2 μm Passively Mode-Locked Thulium-doped Fiber Lasers with Ta2AlC-deposited Tapered, Side-polished and D-shaped Fibers

In this work, mode-locked thulium-doped ber lasers operating in the 2 µm wavelength region were demonstrated using tantalum aluminum carbide (Ta 2 AlC)-based saturable absorbers (SAs) utilizing the evanescent wave interaction. The Ta 2 AlC MAX Phase was prepared by dissolving the Ta 2 AlC powder in isopropyl alcohol (IPA) and then deposited onto three different evanescent eld-based devices, which were the tapered ber, side-polished ber (SPF), and D-shaped ber. Flame-brushing and wheel-polishing techniques were used to fabricate the tapered and D-shaped bers, respectively, while the side-polished ber was purchased commercially. All three SA devices generated stable mode-locked pulses at center wavelengths of 1937, 1931, and 1929 nm for the tapered, side-polished, and D-shaped bers. The frequency of the mode-locked pulses was 10.73 MHz for the tapered ber, 9.58 MHz for the side-polished ber, and 10.16 MHz for the D-shaped ber. The measured pulse widths were 1.678, 1.734, and 1.817 ps for each of the three SA devices. The long-term stability of the mode-locked lasers was tested for each conguration over a 2-hour duration. The lasers also showed little to no uctuations in the center wavelengths and the peak optical intensities, demonstrating a reliable, ultrafast laser system.


Introduction
The discovery of optical bers by Charles Kao and George A. Hockam in 1966 gave the inroads for optical ampli er development with a wavelength range of 1.46 µm to 1.53 µm 1,2 , 1.53 µm to 1.565 µm 3, 4 and 1.565 µm to 1.625 µm [5][6][7][8][9][10] . Consequently, the development of laser con guration such as pulsed 11,12 , dual and multiwavelength [13][14][15][16][17][18][19][20][21][22] , and optical sensors [23][24][25] has been the focus of many research laboratories. There have been numerous activities on pulsed laser operating at wavelengths 1 µm and 1.5 µm, of interest will be to generate short pulses in the 2 µm wavelength region for several applications such as in spectroscopy 26 , gas detection 27 , laser ablation 28 , long-range light detection and ranging (LIDAR) 29 , plastic and glass processing 30 as well as in the medical eld 31 . Lasing in the 2 µm is commonly achieved using thulium-doped bers (TDFs) as the gain medium, as TDFs have a broad ampli cation range of 400 nm, ranging from 1700 to 2100 nm 32 . The 2 µm wavelength region is also of interest as it coincides with the absorption lines of water (H 2 O) and several leading greenhouse gases such as carbon dioxide (CO 2 ) and nitrogen dioxide (NO 2 ) 33 . Although lasing in the 2 µm region has traditionally been demonstrated with continuous wave (CW) outputs, recent advances in ber laser technologies have increased the development of 2 µm pulsed ber lasers that can generate short pulses with pulse durations in the pico-or femtosecond range.
Pulsed laser generation can be achieved either by Q-switching or mode-locking. In the former, short pulses with high output energies can be produced using an optical component incorporated in the laser cavity to modulate the Q-factor. In the latter, the oscillating longitudinal modes present in the laser cavity are phase locked when an optical component is introduced in the optical cavity. Pulse generation in ber lasers could be obtained using two main techniques: active and passive techniques 34 . Active techniques require the use of external modulators such as acousto-optic and electro-optic modulators 35 . It, however, causes the system to be bulky and in exible due to the extra electronic components needed to be used.
In comparison, a passive technique allows for the development of a more compact and versatile system. Saturable absorbers (SAs) are used to saturate the molecules or atoms, whereby the optical absorption decreases as the light intensity increases. Due to this nonlinear optical response of SAs together with a narrow optical bandgap, a high damage threshold, and a wide bandwidth, SAs are suitable devices to generate short pulses using the Q-switching and mode-locking techniques. SAs can be divided into two groups, namely arti cial SAs and real SAs. Nonlinear optical loop mirrors (NOLMs), nonlinear ampli cation loop mirrors (NALMs), or nonlinear polarization evolution (NPE) are examples of arti cial SAs. Arti cial SAs are not suitable for commercialization due to its sensitivity to environmental changes and large size despite their positive attributes of near-instantaneous response time and high modulation depth 36-39 . Semiconductors saturable absorbers mirrors (SESAM), a real SA, were chosen as the SA of choice for nonlinear absorption property that depends on light intensity 40,41 . However, the disadvantages of SESAM are the operating bandwidth is narrow, complex design, costly, and has a low-damage threshold 42 .
In view of those limitations mentioned above, new SA nanomaterials are now the main focus of research in ultrafast laser worldwide. To date, various kinds of materials that exhibit intensity-dependent transmission have been used as SAs, namely graphene 43 , carbon nanotubes (CNTs) 44 , black phosphorus (BP) 45 , transition metal dichalcogenides (TMDs) 46 , and topological insulators (TIs) 47 . Recently, a new type of material named MXenes has been widely explored for various optoelectronic applications due to their unique optical properties 48,49 . It also makes them a great candidate to be used as SAs in generating ultrafast lasers. MXenes are typically obtained from their precursor, the MAX phases 50 . Compared to its counterpart, MAX phases are favorable as it does not require the use of strong etching solutions that contain uoride ions (F − ) such as hydro uoric acid (HF), ammonium bi uoride (NH 4 HF 2 ), or a mixture of hydrochloric acid (HCl) and lithium uoride (LiF), thus minimizing the fabrication process and cost 51 . The MAX phases are also useful for high-temperature applications as they comprise of ternary transitionmetal carbides that have metal and ceramic properties. This makes them to have a good thermal and electrical conductivity, as well as having a high damage threshold 52 54 . These demonstrations show the tremendous potential of MAX phases in generating short pulses in ber lasers, which would allow further exploration of various MAX phases with other combination of the early transition metals.
In addition to these aforementioned SA materials, the structure of the SA devices has a signi cant impact on the SA performance. SA materials could be prepared and integrated into ber laser cavities by several arrangements, which are typically done using the optical deposition method onto ber ferules 55 or substrates and polymer hosts 56,57 . Although these methods allow for a more direct integration, they limit the operation of the lasers at low power due to their poor heat dissipation and low optical damage thresholds. Another method that is attracting a great interest of late is the evanescent eld-based devices, which utilize the nonlinear interaction of the propagating light in the optical ber with the SA materials. Since the light-matter interaction is realized via the evanescent eld, this approach is more e cient. It can provide better functionality by eliminating the issue of heat accumulation. Various types of evanescent eld-based devices have been used as to generate pulses in ber lasers. For example, Mouchel et al. utilized a graphene-coated tapered ber to generate mode-locked pulses in an Er:Yb doped double-clad ber laser with a high average output power of 520 mW when being pumped to a maximum pump power In this work, we explored three different types of evanescent eld-based devices: the tapered ber, the side-polished ber, and the D-shaped ber to generate mode-locked pulses in the 2 µm wavelength region. A Ta 2 AlC MAX phase was rst prepared in the solution form by ultrasonication Ta 2 AlC powder in isopropyl alcohol (IPA), deposited onto the three bers. The MAX phase was composed of tantalum (Ta) as the early transition metal instead of the common titanium (Ti). Each of the SA devices was then individually inserted into a TDFL cavity to generate mode-locked pulses with frequencies between 9 and 11 MHz and pulse widths between 1.678 and 1.817 ps. The pump power of the laser cavity could be increased up to 1 W with all three devices, maintaining the mode-locking operation in the 2 µm without any damage to the SA devices. The results show the role of the evanescent eld-based devices as promising and robust SA devices for the development of high-power ber lasers.

Characterization Of The Taalc Max Phase Solution
The crystalline phase of the Ta 2 AlC MAX Phase was rst assessed using X-ray powder diffraction (XRD).
The measurement was recorded using a Malvern Panalytical Empyrean XRD by utilizing Cu-Kα radiation (λ = 0.1541 nm) as the X-ray source and by scanning in the 2θ range of 10° to 80°. The obtained XRD spectrum is shown in Fig. 1.
It was observed from the XRD pattern that a distinct prominent peak at 2θ = 39.07° together with a few sharp peaks was detected at 2θ = 12.92, 25.84, 33.72, 34.34, 35.25, 39.  The tapered ber was fabricated using the ame brushing method, as shown in Fig. 3 (a). An approximately 0.6 m-long bare ber was rst stripped and then xed onto Newport FCL100 translation stages using ber holders. The stages were used to pull and stretch the ber after it has been softened using oxy-LPG ames so that the width of the optical ber was reduced from a diameter of 125 μm to 6 μm, with a tapered length of about 2.5 cm. Upon completion, the insertion loss of the tapered ber was measured using a light source (LS) and an optical power meter (OPM), giving a value of approximately 4 dB at 2000 nm.
The Ta 2 AlC in the solution form was then deposited onto the tapered ber using the drop-cast method and was left until dry at room temperature. The microscopic image of the fabricated tapered ber taken at 50 times magni cation is shown in Fig. 3 (b), where the tapered waist was reduced to only 6 μm. In Fig.  3 (c), the microscopic image of the tapered ber after the deposition of the Ta 2 AlC was also taken at 100 times magni cation. It shows that the Ta 2 AlC was successfully coated around the waist of the fabricated tapered ber. The graphical illustration of the MAX phase Ta 2 AlC-coated tapered ber is also given in Fig. 3 (d).
Page 6/31 The twin detector measurement technique 65 was used to evaluate the nonlinear optical absorption properties of the Ta 2 AlC-deposited tapered ber. The pulsed laser for the measurement was a FemtoFErb 1950 nm femtosecond ber laser, from Toptica Photonics, with a pulse width of 90 fs and a repetition rate of 30 MHz. To calculate the modulation depth, the non-saturable absorption and the saturation intensity of the Ta 2 AlC-deposited tapered ber, the experimental data obtained were tted using the typical two-level saturation absorption model, which can be expressed as follows 66 : where I is the input intensity, I sat is the saturation intensity and α ns is the non-saturable absorption. For the Ta 2 AlC-deposited tapered ber, the computed modulation depth and the saturation intensity as measured in Fig. 4 were 6.02% and 0.36 MW/cm 2 , respectively.

Ta 2 AlC-deposited Side-Polished Fiber
The side-polished ber (SPF) was commercially obtained from Phoenix Photonics. The polishing depth of the SPF was approximately 1 µm from the edge of the core, and the polished length was 1.7 cm, as given by the manufacturer. The Ta 2 AlC was drop-casted onto the polished region of the SPF, similarly as it was with the tapered ber. Fig. 5 (a) shows the image of the Ta 2 AlC-deposited SPF when being injected with a red-light source. It is observed that scattered light was seen along the polished region of the SPF, con rming that the polished length was approximately 1.7 cm. The deposited Ta 2 AlC at the polished part of the SPF is illustrated in Fig. 5 (b), while the microscopic image of the deposited polished region of the SPF at 100 times magni cation is shown in Fig. 5 (c). The measured insertion loss for the side polished ber Ta 2 AlC was 4.8 dB at 2000 nm. The nonlinear absorption test for the Ta 2 AlC-deposited side polished ber was conducted using the twin detection measurement method as described previously. From Fig. 6, the modulation depth and the saturation intensity were calculated to be 1.09 % and 1.63 MW/cm 2 for the Ta 2 AlC-deposited SPF.

Ta 2 AlC-deposited D-shaped Fiber
The D-shaped ber was fabricated using the polishing wheel method 67 , schematically shown in Fig. 7 (a). Two ber holders and two mechanical alignment stages were used to hold a single-mode ber (SMF-28) above a polishing wheel with a diameter of approximately 1.5 cm. The polishing wheel was wrapped with P800 grit sandpaper and then xed on a motor shaft. The wheel was rotated and gradually raised onto the bottom side of the SMF. The polishing process was carried out while the insertion loss of the Dshaped ber was simultaneously monitored by connecting one end of the ber to a light source (LS) and the other to an optical power meter (OPM). The polishing process was stopped when the insertion loss of the D-shaped ber was measured to be approximately 4.2 dB. After the process was completed, the Dshaped ber was carefully transferred onto a glass slide. The deposition of the Ta 2 AlC solution onto the D-shaped ber was carried out in the same manner as described in the previous section. When injected with a red-light source, scattered light was observed in the polished region of the D-shaped ber, as shown in Fig. 7 (b). The microscopic image of the D-shaped ber's side pro le at 50 times magni cation, as shown in Fig. 7 (c) and Fig. 7 (d), illustrates the deposited Ta 2 AlC onto the D-shaped ber. Fig. 8 shows the nonlinear absorption plot of the Ta 2 AlC-deposited D-shaped ber. From the graph, the modulation depth and the saturation intensity were calculated to be 0.82 % and 1.03 MW/cm 2 , respectively.

Experimental Setup
The experimental setup for the 2 µm mode-locked thulium-doped ber laser (TDFL) using the Ta 2 AlCbased SAs is presented in Fig. 9. A 1565 nm laser source (LS) with a maximum output power of 1 W was used to pump the gain medium through the 1550 nm port of the 1550/2000 nm wavelength division multiplexer (WDM). The gain medium was a 4-meter long TmDF200 thulium-doped ber (TDF) from OFS Inc, connected to the common port of WDM 1 . The absorption of the TDF was 22 dB/m at 1565 nm, and the core and cladding diameters of the TDF were 9 and 125 µm, respectively. The TDF was then connected to the common port of WDM 2 , where the 1550 nm port was used to remove any excess pump.
The 2000 nm port of WDM 2 was connected to the input port of a 2000 nm isolator to obtain a unidirectional operation in the laser cavity. A polarization controller (PC) was connected to the isolator's output to adjust the propagating signal's polarization state. The Ta 2 AlC-based SA, either in tapered ber, a side-polished ber, or a D-shaped ber, was connected to the other end of the PC. The cavity was completed by connecting the other end of the SA device to a 90:10 coupler, where 90% of the signal was circulated back into the cavity, and another 10% was taken as the laser output.
Since there were three different types of SA devices used in this experiment, the total length for each ber was slightly different, resulting in a different net cavity dispersion for each setup. Table 2 gives the calculated group velocity dispersion (GVD) values for each SA device at their respective operating wavelengths. The manufacturer provided the GVDs of the TmDF200, while the GVDs of SMF-28 were estimated from Corning dispersion equation. The net cavity dispersion for each of the mode-locked laser was computed by From the values given in Table 2, the calculated net cavity dispersions for the mode-locked lasers were − 1.85, -1.99, and − 1.88 ps 2 for the cavity with the tapered ber, SPF, and D-shaped, respectively. It indicates that all the mode-locked lasers operated in the anomalous dispersion regime.

Results And Discussions
The experiment was rst conducted without any of the SA devices to observe the operation of the ber laser. As expected, the TDFL without any SA only operated in the continuous wave (CW) regime as no pulses were observed in the oscilloscope as the pump power was increased to a maximum of 1 W. The experiment was then continued by integrating the Ta 2 AlC-deposited tapered ber into the laser cavity.
With ne-tuning of the PC, the fundamental mode-locking of the TDFL was observed at a threshold pump power of 245 mW. The fundamental mode-locking could be obtained until the pump power reached 480 mW. As the pump power was further increased until the maximum, the mode-locked operation was sustained but the laser operated at higher harmonics. As our interest was mainly on the fundamental operation of the mode-locked laser, the laser characteristics were only recorded when the pulsed laser operated at the fundamental frequency.
The characteristics of the mode-locked laser using the Ta 2 AlC-deposited tapered ber are shown in Fig.   10. The optical spectrum recorded in Fig. 10 (a) shows a broad laser spectrum at a center wavelength of 1937 nm, having a 3-dB bandwidth of 2.8 nm. It was apparent that Kelly sidebands were observed in the soliton spectrum when the mode-locking operation was initiated, which was expected as the mode-locked TDFL was operating in the anomalous dispersion regime. From the values obtained from the optical spectrum, the transform-limited pulse width could be estimated by the equation; where c is the speed of light, TBP is the time-bandwidth product, λ c is the center wavelength, and ∆λ is the 3-dB bandwidth. Taking the TBP to be 0.315 for a sech 2 pulse pro le, the center wavelength to be 1937 nm, and the 3-dB bandwidth to be 2.8 nm, the transform-limited pulse width was calculated to be 1.407 ps. Figure 10 (b) shows that the oscilloscope trace of the mode-locked pulses had a repetition rate of 10.73 MHz, which correlates with the cavity round trip time estimated from the cavity length of approximately 19.3 m. A comparable pulse train was achieved by Zhou et al. 59 , where the slight uctuation of the pulse peak intensities was affected by the relatively low sampling rate of the photodetector bandwidth and the low sampling points per pulse. Nonetheless, it was found that the pulse train's amplitude jitter was within reasonable limits. The radio frequency (RF) spectrum of the modelocked pulse is shown in Fig. 10 (c), whereby a sharp peak was observed at about 10.73 MHz with a measured signaltonoise ratio (SNR) of ~ 55 dB. Figure 10 (d) shows the autocorrelation trace of the modelocked pulse, where the pulse width was measured to be 1.678 ps when tted with a sech 2 tting. It was only about 19% longer than the calculated transform-limited pulse width. The corresponding TBP was 0.375, indicating that the pulse width is only slightly chirped.
The experiment was then further conducted by replacing the Ta 2 AlC-deposited tapered ber with the Ta 2 AlC-based side-polished ber that has been described in Sect. 3.3. The fundamental mode-locking operation was achieved at a threshold pump power of 351 mW. The laser spectrum is given in Fig. 11 (a) also shows a typical soliton spectrum with distinct Kelly sidebands. However, it was observed that the sidebands were uneven, with the longer being higher than the shorter-wavelength sidebands. It is highly likely due to the optical ber birefringence ltering effect in the cavity, as was theoretically and experimentally proven by Man et al. 72 . The center wavelength and the 3-dB bandwidth of the TDFL were recorded to be 1931 nm and 3.1 nm, respectively. As for the frequency of the mode-locked pulses, the oscilloscope trace shown in Fig. 11 (b) shows a repetition rate of 9.52 MHz. The frequency was lower compared to the mode-locked laser with the Ta 2 AlC-deposited tapered ber, which was due to the slightly longer length of the SPF. The RF spectrum of the mode-locked laser in Fig. 11 (c) shows a sharp peak at around 9.52 MHz, having an SNR of ~ 50.5 dB. From the autocorrelation trace of the mode-locked pulse shown in Fig. 11 (d), the pulse width was measured to be 1.743 ps, tted with a sech 2 pro le. The corresponding TBP was then calculated to be 0.434, also indicating that the pulse was chirped.
The TDFL cavity was further tested by inserting the fabricated D-shaped ber with the Ta 2 AlC solution.
The mode-locked pulses were generated at a pump power of 380 mW. The output characteristics of the mode-locked laser are shown in Fig. 12. From the optical spectrum plotted in Fig. 12 (a), the mode-locked laser had a center wavelength of 1929 nm with a 3-dB bandwidth of 2.2 nm. The presence of minor dips in the optical spectrum could be attributed to water absorption lines in 2 µm 73 . The mode-locked pulse had a repetition rate of 10.16 MHz with a 9.84 ns interval between peaks, measured from the oscilloscope trace in Fig. 12 (b). It tallies well with the fundamental frequency of the mode-locked laser, which was estimated by f = c/nL where c is the speed of light, n is the refractive index of an optical ber, and L is the length of the cavity. By taking n to be approximately 1.44 at 2000 nm and L to be 20.43 meters, the fundamental frequency was about 10.2 MHz. Figure 12 (c) shows the RF spectrum with a peak that corresponds to the repetition rate of the mode-locked laser, in which it has an SNR value of ~ 47 dB. The autocorrelation trace of the pulse recorded using the autocorrelator is shown in Fig. 12 (d). When tted with a sech 2 tting curve, the pulse width was measured to be 1.817 ps.
The stability of the mode-locking operation with all three SA devices was evaluated by conducting a longterm stability test over two hours. The laser output for each of the three SA devices was monitored at every 10-minute interval, in which their optical spectrum was recorded and plotted in Fig. 13. As seen from Fig. 13 (a), the mode-locked TDFL with the Ta 2 AlC-deposited tapered ber exhibit a steady output intensity, and the contour plot shows no changes in the central wavelength of the output spectrum. For Ta 2 AlC-deposited SPF, the output spectrum displayed in Fig. 13 (b) also shows a very stable output as the central wavelength and the Kelly sideband exhibit no changes throughout the stability test. Meanwhile, the D-shaped ber demonstrates a steady output as depicted in Fig. 13 (c).
The pump power against the output power of the mode-locked lasers is plotted in Fig. 14. As seen from the graph, the average output power of the laser with each type of SA device increased almost linearly after the mode-locking operation was initiated. The mode-locking threshold was 245 mW for the TDFL with the Ta 2 AlC-deposited tapered ber, 351 mW for the Ta 2 AlC-deposited SPF, and 380 mW the Ta 2 AlCdeposited D-shaped ber. At this pump power, the fundamental mode-locking (FML) was observed. The FML operation was sustained until a pump power of 480 mW for the Ta 2 AlC-deposited tapered ber and Page 11/31 pump power in which the FML was sustained, the average output power recorded for the mode-locked TDFL with the tapered ber, SPF, side polished, and D-shaped ber was 1.91 mW, 0.8 mW 1.37 mW, respectively. When the pump power was further increased beyond these pump power, harmonic modelocking was observed and could be maintained up until the maximum pump power of 1 W. At the maximum pump power of 1 W, the maximum average output power obtained were 3.48 mW, 2.27 mW and 2.71 mW for the TDFL with the tapered ber, SPF and D-shaped ber, respectively. The TDFL with the Ta 2 AlC-deposited tapered ber had the highest output power as the structure of the tapered ber was maintained, only its dimension was reduced 74 .
In contrast, the mode-locked TDFL with the Ta 2 AlC-deposited SPF and D-shaped ber had a lower output power since the ber structure had been modi ed during the grinding or polishing process. It caused a higher amount of light to escape easily by scattering light due to imperfection of the surfaces. Nevertheless, all the three SA devices could operate even when the pump power was increased until a maximum pump power of 1 W, without being optically damaged. Compared with materials embedded in polymer hosts, SAs in the lm form typically had a lower damage threshold and can be easily burnt when the power was high 75 . It limits the application of polymer-based SAs in ultrafast ber laser systems, eliminating the possibility of power-scaling of ber lasers.
The performance of the mode-locked TDFLs with each of the three SA devices is summarized in Table 3. Table 3 Summary of the optical characteristics of the TDFL with each mode-locking device in the fundamental operation.  Table 3, the lowest pump power needed to initiate the mode-locking operation was that of the Ta 2 AlC-deposited tapered ber at 245 mW. This was followed by the Ta 2 AlC-deposited SPF and the Dshaped ber at the pump power of 351 mW and 380 mW, respectively. A low pump power threshold to induce the mode-locking operation was favorable as it could reduce the energy consumption. The modelocked TDFL using the Ta 2 AlC-deposited tapered ber generated the highest maximum average output power, which was as high as 3.48 mW. Compared to the output power of the TDFL using the Ta 2 AlCdeposited SPF and D-shaped ber, the generated output power was only 2.71 mW for the former and 2.27 mW for the latter. It was lower by about 22% from the output power generated by the TDFL with the Ta 2 AlC-deposited tapered ber. The corresponding peak power could be calculated by dividing the average output power with the repetition rate and then dividing the value with the pulse width. It gives peak power values of 106 W, 43 W, and 82 W for the TDFL with the Ta 2 AlC-deposited tapered ber, SPF, and D-shaped ber. The lowest and highest repetition rate recorded was 9.52 MHz with the Ti 2 AlCdeposited SPF and 10.73 MHz with the Ta 2 AlC-deposited tapered ber. The difference in the repetition rate was only due to the length of the ber used for the evanescent eld-based bers. In this regard, a higher repetition rate of the mode-locked TDFL could be obtained by having a shorter cavity length. The SNR values recorded from the RF spectrum for all three cases were more than 47 dB, which indicates that all three evanescent eld-based SAs could generate stable mode-locked pulses. The shortest pulse width was recorded to be 1.678 ps, obtained using the Ta 2 AlC-deposited tapered ber. The pulse widths obtained using the Ta 2 AlC-deposited SPF and D-shaped ber were slightly longer, with values of 1.734 ps and 1.817 ps being measured. Overall, it is seen that all the evanescent eld-based SA devices could generate mode-locked pulses in the 2 µm region, with the Ta 2 AlC-deposited tapered ber providing the best performance in terms of the low mode-locking threshold, the highest output, and peak power, as well as having the shortest pulse width.

Methods
Preparation of Ta 2 AlC MAX Phase.
The Ta 2 AlC was purchased from Forsman Scienti c (Beijing) Co., Ltd in powder form with a purity of ≥ 98 %. It was prepared directly by dissolving 30 mg of Ta 2 AlC powder in 3 ml isopropyl alcohol (IPA) and then sonicated for 1 hour.

Conclusion
Mode-locked thulium-doped ber lasers (TDFLs) operating in the 2 µm were successfully demonstrated using three different evanescent eld-based devices: tapered ber, side-polished ber (SPF), and also the D-shaped ber. The tapered ber was fabricated using the ame-brushing method, while the D-shaped ber was polished using the wheel-polishing technique. Meanwhile, the SPF was obtained commercially.
A new type of MAX phase, the Ta 2 AlC, was drop-casted onto the three bers and then inserted separately into the TDFL cavity. The MAX phase was composed of tantalum (Ta) as the early transition metal instead of the common titanium (Ti). Stable mode-locked pulses were obtained for all cases using the three SA devices. The stability measurements showed little to no uctuations in the center wavelength and the peak optical power of the mode-locked TDFLs. The SNR values were also recorded to be more than 47 dB for all three cases, further proving the stability of the generated mode-locked pulses. It was observed from the results that the Ta 2 AlC-deposited ber had the best performance as it can generate the highest output power of 1.91 mW, the highest peak power of 106 W, and had the shortest pulse width of 1.678 ps. These demonstrations show the potential of evanescent eld-based devices to be used as SAs for the development of high-power ultrafast ber lasers sensing.