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Abstract
We report simple and compact all-fiber erbium-doped soliton and dispersion-managed soliton femtosecond lasers mode-locked by the MXene Ti3C2Tx. A saturable absorber device fabricated by optical deposition of Ti3C2Tx onto a microfiber exhibits strong saturable absorption properties, with a modulation depth of 11.3%. The oscillator operating in the soliton regime produces 597.8 fs-pulses with 5.21 nm of bandwidth, while the cavity with weak normal dispersion (~0.008 ps2) delivers 104 fs pulses with 42.5 nm of bandwidth. Our results contribute to the growing body of work studying the nonlinear optical properties of MXene that underpin new opportunities for ultrafast photonic technology.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers have become a mature technology, facilitating a broad range of applications, including high-precision fiber sensing technology, high-capacity and high-rate wavelength division multiplexing fiber communication systems, and high-power lasers for micromachining [1,2]. While many schemes exist to generate ultrashort pulses in fiber lasers, saturable absorbers (SAs), an ultrafast optical switch exhibiting an intensity-dependent transmission, have been widely deployed in many systems as they enable a wide space of parameters to be accessed without the implementation of complicated and expensive external modulators.
Various types of saturated absorbing materials are used for pulsed laser generation, including organic dyes, Kerr prisms, metal nanoparticles, ion-doped crystals, and semiconductors of various structures [3,4]. The most commonly used saturable absorbing devices on the market are semiconductor saturable absorber mirrors (SESAMs); however, their fabrication process is complicated, expensive, and limited by semiconductor materials, and these devices only perform well in the near-infrared range. These limitations are driving research into new candidates with exceptional optical properties for SA applications. In 2003, Set et al. first reported a SA that incorporated carbon nanotubes (CNTs) for passively mode-locked lasers [5]. Since then, particular interest has been focused on nanomaterials as the reduced dimensionality of the materials lead to distinct and complementary properties and new physical phenomena [6–8]. In 2009, a graphene-based mode-locking laser was reported [9]. Graphene is a band-free semi-metal semiconductor material with ultra-high electron mobility and broadband optical absorption properties [9–12]. However, the energy band structure without band gap limits the application and development of graphene in the field of optoelectronics. Apart from CNTs and graphene, many other nanomaterials have been investigated in fiber laser systems as a SA device, such as topological insulators (TIs, e.g., Bi2Te3, Bi2Se3 and Sb2Te3), transition metal dichalcogenides (TMDCs, e.g., MoS2, WS2 and MoSe2), and black phosphorus (BP) [13–15]. TIs have a complex saturation absorption mechanism and are broadband nonlinear optical response materials [16–20]. TMDCs have special energy band structures, semiconductor or superconducting properties, and excellent mechanical properties [21–23]. BP and graphene have similar properties, and a single layer of BP not only has a conductivity comparable to that of graphene, but also has an energy gap, unlike graphene [24–28]. However, the lack of fine-grained material manufacturing remains an obstacle to the commercialization of two-dimensional (2D) BP materials, and a deeper understanding of their optical performance is required [29].
Recently, rapid growing interest has been focused on MXene Ti3C2Tx materials due to their remarkable optoelectronic and optical properties. These nanomaterials form a class of metal carbide and metal nitride materials with a 2D layered structure resembling potato chips that are stacked on top of each other [30–33]. The generic chemical formula of MXene materials is Mn+1XnTx(n = 1–3), where M represents an early transition metal such as Sc, Ti, Zr, V, Nb, Cr, or Mo; T usually represents an element from the third or fourth main group; and X represents C or N. At present, such materials are attracting worldwide attention in many fields (such as energy, optics, catalysis, etc.) [4,34]. MXene has large surface area and good electrical conductivity and stability, as well as highly tunable and tailorable electronic/optical properties. MXene has been found to exhibit nonlinear saturable absorption, with increased transmittance at higher light fluences [33]. For instance, Ref [30] reports that the zero-gap band structure of Ti2C3 (Eg< 0.2 eV of Ti3C2Tx), indicating its potential for use in broadband optical devices. Zhang et al. have confirmed the application of MXene Ti3C2Tx in ultrafast photonics, with promising performances operating in erbium-doped fiber lasers [33,35,36].
So far, most of the reported mode-locked fiber lasers using nanomaterial-based SA devices have been implemented in an all-anomalous dispersion regime. The pulse energy and duration of a dispersion-management-free soliton laser cavity are limited by the soliton area theorem [37]. The concept of using an ultrashort laser cavity to create ultrafast pulses is limited by the minimum length of the gain fiber and other fiber components. Another common solution is the so-called stretched-pulsed cavity configuration. The stability of the pulse sequence of stretched-pulse fiber lasers is not only limited by the net cavity dispersion, but also by the saturable absorber parameters, in particular, the modulation depth and recovery time [38]. Table 1 summarizes the output performance of the shortest-pulse Er-doped fiber lasers based on SA devices and shows that the SAs from the materials are undergoing a revolution.
Table 1. Output Performance Comparison of the Shortest-Pulse Er-Doped Fiber Lasers Based on CNT, graphene, TIs, TMDCs, BP, or MXene Ti3C2Tx SA Materials
In this article, we demonstrate ultrashort pulse generation from a soliton mode-locked fiber laser and a stretched-pulse mode-locked fiber laser based on a MXene SA. The MXene-based device exhibits strong nonlinear saturable absorption with an optical modulation depth of 11.3% and saturation intensity of 1.94 mW at 1558 nm. Using this MXene-based SA device in an all-fiber erbium-doped soliton femtosecond laser, we demonstrate stable mode-locked operation at 1564 nm, with 597.8 fs pulses and a spectral width of 5.21 nm. Through intracavity dispersion management, a stretched-pulse fiber laser generates 104 fs pulses, which are approximately six times shorter than the soliton pulses; the net cavity dispersion is ~0.008 ps2, in accordance with the theoretical value, and the 3 dB width is 42.54 nm at 1550 nm. These are among the shortest pulses ever generated from a fiber laser based on MXene. Our work highlights the potential of MXene for high-performance femtosecond pulse generation in the near future.
2. MXene SA fabrication and characterization
MXene Ti3C2Tx was prepared via the aqueous acid etching method [35,39,40]. First, the raw material Ti3AlC2 powder was manually ground with deionized (DI) water. The resulting Ti3AlC2 DI solution was then mixed with 40 wt.% hydrofluoric (HF) acid in a volume ratio of 1:15. In general, milder etching and a single-layer spalling environment are beneficial to the synthesis of large-scale, less-defective MXenes. However, the Ti3C2Tx sheets prepared in a very mild environment still have atomic vacancies, and the density of vacancies is strongly related to the HF concentration used for etching. Similarly, the etching temperature and time will also affect the layer structure and defects in MXenes. Layering was achieved using a magnetic stirrer. The obtained deposit was flushed with DI water to obtain pH > 6 and dried in a vacuum oven. Water bath ultrasonic was employed to assist in exfoliate MXene. The MXene powder was dispersed in IPA and treated with water bath ultrasonic (below 20°C) for 10 hours. After that, the suspension liquid centrifuged at a speed of 3000 rpm for 30 minutes, the supernatant was centrifuged at a speed of 18000 rpm for 30 minutes. After removing the supernatant, the precipitate was dispersed in water to obtain the MXene dispersion.
The characterization of the sample, including SEM (scanning electron microscope), TEM (transmission electron microscopy) and AFM (atomic force microscopy) have been carried out to investigate the morphology of MXene. As shown in Fig. 1(a), the book-like SEM image of MXene shows excellent stripping effect after etching process. The TEM image of the prepared MXene nanosheets is shown in Fig. 1(b), with an estimated size distribution of MXene nanosheets of 50-200 nm. The AFM image of the sample is shown in Fig. 1(c), where six MXene nanosheets have been marked with different colored lines. The thickness and size profile of MXene nanosheets are in Figs. 1(d) and 1(e): from line 1-6, the sizes are 208.6, 107.8, 227.8, 258.6, 123.1 and 220 nm and the corresponding thickness are 2, 3, 2, 2, 2 and 2 nm, respectively, which can be used to indicate that our samples are 2-3 layer thick [35]. We then characterized the energy dispersive spectrometry (EDS) to study the elemental changes during the transformation from the MAX phase to the MXene phase. Through the scanning transmission electron microscopy (STEM) image and elemental mapping images, the elements in the Ti3C2Tx nanosheets are visually revealed, indicating that the Ti, C and F are uniformly distributed (Fig. 1(g)-1(i)). The outlines of the Ti and F elemental mapping images are consistent with that of the Ti3C2Tx nanosheet in Fig. 1(f). Compared with the MAX phase (Ti3AlC2), MXene (Ti3C2Tx) has additional F elements, due to the HF corrosion process. However, the C elemental mapping image is incongruent with the Ti3C2Tx nanosheet because of the interference of the carbon supported film.
Fig. 1 (a) SEM image of the delaminated Ti3C2Tx after HF etching; (b) TEM image of MXene Ti3C2Tx; (c) AFM of Ti3C2Tx nanosheets, and the height profiles of the six nanosheets marked with colored lines are shown in (d) and (e); (f) Scanning transmission electron microscope (STEM) of a Ti3C2Tx nanosheet; (g)-(i) are EDS images of Ti3C2Tx corresponding to C, Ti and F elements, respectively.
We used home-made machine to make microfibers and pull the bare single-mode fiber (SMF-28) under an oxy-hydrogen flame. In this fabrication process, the 1550-nm laser source was propagated into the microfiber, and the power meter monitored the power change in real time. The fiber was uniformly straightened to 10 mm at both ends, and the drawing speed was 0.25 mm/s. The power consumption at the end of production was 0.1 dB. The microfiber was fixed on the glass slide with ultraviolet glue, and the area near the taper was kept clean. The tapered fiber, with a diameter of ~13 μm at the taper, was observed directly using a microscope. The diagram of the optical deposition method is shown in Fig. 2(a). A 980 nm pump light source with a power of 60 mW was used in the process. MXene is dropped on the microfiber with a pipette, the volume of the droplet is ~5-10μL.The material is adsorbed on the surface of the microfiber through the action of an evanescent filed. The adsorption process of the material was observed in real time using a microscope. Real-time observation of the loss of the fiber facilitates control of the depth of deposition. When there is too much material, the power loss will increase. In this work, the power loss after deposition is 3 dB. Figure 2(b) shows the optical microscope image of the microfiber deposited with MXene. The film thickness is characterized by step profiler (DektakXT) testing methods. MXene-based SA has a material thickness of ~3μm.
Measurement of the saturable absorption of the MXene-based SAs was carried out using a balanced twin-detector (Fig. 3(a)). An ultrashort pulsed fiber laser (800 fs pulse duration, 1558 nm wavelength, 45.9 MHz pulse repetition frequency) was used as the pump light source. After an adjustable attenuator (Thorlabs EVOA1550A), the output was split using a 50:50 fiber coupler; one beam was used for detecting absorption and the other for power monitoring as a reference signal. By continuous adjustment of the attenuator, the transmitted power as a function of the incident optical power was recorded for the MXene-SA device, via photodetectors (PD1 and PD2 in Fig. 3(a)). With increasing peak power intensity, the transmittance of the MXene SA tends to be constant, confirming saturable absorption. A nonlinear transmission curve (Fig. 3(b)), indicative of saturable absorption, is apparent. The following formula was used to fit the experimental data:
where is the transmittance, is the modulation depth (maximum value of the transmission shifted to be 100% but without normalization), is the power of the input light, is the saturation power, and is the non-saturated loss. As shown in Fig. 3(b), the modulation depth and the saturation power of the MXene are estimated to be 11.3% and 1.94 mW, respectively. The strong saturable-absorption property of the MXene-SA at 1558 nm indicates that this device could be used as an ultrafast optical switch for ultrashort pulse generation.3. Experimental setup and results
3.1 MXene-SA in a soliton mode-locked fiber laser
A microfiber-based MXene-SA was implemented in a soliton mode-locked fiber laser. The fiber laser was constructed using an Er-doped fiber (YOFC, Er1022, which has a group velocity dispersion (GVD) of 7.65 ps2/km) and a single-mode fiber. As shown in Fig. 4, the laser cavity structure is compact, low-cost, and easy to integrate within a larger system; it consists of a ring cavity, including a polarization-independent optical isolator (ISO in Fig. 4), polarization controller, and the fiber amplifier. The fiber amplifier consists of a ~3.5 m length of Er1022 fiber, co-pumped by a 980 nm laser diode through a 980/1550 wavelength division multiplexer (WDM). The pigtail fiber of the WDM includes a piece of Corning HI1060 fiber. The polarization controller (PC) enables a thorough and continuous adjustment of the net cavity birefringence, and the isolator ensures unidirectional propagation. A fiber optical coupler (OC) with a split ratio of 20:80 was used, so that 20% of the power was used to measure the output of the fiber laser in the time and frequency domains. The total cavity length is ~11 m. The anomalous dispersions of the SMF-28 and HI1060 fibers are −22 ps2/km and −7 ps2/km, respectively. The net cavity dispersion is calculated to be ~-0.14 ps2, which ensures that the laser operates in the average-soliton regime. The performance of the mode-locked fiber laser was measured using an optical spectrum analyzer, a home-built autocorrelator, an oscilloscope, and a radio-frequency (RF) analyzer with a high-speed photodetector.
Soliton mode-locking is observed at the fundamental repetition frequency of the cavity (17.9 MHz) at a pump power of 118.5 mW. Figure 5(a) shows a typical pulse train, with a fundamental cavity repetition time of 55.53 ns corresponding to a total cavity length of 11 m. The time interval between two pulses corresponds to the fundamental repetition frequency. The spectral profile of the output is shown in Fig. 5(b), centered at 1564.24 nm with a full width at half maximum (FWHM) of 5.21 nm. The spectrum displays soliton sidebands, demonstrating the formation of conventional solitons. Soliton mode-locking was achieved in the anomalous dispersion regime utilizing MXene-SA. The output power of the cavity was measured to be 283 μW. The home-built autocorrelation instrument was used for measurement of the autocorrelation of the soliton pulse. Figure 5(c) shows the autocorrelation trace alongside a sech2 fit; the corresponding soliton pulse duration is 597.8 fs. The time-bandwidth product (TBP) was calculated to be ~0.382, which is 1.2 times larger than that of the transform-limited pulse, indicating that the pulse is slightly chirped. The RF spectrum (Fig. 5(d)) displays a signal-to-background contrast of 55.2 dB, without any harmonics, which indicates low-amplitude fluctuation and stable soliton mode-locking.
Fig. 5 Output performance of the soliton Er-doped fiber laser based on MXene-SA: (a) typical output pulse train with a spacing of 55.53 ns; (b) measured optical spectrum with a 3 dB spectral width of 5.21 nm at 1564 nm; (c) autocorrelation trace of the 597 fs (deconvolved) output pulses together with a sech2 fit; (d) RF spectrum showing the signal-to-background contrast of 55.2 dB, with a resolution bandwidth of 10 kW.
However, the generated pulses were optical solitons, with durations strongly limited by the soliton area theorem. In order to obtain a shorter pulse width in a MXene-SA mode-locked fiber laser, it is necessary to manage the cavity dispersion and enforce the so-called stretched-pulse regime, where the net cavity dispersion is designed to be close to zero. Here, the EDF1007 (YOFC) fiber is selected for its large group velocity dispersion coefficient to achieve the dispersion management in the laser cavity.
3.2 MXene-SA in a stretched-pulse mode-locked fiber laser
A schematic of the stretched-pulse mode-locked fiber laser is depicted in Fig. 6. Mode-locking can be achieved by the MXene-SA device. A 4.78 m EDF1007 (GVD = 22.7 ps2/km) is utilized as the gain medium, which is pumped by a 980 nm laser diode through a 980/1550 WDM. The WDM has a 0.9 m HI1060 pigtail fiber with a GVD of −7 ps2/km at 1550 nm. Pockels cells (PC1 and PC2 in Fig. 6) are utilized to adjust the intracavity polarization due to the limitation of the length of the single-mode fiber in the cavity. A polarization-independent optical isolator ensures unidirectional propagation, and a fiber OC is used with an output ratio of 20% for both spectral and temporal diagnostics. The tail fiber of the other devices in the fiber laser cavity is a single-mode fiber (SMF) with a GVD of −22 ps2/km. The total length and net dispersion of the cavity are ~10 m and ~0.008 ps2 respectively; the net dispersion is the same as that typically reported for stretched-pulse lasers [41].
Mode-locking was observed when the pump power increased to 234.9 mW, with the appropriate adjustment of PC1 and PC2. A typical pulse train is shown in Fig. 7(a) with a time interval between two pulses of 50.37 ns. The output power and fundamental repetition frequency of the cavity are 1.3 mW and 20.03 MHz, respectively; the single pulse energy is 65 pJ. Figure 7(b) presents a typical spectrum of the mode-locked ultrashort pulse; the center wavelength is 1550 nm, and the spectral FWHM is 42.54 nm. The autocorrelation trace with a Gaussian fit is shown in Fig. 7(c); these experimental data were recorded using an autocorrelation function analyzer (Femtochrome FR-103). The pulse duration is estimated to be 104 fs; considering the theoretical TBP value of a transform-limited pulse (0.44 for a Gaussian pulse), it should be possible to further reduce the pulse duration to approximately 84 fs. This difference in duration between our measured pulse and the transform-limited pulse could be attributed to uncompensated higher-order dispersion [42], and indicates that the mode-locked pulses were weakly chirped; the TBP is calculated to be 0.55 for our pulses. The fundamental radio frequency spectrum displays a signal-to-background contrast of ~62.4 dB (Fig. 7(d)), indicating good mode-locking performance.
Fig. 7 Output performance of the stretched-pulse Er-doped fiber laser based on the MXene-SA: (a) typical output pulse train, with a fundamental period of 50.37 ns; (b) optical spectrum with FWHM of 42.54 nm at 1550 nm; (c) autocorrelation trace of the output pulses and Gaussian fit, with a pulse duration of 104 s (deconvolved); (d) RF spectrum at the fundamental frequency of 20.03 MHz with a signal-to-background contrast of 62.4 dB (measured resolution bandwidth is 1 kHz).
The evolution of the pulse duration and spectral width in one roundtrip is shown in Fig. 8. We performed a numerical simulation of the stretched-pulse mode-locked fiber laser based on the modified nonlinear Schrödinger equation (NLSE). Starting from the amplifier, as indicated in the dispersion map in the upper of the Fig. 8, the cavity is divided into three segments: L1 is the 4.78 m Er-doped fiber (the only element with normal dispersion), L2 is the 4.29 m SMF-28 fiber (the pulse is further compressed by a length of external fiber with anomalous dispersion), and L3 is the 0.93 m HI1060 fiber. The position of the OC in the cavity is at 5.18 m, and the position of the SA in the cavity is at 6.68 m. The minimum pulse duration is achieved at the point where the normal dispersion and nonlinear effect are compensated by the anomalous dispersion. The output pulse from the OC can be further compressed by a length of the external fiber with anomalous dispersion (fiber pigtail of the OC). The optimal length of the fiber pigtail of the OC in this experiment is ~1.6 m, with an achievable shortest pulse duration of 105 fs. The simulation results are in a good agreement with experimental results for pulse durations.
Fig. 8 Variations of the pulse duration and spectral width in a roundtrip of the stretched pulse fiber laser cavity
4. Conclusion
In summary, we have demonstrated all-fiber erbium-doped soliton and dispersion-managed lasers, with a soliton-pulse of 597 fs and stretched-pulse of 104 fs, mode-locked using a microfiber-based MXene SA device fabricated via an optical deposition method. We have thus demonstrated the shortest pulse duration achieved using MXene in a fiber laser to date (104 fs). This work opens a new route for the application of 2D MXene nanomaterials to nonlinear photonics, emphasizing the application of a MXene-SA in ultrafast lasers.
Funding
Fundamental Research Funds for the Central Universities; National Natural Science Foundation of China (NSFC) (61505005, 51778030, 61435010, 61875138, 61805146); Science and Technology Development Fund (007/2017/A1, 132/2017/A3); Macao SAR, China; Shenzhen Science and Technology Innovation Commission (JCYJ20170818093453105); China Postdoctoral Science Foundation (2018M643165).
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