nonlinear optical property were mainly focused on 1.5-μm regime. Therefore, we believe that it would be technically meaningful to perform another systematic investigation on the applicability of Sb₂Te₃-based SAs for the implementation of 2-μm pulsed fiber lasers.

Generally, two-dimensional (2D) materials could be prepared via mechanical exfoliation (ME), liquid phase exfoliation (LPE), chemical vapor deposition (CVD), or magnetron-sputtering deposition (MSD).^[33] 2D material obtained with LPE or ME methods usually have an uncontrollable size and random thickness, which are detrimental for the performance of a SA. CVD method could prepare 2D material with a large area and certainly atomic thickness,^[34] but the preparation process is relatively complex (e.g., the 2D material films require to be carefully transferred onto target substrates and difficult to remove transfer residues completely). Compared with ME, LPE and CVD method, the MSD is a simpler method to grow 2D material on target substrates directly and suitable for mass-production, which are critical for their practical applications. Therefore, magnetron sputtering deposited Sb₂Te₃ was selected as the SA in the work. In order to enhance the reliability of the Sb₂Te₃ SA, the Sb₂Te₃ material was deposited directly on the waist of the microfiber. The microfiber-based Sb₂Te₃ SA with evanescent wave interaction is attractive for high power tolerance, which is benefit to generate high output power. Moreover, the whole microfiber (up to centimeter order) was completely coated tightly with the Sb₂Te₃ film, which ensures the strong nonlinear optical response in material together with the longer interaction length.

In this paper, we developed a type of microfiber-based Sb₂Te₃ SA with the MSD method, and then incorporated it into thulium doped fiber laser to generate stable ultrafast pluses at 2 μm. Stable soliton pulses emitting at 1930.07 nm were obtained with pulse duration of 1.24 ps, a 3-dB spectral bandwidth of 3.87 nm, an average output power of 130 mW, and signal-to-noise ratio (SNR) of 84 dB. To our knowledge, this is the first demonstration of Sb₂Te₃-based SA in fiber lasers at 2-μm regime.

The preparation process of microfiber-based Sb₂Te₃ SA was briefly explained as follows. Firstly, the microfibers with effective fused zone length of about 1 cm and waist diameter of about 20 μm were obtained by stretching single mode fibers (SMF-28e) on a taper drawing machine. Then Sb₂Te₃ film was deposited onto the waist of microfibers by MSD technique. Briefly, the microfibers and the Sb₂Te₃ target with purity of 99.99% were placed into the vacuum chamber. A mechanical pump is firstly used to ensure that the vacuum degree reached 1.0 × 10⁻¹ Pa in the vacuum chamber. Then the vacuum degree was pulled to 9.8 × 10⁻⁴ pa by using a molecular pump. After that, ionized Ar is excited by a pulsed direct-current (DC) power to bombard the Sb₂Te₃ target. The inspired Sb₂Te₃ plasma plume is then deposited onto the microfibers gradually. During the depositing process, the microfibers are rotated with the speed of 20 r/min to ensure the uniformity of Sb₂Te₃ film. For checking the Raman shift property of the Sb₂Te₃ film grown by MSD method, the thickness-dependent Raman spectrum is measured with a Raman spectrometer (Horiba, Raman Evolution hr800) excited by a 488-nm laser. Figure 1(a) shows the typical Raman spectra of Sb₂Te₃ films in the range from 100 cm⁻¹ to 500 cm⁻¹. The main peaks are located at about 118 cm⁻¹, 250 cm⁻¹, and 450 cm⁻¹, which are in good agreement with the Raman peaks observed in V–VI compound semiconductors.^[35] The peak at 118 cm⁻¹ has been identified as the E_g vibrational mode. The weak shoulders at 138 cm⁻¹, 159 cm⁻¹, 189 cm⁻¹, and 371 cm⁻¹ are probably from the secondary phase such as (Sb, O) in the film. In addition, its nonlinear absorption coefficient β and the nonlinear refractive index n₂ are determined with the Z-scan technique. A mode-locked Ti:sapphire oscillator-seeded regenerative amplifier is used to generate femtosecond pulses with a central wavelength of 800 nm, pulse duration of 100 fs, and repetition rate of 1 kHz. The open aperture (OA) Z-scan measurement is shown in Fig. 1(b). Typical saturable absorption curve is observed, demonstrating the effective saturable absorption at 800 nm. As for OA Z-scan, the normalized transmittance can be expressed as^[36] 1 where, I₀ is peak intensity at the focus, z is the position of the sample with respect to the focal position, z₀ is the diffraction length of the beam. The effective length of the sample L_eff could be obtained with the following equation 2 where L is the sample thickness, α₀ is the line absorption coefficient. As a result, the fitting parameter β could be unambiguously deduced by fitting the experimental data with Eq. (1). Besides, the closed aperture (CA) Z-scan measurement is also measured to estimate the nonlinear refractive index of Sb₂Te₃ film. The typical CA/OA curve is shown in Fig. 1(c). The Sb₂Te₃ film showed the feature of peak-valley profile, meaning that the sample is self-defocusing with a negative nonlinear refractive index. The experimental data can be fitted by using the following equation^[37] 3 where T(x) is the normalized transmittance, x = z/z₀. The only fitting parameter Δϕ could be obtained through Eq. (3). Therefore, the refractive index n₂ can be deduced by using the following equation 4 where k = 2π/λ is the wave vector. In our experiments, the parameters of β and n₂ were estimated to be about 9.0 × 10⁴ cm/W and −8.0 × 10⁸ cm²/W according to the actual experimental data points, respectively. The resulting value of β for Sb₂Te₃ is much higher than those of graphene oxide,^[38] MXene,^[39] transitionmetal dichalcogenide MoS₂,^[40] and black phosphorus,^[41] indicating a strong optical switch capability of Sb₂Te₃. The higher β value of Sb₂Te₃ also indicated that it could be exploited as potential material for nanophotonic devices, especially optical limiter.^[42] Furthermore, the recovery time of Sb₂Te₃ material is measured by using a pump-probe experiment. Figure 1(d) presents carriers dynamic relaxation process with the pump and probe wavelength of 400 nm. Its dynamic relaxation process exhibits an exponential decay. The process is fitted by exponential decay function of y = A * exp(−(x − x₀)/τ) + y₀, where τ represents the fast relaxation time during dynamic relaxation. The fast relaxation time is calculated to be about 5 ps with the theoretically fitting, showing the ultrafast response of the Sb₂Te₃ material.

Fig. 1.

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	Fig. 1. (color online) (a) Raman spectrum of Sb2Te3 film. (b) The normalized open-aperture Z-scan trace. (c) The normalized CA/OA Z-scan trace. (d) Pump–probe measurement of the carrier lifetime of Sb2Te3.

Figures 2(a)–2(b) were the optical images of the microfiber coated with the Sb₂Te₃ material. The Sb₂Te₃-coated areas are clearly seen from the scattered light with a 650 nm visible light guided through the microfiber, indicating the strong Sb₂Te₃-light interaction through evanescent wave coupling effect. The Sb₂Te₃ SA will be slowly oxidized when it is exposed to the air, which is greatly detrimental for the performance of the SA. In order to solve this problem, as shown in Fig 2(c), we have insulated the tapered region coated with Sb₂Te₃ film from the air by packaging it into a small copper tube which is filled with nitrogen to ensure the high reliability and stability of the SA. The nonlinear optical absorption properties of microfiber-based Sb₂Te₃ SA is measured by a typical balanced twin-detector method with a configuration shown in the inset of Fig. 2(d). A homemade mode-locked fiber laser operating at central wavelength of 1864 nm with 1.3-ps pulse duration and fundamental frequency of 10.393 MHz was utilized as the test laser source. As shown in Fig. 2(d), the nonlinear saturable absorption curve is fitted by the two-level saturable absorber model 5 where α(I) is the absorption, I is the optical intensity, α_s is the modulation depth, α_ns is the nonsaturable loss, and I_sat is the saturable intensity. In our experiment, the microfiber-based Sb₂Te₃ SA shows remarkable nonlinear properties with α_s, α_ns, and I_sat of 38%, 31.2%, and 3.3 MW/cm², respectively. Besides, the linear transmission of the fabricated absorber is also measured as ∼ 27.7% at 1900 nm by using the homemade amplified spontaneous emission (ASE) of Tm-doped gain fiber, and this result is basically in agreement with the fitting value obtained from the nonlinear saturable absorption measurement.

Fig. 2.

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	Fig. 2. (color online) Optical microscope images of the waist region of sample without (a) and with (b) the guiding 650-nm light. (c) A photograph of the prepared Sb2Te3 SA. (d) Saturable absorption property of SA. The inset was the schematics of the balanced twin-detector equipment.

We employed the microfiber-based Sb₂Te₃ SA in thulium-doped fiber (TDF) laser to further investigate its nonlinear optical properties. The experiment setup schematics is shown in Fig. 3. A 1550 nm/2000 nm wavelength-division multiplexer (WDM) is used to deliver the 1550-nm laser into the ring cavity. A piece of TDF (Nufern, SM-TSF-9/125) with the length of 3.2 m is used as the gain medium. An optical coupler (OC) with 50/50 splitting ratio is utilized to extract output pulses from the cavity. The total cavity length is about 14.2 m. A polarization controller (PC) is used to adjust the state of the polarization of the oscillating beam within the cavity, and a polarization insensitive optical isolator (PI-ISO) is used to prevent back reflection in the cavity and ensure unidirectional laser operation. Besides the TDF, the other fibers including the tail fibers of each element are SMF-28e fibers. All components are polarization-independent in the fiber laser systems, which can avoid the self-started mode-locking operation induced by nonlinear polarization evolution (NPE). It should be noted that the direction of the operating laser is opposite to that of the pump laser, which avoids the output of residual pump power.

Fig. 3.

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	Fig. 3. (color online) Schematic diagram of passively mode-locked thulium doped fiber laser

The characteristics of the output light are monitored by an optical spectrum analyzer (Yokogawa AQ6375B) and an oscilloscope (ROHDE & SCHWARZ RTO2024) with a photo detector (EOT ET-5000 F), respectively. A radio frequency (RF) spectrum analyzer (ROHDE & SCHWARZ FSV13) is employed to record the RF spectrum of the mode-locking operation. In addition, the pulse duration is measured with a commercial autocorrelator (APE Pulsecheck).

Self-starting mode-locking operation is observed when the pump power reaches 400 mW. Compared with normal Er-doped fiber lasers,^[43,44] our TDF laser exhibits a little higher lasing threshold due to the relatively large insertion loss caused by the small waist diameter of the microfiber which ensures sufficient evanescent field interaction to obtain stable mode-locking pulse operation. During operation, the pump power can be increased up to 1.5 W without losing the mode locking. The mode-locking performance is characterized at the maximum pump power of 1.5 W. Figure 4(a) shows the typical spectrum of mode-locking centered at 1930.07 nm with a 3-dB spectral bandwidth of 3.87 nm. The presence of Kelly sidebands on the spectrum confirmed that the mode-locked laser is operating in the soliton regime for typical all-anomalous dispersion lasers. The spectrum of output pulse is also measured from 1200 nm to 2000 nm and no signal is observed at 1550 nm, verifying that there is no output of residual pump power. Figure 5(b) shows the RF spectrum of the laser at a fundamental repetition rate of 14.51 MHz measured in a 0.6-MHz span with a 10-Hz resolution. The signal-to-noise ratio (SNR) is 84 dB. A wide 1-GHz span with a resolution of 1 kHz presented in the inset of Fig. 5(b) indicating the low fluctuations and the high stability of the mode locked pulse operation. The autocorrelation of the laser output pulse together with a sech² fitting is depicted in Fig. 5(c). The pulse duration after deconvolution is 1.24 ps. The time-bandwidth product (TBP) is calculated to be about 0.386, implying that the output pulses of our laser are slightly chirped. Additionally, the inset shows the autocorrelation trace with a large range of 50 ps, confirming the single-pulse operation without any pre- or post-pulses. The output average power is recorded in Fig. 5(d). The laser system produced a maximum average power of 130 mW at the pump power of 1.5 W, corresponding to the maximum pulse energy of 8.96 nJ. The slope efficiency is 11%. The large pulse energy is attributed to the microfiber-based Sb₂Te₃ SA with evanescent wave interaction which can avoid the transmittance of light through the SA material directly so as to prevent the dispersion caused by SA material efficiently and attractive for high power tolerance. The inset shows the oscilloscope trace of the pulse-train with the pulse interval of about 68.9 ns, well consistent with the cavity length.

Fig. 4.

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Fig. 4. (color online) (a) Optical spectrum with the bandwidth of 3.87 nm. (b) Radio frequency (RF) spectrum at fundamental frequency of 14.51 MHz with 10-Hz resolution. The inset was the RF spectrum of 1-GHz span. (c) Autocorrelation trace for output pulse with a pulse duration of 1.24 ps with sech2 fit. The inset shows the autocorrelation trace with a large range of 50 ps. (d) Relationship between the pump power and output power. The inset is the oscilloscope trace of the pulse-train.

Moreover, in order to examine the influence of the microfiber-based Sb₂Te₃ SA in thulium-doped mode-locked fiber laser system, we removed the Sb₂Te₃-SA and placed another uncoated microfiber with the same size. In this case, even if the PC is rotated and the pump power is adjusted gradually and carefully in a large range, the passively mode-locked pulse cannot be observed. The contrastive results demonstrate that the Sb₂Te₃-SA is responsible for the mode-locked operation.

The comparisons of mode-locked lasers operating at 2 μm based on different low-dimension materials are listed in Table 1. The output average power and the pulse energy are much larger than that mode locked by other low-dimensional materials, indicating that Sb₂Te₃ has significant advantages in obtaining high-power mode-locked fiber laser. These comparisons suggested that Sb₂Te₃ material may be served as an excellent SA at 2 μm that can successfully compete with other extensively studied low-dimensional materials such as carbon nanotube and graphene.

Table 1.

Table 1.

Table 1. Comparison of passively mode-locked fiber lasers around 2 μm. . SA λ/nm Δλ/nm τ/ps Pavg/mW E/nJ Reference Carbon nanotube 1927 52.8 0.152 4.85 0.19 [4] Graphene 1910 5.14 0.773 115 6 [21] WSe2 1863.96 3.19 1.16 32.5 2.86 [19] Black Phosphorus 1910 5.8 0.739 1.5 0.04 [22] Bi2Te3 1935 5.6 0.795 1 0.36 [32] Sb2Te3 1930.07 3.87 1.24 130 8.96 this work

Table 1. Comparison of passively mode-locked fiber lasers around 2 μm. .

In conclusion, microfiber-based Sb₂Te₃ SA is prepared by MSD technique and its nonlinear optical properties are investigated. The microfiber-based Sb₂Te₃ SA is embedded into thulium-doped fiber laser with typically ring cavity structure. Stable soliton pulses emitting at 1930.07 nm are obtained with pulse duration of 1.24 ps, a 3-dB spectral bandwidth of 3.87 nm, an average power of 130 mW, and SNR of 84 dB. To our knowledge, this is the first demonstration of Sb₂Te₃-based SA in fiber lasers at 2-μm regime. Our results suggested that microfiber-based Sb₂Te₃ SA can be used as an excellent photonic device for ultrafast pulse generation in 2-μm regime, and MSD technique opens a promising way to produce high-performance SA with large modulation depth, low saturable intensity, and high power tolerance, which are beneficial for high power and ultrafast pulse generation with high stability.