164 fs mode-locked erbium-doped fiber laser based on tungsten ditelluride

1 Introduction

Ultrafast fiber lasers have attracted considerable attention in medical treatment, material processing, femtosecond time spectroscopy, nano-scale imaging and communication due to high pulse energy, low thermal effect and outstanding spatial/temporal resolution [1]. In particular, passive mode-locking method, which employs SAs as key mode-lockers has been recognized as an economical and efficient method to implement femtosecond pulses [2], [3], [4]. Therefore, the explorations of some SAs with excellent performance may become the point of penetration in the development and innovation of ultrafast lasers.

So far, in addition to semiconductor saturable absorber mirror (SESAM) which has been commercialized, two-dimensional (2D) nanomaterials have become candidates for the next generation of SAs, because of their unique electronic and optical properties [5], [6], [7], [8]. With the deepening of research, the types of SA materials continue to expand, such as graphene, topological insulators, black phosphorus, TMDs, etc. [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. As the representative materials of TMDs, molybdenum disulfide (MoS₂), and tungsten disulfide (WS₂) have made impressive achievements in laser as SAs. It has been reported that the third-order nonlinear susceptibility of MoS₂ is higher than that of graphene at 800 nm [33]. Moreover, both mode-locked or Q-switched lasers based on MoS₂ and WS₂ have been realized in the wide band of 1–2 μm. Undoubtedly, the diversity of TMDs makes them occupy the essential status in the exploration of SA materials.

Tungsten ditelluride (WTe₂), is also an important member of TMDs, inherits the thickness-dependent band gap structure of TMDs. The band gap of bulk and monolayer WTe₂ are 0.7 and 1.18 eV, which is lower than that of commonly used MoS₂ or WS₂. This small band gap predicts high electron mobility and facilitates the application of broadband absorption [34]. In addition, the fast relaxation process of WTe₂ occurs within 1 ps as demonstrated in Ref. [35]. SAs with fast response timesare less affected by amplified spontaneous emission [36]. Moreover, the fast relaxation time of WTe₂ makes it suitable for the generation of ultrashort pulses. Although Q-switched laser based on WTe₂ has been realized in the previous work [13], the advantages of femtosecond mode-locked laser in small thermal effect, high temporal and spatial resolution inspire us to further explore the application potential of WTe₂ in mode-locked lasers.

In this work, an erbium-doped fiber laser (EDFL) based on WTe₂ is demonstrated. The WTe₂ is fabricated by the magnetron-sputtering technology (MST). WTe₂ SA adopts the structure of microfibre, which strengthens the interaction between light and materials, and avoids the direct photoablation damage of materials. The prepared WTe₂ SA shows nonlinear absorption coefficient of −3.78×10⁻⁵ cm/W and modulation depth of 37.95%. After the WTe₂ SA is integrated into the ring cavity, a sub-170 fs mode-locked laser is achieved at 1557.71 nm. High signal-to-noise ratio (SNR) and small standard deviation of output power indicate the stability of EDFL. Our research highlights the potential of WTe₂ in the realization of ultrafast pulses and paves the way for its further application in the field of ultrafast photonics.

2 Preparation and characterization of WTe₂ SA

In the preparation process, WTe₂ is coated on microfiber to form WTe₂ SA. The microfiber used is uniformly stretched from single-mode fiber (SMF 28e), its waist diameter and effective interaction length are 15 μm and 1 cm. Considering the difficulty of adhesion in the fragile microfiber, MST is suitable for the preparation, which not only has a high film formation rate, but also prepares films with great uniformity and large scale. The detailed preparation process is as follows. First, the microfiber and WTe₂ target with the purity of 99.99% are fixed in a vacuum chamber, the vacuum degree of that chamber is 9×10⁻⁴ Pa. Then, argon ion ionized at high pressure bombards the target surface. Because of the high energy of sputtered target atoms, the diffusion ability of atoms during deposition is strong, which leads to the compactness of the deposited structure and the strong adhesion between the film and the substrate. Meanwhile, the microfiber rotates at a constant speed of 20 r/min to guarantee the uniformity of coating.

The nanoscale observation of surface morphology of prepared WTe₂ is realized by scanning electron microscope (SEM). As shown in Figure 1A and B, WTe₂ particles are compact and uniformly arranged. The thickness of WTe₂ film is 100 nm as shown in Figure 1C. Furthermore, the linear transmission of the WTe₂ is shown in Figure 1D, the transmittance near 1550 nm is about 61.6%. We found that WTe₂ shows absorption in a wide wavelength range from 1100 to 1800 nm. Therefore, the corresponding relationship between band gap and summation of phonon energy can be given by the Tauc plot [37], [38]. From Figure 1E, the band gap of the prepared WTe₂ is about 0.2 eV, it is probably due to the phase type of the WTe₂. According to Ref. [34], bulk WTe₂ in the Td (or 1T) structure has a 0.21 eV band by using the density functional theory (DFT). From the Raman spectrum of WTe₂ mentioned in the previous work [13], the vibration modes of WTe₂ here is consistent with that of Td-WTe₂ [39], which indicates that the phase of WTe₂ in our experiment is Td. Thus, the band gap of 0.2 eV is attributed to the Td phase of the bulk WTe_2. The nonlinear absorption properties of WTe₂ film is investigated by an open-aperture (OA) Z-scan measurement. A 100-fs Ti: sapphire amplifier at 800 nm with a repetition rate of 1 kHz pumps the measurement system. The excitation power of the mentioned driving laser is 0.4 mW. The traditional OA Z-scan data and fitting trace are shown in Figure 1F. The nonlinear absorption coefficient (β) of WTe₂ is −3.78×10⁻⁵ cm/W. The power-dependent nonlinear absorption of WTe₂ is exhibited by balanced twin-detector measurement. The light source is a 600 fs home-made nonlinear polarization evolution mode-locked laser with a repetition rate of 120 MHz at 1550 nm. Typical power-dependent nonlinear absorption trends and fitting curve are indicated in Figure 1F. The modulation depth (α_s) of WTe₂ SA is up to 37.95%, other details of saturation intensity (I_sat) and non-saturable loss (α_ns) are listed in Figure 1G. The modulation depth is slightly improved compared with the previous work [13], which is related to the increase in the relaxation time and the enhancement of light-matter interaction caused by the increase in thickness [40], [41], [42], [43]. The nonlinear behavior of WTe₂ and some commonly used SAs are listed in Table 1. From Table 1, Sb₂Te₃ has the strongest nonlinearity. In addition, the nonlinear absorption of MoTe₂ and WTe₂ are remarkable, while the modulation depth of WTe₂ is slightly better than that of MoTe₂.

Figure 1:

Characterization and nonlinear characteristics of WTe₂.

(A) SEM image of WTe₂-coated microfiber, (B) surface morphology, (C) vertical thickness, (D) linear absorption spectra, (E) Tauc plot, (F) OA Z-scan, (G) power-dependent absorption of WTe₂.

3 Experiment

The experimental device diagram of the WTe₂-based EDFL is shown in Figure 2, including a laser diode (LD), a wavelength division multiplexer (WDM), a section of erbium-doped fiber (EDF), an optical coupler (OC), a polarization controller (PC), and an isolator (ISO). The LD pumps the whole system through a 980/1550 nm WDM. As the gain fiber, EDF amplifies the pulse. PC generates stress birefringence through the mechanical extrusion of the fiber cross section. The ISO is used for guaranteeing orderly unidirectional laser transmission. A small portion of the light is exported by a 20:80 OC for real-time observation and data recording. The length of the whole ring cavity is 2.98 m, which contains the EDF of 0.58 m and single-mode fiber of 2.25 m.

Figure 2:

Experimental device diagram of the WTe₂-based EDFL.

4 Results and discussion

Although experimental devices similar to previous work are adopted [13], an additional 50 cm single-mode fiber was added, and another SA showing different nonlinearity was used. When the pump power reaches the mode-locked threshold of 244 mW, self-starting mode-locking is observed after fine-tuning the PC. And when it slowly increases to 630 mW, the main operating parameters of achieved laser are measured by the light exported through OC. The obtained mode-locked pulses are uniform and own a fixed time interval of 14 ns as shown in Figure 3A. The autocorrelation trace of the single pulse fitted by sech² profile is shown in Figure 3B, the pulse duration is 164 fs. The coefficient 1.54 is the conversion coefficient of pulse duration indicated in autocorrelation curve and actual pulse duration. Subsequently, long-term spectral samplings are presented in Figure 3C, informing the mode-locked system works at 1557.71 nm with a 3 dB bandwidth of 31.42 nm. The time-bandwidth product (TBP) is 0.6366. As shown in Figure 3D, the fundamental frequency at 71.3 MHz owns the SNR up to 75 dB with a resolution of 1 Hz. The illustrations show the distribution of frequency multiplications over a range of 800 MHz.

Figure 3:

The operating parameters of achieved laser.

(A) Oscilloscope signals; (B) Pulse duration; (C) Spectrum; (D) Radio frequency (RF) spectrum.

The law of output power increasing with pump power is shown in Figure 4A, the slope of each point is approximately the same. The maximum output power is 36.7 mW. In the laboratory environment, we monitored its output power stability for about 8 h, and the output power of the laser is recorded once a second. The data of nearly 30,000 points are almost distributed in a straight line in Figure 4B, and the standard deviation of the samples is 0.56 mW. From the perspective of high SNR and small standard deviation, the system is relatively stable.

Figure 4:

Output power and stability of laser.

(A) The law of output power increasing with pump power; (B) Long time output power monitoring.

Further, comparisons of mode-locked lasers based on some SAs have also been launched in Table 2. The results in Table 2 are all based on the MST method and the structure of the tapered fiber. After eliminating the interference of preparation method and the structure of SA, we found that the pulse duration of WTe₂-based laser is relatively short. The results highlight the potential of WTe₂ in the realization of ultrafast pulses.

5 Conclusion

In conclusion, the ultrafast EDFL based on the WTe₂ SA has been implemented. The proposed WTe₂ SA has been prepared by coating WTe₂ on microfiber with MST. This WTe₂ SA has shown impressive optical nonlinearity with nonlinear absorption coefficient of −3.78×10⁻⁵ cm/W and modulation depth of 37.95%. After integrating it into the ring cavity, a sub-170 fs mode-locked laser has been achieved at 1557.71 nm. The laser has the maximum output power of 36.7 mW and SNR of 75 dB, and the stability of the output power has been maintained during the monitoring for nearly 8 h. In addition, after eliminating the interference of preparation method and SA structure, the proposed WTe₂-based laser is not inferior, especially in the realization of ultrashort pulses. Our work shows the potential of WTe₂ in ultrafast photonics, and opens a possible way to achieve high-performance laser with high power and ultrafast pulse.