Ultrafast fiber lasers, with their ultra-short pulse duration and ultra-high peak power, have significant applications in optical communication, medical diagnostics, precision measurement, astronomical detection, fundamental scientific research, and other fields^[1-4]. Among the available ultrafast pulse generation technologies, a passively mode-locked fiber laser based on a saturable absorber (SA) has become one of the most effective methods due to its compact structure, low cost, and high compatibility^[5-10].

From the SA point of view, numerous SAs have been explored to conduct mode-locked operations. Some are effective SAs, such as nonlinear polarization rotation (NPR)^[5], nonlinear optical loop mirror (NOLM)^[7], and the nonlinear Kerr effect^[8-9]. Others are real SAs such as semiconductor saturable absorber mirrors (SESAM)^[10], and SAs based on two-dimensional (2D) materials including, but not limited to, single-walled carbon nanotubes^[5], graphene^[6], and other 2D layered materials^[11-15].

Each SA has both advantages and disadvantages. NPR and NOLM have all-fiber structures and can sustain higher light intensity. However, the pump power threshold for mode-locked operation is high. Besides, they are highly sensitive to environmental disturbance, which results in low system stability and limits practical applications. Comparatively, SESAM is one of the most successfully commercialized real SA devices so far. However, SESAM suffers from high costs, complex preparation, limited operating bandwidth, and a low damage threshold^{[6, 10]}. Fortunately, spurred by the advent of graphene^[16-17], a wide range of 2D materials have been recognized and demonstrated to be excellent SAs. Sufficient research has shown that applying 2D material-SAs as broadband, cost-efficient, and widely used optical modulators for ultrafast laser generation is a fast-developing field with broad commercial prospects^[18-21].

MAX phase materials (MAX-PM) are layered ternary carbides or nitrides with ceramic and metallic properties^[22-26]. Generally, the formula for MAX-PM is M_n+1AX_n, where M represents one kind of transition metals such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, etc. X is a carbon or nitrogen with n = 1, 2, 3, and A represents an element belonging to group III, IV, V, or VI, such as Al, Ga, In, Si, Ge, Sn, P, As, S, etc. Besides, the structure of MAX-PM is characterized by alternating layers of M and A atoms, forming a tightly packed hexagonal layered structure with X atoms filling octahedral voids. Due to the special atomic arrangement, MAX-PM is a combination of metal and ceramic. In its ceramic state, MAX-PM has exceptional resistance to oxidation and high temperature. Conversely, in its metallic state, it has high-temperature plasticity and excellent electrical and thermal conductivity. These hybrid characteristics make it highly desirable for applications in nuclear engineering, high-temperature devices, and the aerospace industry.

On the other hand, the emergence of MAX-PM has undoubtedly promoted the development of SAs. The excellent nonlinear saturable absorption features, such as impressive modulation depth, flexibly tunable bandgap, and high electron density around the Fermi level, make MAX-PM a formidable contender in the SA family^[27-32]. In particular, Nb₄AlC₃ is a member of the MAX-PM family. In addition to the commonality of MAX-PM, the valence and conduction bands of the Nb₄AlC₃ overlap significantly, and the band gap at the Fermi energy level is zero. Such properties endow Nb₄AlC₃ with good photoelectric response properties^{[27, 29]}. Furthermore, Nb₄AlC₃ has high antioxidant properties involving the formation of a protective layer due to atmospheric oxidation of aluminum inside the Nb₄AlC₃^[26-28]. Considering the above analysis, an environmentally stable Nb₄AlC₃-based optoelectronic device is expected. However, Nb₄AlC₃ material has rarely been researched in the field of nonlinear optics so far.

In this study, passively mode-locked erbium-doped fiber (EDF) lasers were successfully modulated by a tapered-fiber-structured Nb₄AlC₃-SA. The Nb₄AlC₃ dispersion was prepared using the liquid phase exfoliation method. Then, tapered-fiber-structured Nb₄AlC₃-SA was fabricated, and the prepared Nb₄AlC₃’s saturation intensity and modulation depth were tested. They were found to be 2.02 MW/cm² and 1.88%. A conventional soliton (CS) mode-locked EDF laser was achieved with output central wavelength, pulse duration, and pulse repetition rate of 1565.65 nm, 615.37 fs, and 24.63 MHz, respectively. To the authors’ knowledge, this is the first report of Nb₄AlC₃-SA applied as a mode-locker. This confirms that Nb₄AlC₃ possesses excellent nonlinear saturable absorption properties, outstanding modulation capability, and provides valuable references for further research of air-stable ultrafast photonic devices with Nb₄AlC₃ materials.

In recent years, top-down or bottom-up methods, including micromechanical exfoliation (ME), chemical vapor deposition (CVD), and liquid-phase exfoliation (LPE), have been successfully applied to prepare mono- or few-layer 2D material nanosheets^[33-36]. Each method has its advantages and limitations in practical applications. ME can produce any kind of 2D material nanosheets at high quality, but the production efficiency and size are limited. CVD, a bottom-up preparation method, can synthesize large-scale monosheet films with high purity, large area, and uniform thickness. However, the method is expensive and complex, and the preparation is often accompanied by the follow-up process of film transfer. LPE, always assisted by high-intensity ultrasonication, is a simple and cost-effective method to fabricate nanosheets at ambient conditions. For the LPE method, forceful solvent-2D nanoflake-interaction (internal force) and sonication energy (external force) are critical factors in exfoliation efficiency^[34-35]. In this paper, the Nb₄AlC₃ nanosheets were prepared using the LPE method.

Tapered fibers are widely adopted to fabricate SA devices. On the one hand, high dispersion and nonlinearity of optical fiber can be introduced by changing the size and structure in the tapering process, which facilitates the laser system to compress pulse duration efficiently^[37-39]. On the other hand, large-area thin 2D nanosheet film integrated with a tapered fiber structure can offer tight optical confinement for enhancing the light-material interaction, thereby strengthening the modulation effect. Besides, utilizing the evanescent field of the tapered fiber protects the 2D material SA from thermal damage.

In this study, a tapered-fiber-structured Nb₄AlC₃-SA was prepared for the proposed mode-locking. First, the Nb₄AlC₃ nanosheets were prepared by the LPE method, and a tapered fiber was processed from a piece of commercial fiber (SM-28). Then, the SA was fabricated by coating the Nb₄AlC₃ nanosheets onto the twisted part of the tapered fiber. 20 ml of deionized water was mixed with 10 mg of Nb₄AlC₃ powder. Then, the mixture underwent ultrasonication with a repetition rate and power of 40 kHz and 300 W at a room temperature of 25 °C for 12 h. The AS-prepared Nb₄AlC₃ solution subsequently underwent high-speed centrifugation at 1500 rpm for 10 min. The 70% supernatant was collected for characterization and SA fabrication. Like our previous work^[39], the tapered fiber was fabricated by a hydrogen-oxygen flame-based fiber processing machine from a piece of standard single-mode fiber (SM 28). The waist diameter of 15 μm and tapered length of 1 cm were precisely controlled by the preset program. Then, using a home-made fiber holder, the tapered part was exposed to ambient air, rotated vertically in the direction of the optical fiber, and tilted repeatedly along the fiber direction simultaneously. After the as-prepared Nb₄AlC₃ nanosheets supernatant was slowly dropped onto the surface of the waist area and evaporated until dry, a tapered-fiber-structured Nb₄AlC₃-SA was obtained.

Subsequently, the Nb₄AlC₃ nanosheets were characterized (Fig. 1, color online). Fig. 1(a) depicts the scanning electron microscopy (SEM) (Czech Republic TESCAN MIRA LMS) with a resolution of 500 nm. The surface texture was found to have a good laminar structure. Fig. 1(b) presents the transmission electron microscope (TEM) (Japan JEOL JEM 2800). The clear lattice structure confirms the successful exfoliation of the Nb₄AlC₃ powder. The X-ray diffraction (XRD) (AXS D8 Advance, Bruker, Billerica, MA, USA) pattern is shown in Fig. 1(c). It was observed that all diffraction peaks are consistent with previous reports^{[28, 31]}, which confirms the Nb₄AlC₃ truth of the sample. Raman spectroscopy can effectively distinguish different substances and accurately identify the constituents of substances, so the Raman spectrum of the Nb₄AlC₃ sample was tested and is shown in Fig. 1(d). In this figure, the four typical peaks of the Nb₄AlC₃ sample are apparent and analogous to the previous report^{[29, 32, 40]}.

The intensity-dependent nonlinear saturable absorption property of Nb₄AlC₃-SA was investigated using a balanced twin-detector. As shown in Fig. 2(a) (color online), a home-made fiber laser with center wavelength, pulse duration, and repetition rate of 1560 nm, 500 fs, and 10 MHz was used as the light source. After an attenuator, the injected light was equally divided into two beams by a 50/50 optical coupler. The signal light passing through Nb₄AlC₃-SA was tested by power meter I, while the reference light was tested by power meter II. Fig. 2(b) (color online) shows the recorded data as blue dots. It was observed that the transmission efficiency of the SA increased with pulse intensity, which is a typical characteristic of nonlinear saturable absorption. When the light intensity reaches 1.35 MW/cm², the transmission reaches the maximum, indicating the saturation of the SA. The saturation intensity and modulation depth can be determined by the formula^[41]:

$ T\left(I\right)=1-{T}_{{\mathrm{ns}}}-\Delta {T}\cdot\mathrm{e}\mathrm{x}\mathrm{p}\left(-\frac{I}{{I}_{{\mathrm{sat}}}}\right) \quad,$ (1)

where T(I) and ∆T are the transmission and modulation depth, I and I_sat are the laser intensity and the saturation intensity, and T_ns is the non-saturable loss. Finally, the prepared SA’s saturation intensity and modulation depth were deduced to be 2.02 MW/cm² and 1.88 %.

The results demonstrated above provide a good understanding of the nonlinear optical properties of Nb₄AlC₃ nanosheets. This exploration of these properties offers a good reference for further development in photonics and photoelectric applications of Nb₄AlC₃ materials.

Using the same detector in [39], the linear transmittance of the prepared Nb₄AlC₃-SA was studied, with the results depicted in Fig. 3 (color online).

The red and black curves in the inset represent the output amplified spontaneous emission (ASE) spectra with and without Nb₄AlC₃-SA inserted into the optical path, respectively. The quotient of the former data divided by the latter data is the linear transmittance, which is presented as a blue curve. Ultimately, the linear transmittance at 1565.65 nm was determined to be 88.64%, indicating that the prepared SA is suitable for photonics devices with low insertion loss.

The alignment-free experimental setup of the mode-locked EDF laser using Nb₄AlC₃-SA as a modulator is schematically depicted in Fig. 4. It had a ring cavity configuration with a total length of about 8.4 m. A 980 nm LD with a maximum output power of 500 mW was employed as a pump source, and a 980/1550 nm wavelength division multiplexer (WDM) was used to couple the pump light into the cavity. An in-line polarization controller (PC) was used to fine-tune the birefringent environment or adjust the polarization state of the cavity, but it was not fundamental to realizing the mode-locking operation. In addition, a polarization-independent isolator (PI-ISO) was used to ensure the unidirectional running of the laser. A 40-cm-long piece of EDF (Liekki Er-110-4/125) with a −46 ps/nm/km dispersion parameter was used as the gain medium. The net dispersion of the whole ring cavity was calculated to be about −0.15 ps², facilitating CS pulse shaping through the interaction of self-phase modulation and anomalous group velocity dispersion (GVD). 10 % of the signal was extracted from the cavity by an optical coupler (OC). The output performance was monitored by a digital oscilloscope (OSC) (Wavesurfer 3054, LeCroy, Teledyne, USA) with a 3-GHz photo-detector (PD 03), an optical spectrum analyzer (OSA) (AQ6370B, Yokogawa, Tokyo, Japan), a radio frequency spectrum analyzer (FPC1000, Rohde & Schwarz, Jena, Germany) and a power meter (PM3, Molectron, Barrington, NJ, USA). The pulse duration was measured by an optical autocorrelator (AC) (Femtochrome FR-103 XL, Berkeley, USA).

When the Nb₄AlC₃-SA was absent from the cavity, the system only operated in a continuous wave (CW) regime in the entire adjustment range of the PC and pump power, which means that the devices currently used in the ring cavity can not provide enough modulation for pulse generation. By incorporating the tapered-fiber-structured Nb₄AlC₃-SA into the ring cavity, with the PC’s orientations being fine-tuned and the pump power being increased beyond the mode-locked threshold, the system evolves into CS operation on its own (called self-starting) and remains stable in the pump power range of 100-340 mW.

Fig. 5 (color online) shows the spectral and temporal performance of the CS operation. As illustrated by Fig. 5(a), the out spectrum centered at 1565.65 nm with a 3 dB bandwidth of 7.78 nm. Several pairs of Kelly sidebands were symmetrically distributed on both sides of the center wavelength, verifying the typical CS characteristics. Studying the formation mechanism of the Kelly sidebands found that the losses existing in the laser cavity promote the dispersion wave generation and that when the phase difference between the soliton and the dispersion wave reaches 2π, strong interference occurs, and Kelly sidebands are generated. The wavelength location of the Kelly sidebands usually matches the dispersion as described by the following formula^[42]:

$ \Delta \lambda =\frac{{\lambda }^{2}}{0.576{\text{π}} c\tau }\sqrt{-1+\frac{4{\text{π}} {\left(0.565\;7\tau \right)}^{2}}{k''L}}\quad, $ (2)

where c is the velocity of light (the same in the following (3) and (4)), k′′ is the average GVD of the total cavity, L is the cavity length, τ is the half-height full width of the pulse, ∆λ is the wavelength offset of the Kelly sideband from the center wavelength of the CS, and λ is the central wavelength of the CS spectrum. The spectral interval ∆λ is inversely proportional to the net dispersion of the resonant cavity, which indicates that when the net dispersion of the cavity gradually approaches zero, the ∆λ gradually becomes wider and eventually exceeds the gain spectral bandwidth. Consequently, stretched soliton (SS) with a square spectral top will be generated.

The pulse sequence recorded within 600 ns is presented in Fig. 5(b). The temporal interval between adjacent pulses was measured at 40 ns, as confirmed by the repetition rate of 24.63 MHz (f=1/T) in the RF spectra depicted in Fig. 5(c). The signal-to-noise ratio (SNR) was measured at 56.3 dB. Combining the broadband RF spectrum with good flatness, as shown in an inset in Fig. 5(c), demonstrates the high stability of the mode-locked operation. According to the following formula:

$ L=\frac{c}{nf} \quad,$ (3)

where n, L, and f are the refractive index, cavity length, and the CS pulse repetition rate, respectively, the cavity length is theoretically calculated to be 8.34 m, consistent with the actual length of 8.4 m.

As presented in Fig. 5(d), with a sech² pulse profile fitting, the pulse duration was determined to be 615.37 fs. According to the formula:

$ TBP=\frac{c\Delta \lambda \Delta t}{{\lambda }^{2}}\quad, $ (4)

where λ, Δλ, and Δt are the center wavelength, 3 dB bandwidth, and pulse duration of the CS, respectively, the time-bandwidth product (TBP) is calculated to be 0.585, meaning the existence of chirp.

Fig. 6(a) (color online) illustrates that both average output power and pulse energy maintained a linear growth trend with the increase of pump power. At the pump power of 340 mW, the average output power reached 6.45 mW, corresponding to a pulse energy of 0.262 nJ. To further confirm the long-term stability of the laser system, the output spectra at the highest power level were monitored for four hours at half-hour intervals. As shown in Fig. 6(b) (color online), no apparent changes were observed, demonstrating the system’s excellent stability at room temperature.

Table 1 compares current and previous MAX-PM mode-locked EDF lasers relatively comprehensively. It can be seen that this paper reveals superior performance, particularly outstanding in terms of pulse duration. The results demonstrate the feasibility of using Nb₄AlC₃-SA as a mode locker in fiber lasers, which fits well with the authors’ original expectations.

In the experiment, by using pure tapered fiber (directly exposed to the air) or removing the Nb₄AlC₃ SA, CW was always observed despite the fact that the PC and the pump power were tuned over the entire range. In contrast, modulated operation was readily achieved by depositing the Nb₄AlC₃ nanosheets on tapered fiber. No additional nonlinear response from other devices was observed during the experiment. These results show that saturable absorption is purely caused by the as-prepared Nb₄AlC₃-SA.

During the experiment described in this paper, although the mode-locking phenomenon disappeared at pump power higher than 340 mW, when the pump power was modulated from 500 mW back into the 100−340 mW range, the mode-locking always recovered. These results verify that the Nb₄AlC₃-SA has a bleaching pump power of around 340 mW and a much higher optical damage threshold.

A stable CS mode-locked EDF laser with a pulse duration of 615.37 fs was obtained based on the tapered-fiber-structured Nb₄AlC₃-SA. This paper systematically verifies that Nb₄AlC₃-SA possesses excellent nonlinear saturable absorption properties, opening new possibilities for further investigation of SAs.