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Abstract
In our work, a InSe-PVA film-type modulator was successfully prepared. Its nonlinear optical properties were investigated and its application for obtaining a large-energy mode-locked Er-doped fiber laser was also demonstrated. Stable mode-locked operation with a maximum pulse energy of 20.4 nJ at a pulse repetition rate of 586.3 kHz was generated. The maximum average output power was 11.96 mW under the pump power of 560 mW. Our findings suggest that InSe has competitive performance in acting as ultra-fast modulator in comparison with commonly used two-dimensional materials. In addition, our experiment demonstration will provide useful guidance for future investigations of large-energy mode-locked operations and the applications of InSe-based ultra-fast optical devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulsed fiber lasers have been investigated extensively due to their wide industrial and scientific applications as well as its abundant nonlinear optical components [1–17]. Especially in the past decade, investigations of various kinds of new saturable absorber (SA) materials have promoted the development of ultra-fast fiber lasers significantly. Compared with traditional semiconductor saturable absorber mirrors (SESAMs), new SA materials, including quantum dots [1–2], metal nanoparticles [3–6], two-dimensional (2D) materials [7–35] and so on, have been proved to exhibit obvious advantages and excellent nonlinear absorption properties. Among them, 2D materials have attract much more attention due to their more advantageous characteristics including wide absorption range, fast recovery time, high damage threshold and so on [7–35]. These properties make two-dimensional materials become ideal choices for demonstrating pulsed fiber lasers. In fact, 2D materials such as graphene [7–12], topological insulators (TIs) [13–17], transition metal dichalcogenides (TMDs) [18–31], black phosphorus (BP) [32–33], MXene [34–35] etc have already been used as SAs and shown excellent nonlinear absorption performances.
Recently, Indium Selenide (InSe, In2Se3) have shown perfect nonlinear absorption properties and been employed as SAs for demonstrating pulsed fiber lasers [27–31]. In 2018, an α-In2Se3 crystal was mechanically exfoliated and transferred directly onto a fiber ferrule to serve as a SA for achieving a passively mode-locked thulium-doped fluoride fiber laser [27], the generated mode-locked operation had a repetition rate of 6.93 MHz and a pulse width of 5.79 ps. The same group also reported an In2Se3-based passively Q-switched tunable pulse operation [28]. In additon, Based on a magnetron-sputtering deposition method, α-In2Se3 wideband optical modulator was presented by Yan et al, By employing the α-In2Se3 as SAs, passively mode-locked Er- and Tm-doped fiber lasers were successfully obtained, the pulse duration were 276 fs and 1.02 ps respectively [29]. Besides, the nonlinear absorption applications of InSe have also been reported. Based on InSe as SAs, passively Q-switched and mode-locked Er-doped fiber lasers were reported by Yang et al, the largest Q-switched pulse energy was 112.97 nJ and the narrowest mode-locked pulse duration was 2.96 ns under the repetition rate of 1.74 MHz [30]. In their another work, InSe was employed for achieving a Yb-doped fiber laser, stable mode-locked operation with a maximum output power of 16.3 mW and a minimum pulse width of 1.37 ns was obtained, the corresponding pulse energy was as high as 9.26 nJ [31]. Reported works suggested that Indium Selenide (InSe, In2Se3) exhibited excellent nonlinear saturable absorption characteristics and had good performance in acting as SAs for demonstrating pulsed fiber lasers. Additionally, as reported, InSe consists of typical 4 covalently bonded Se-In-In-Se atomic planes. Layers are held together by van der Waals interactions at an interlayer distance of 0.83 nm [36–39]. The band gap value of InSe varies form 1.2 to 1.4 eV. In addition, InSe also has the properties of large electronic conductivity, large photo-response, dramatic nonlinear effect, high damage threshold and so on [36–41], which ensure InSe to have excellent performance in acting as modulation devices.
In comparison with traditional mode-locked operations, large-energy mode-locked operations have specially wide applications in the fields of industrial processing, seed sources, medical treatment and scientific researches and so on [7,14]. As is discussed that the interaction and balance between the laser gain, cavity loss, total dispersion and various of nonlinear optical progresses in the laser cavity lead to the formation of different solitons (traditional soliton, dark soliton, dissipative solitons, etc) [42–43]. Thereinto, in anomalous dispersion region, the energy of traditional soliton was limited to be 0.1 nJ level. In addition, high peak power will lead to various of nonlinear optical progress in the laser cavity and the evolution of the pulse sharps. Thus, for demonstrating large-energy mode-locked pulse operations, a wide pulse width is a essential factor, which will reduce the effect of peak power. In addition, dispersion will broaden the width of the pulse, thus, in our experiment, a simple single-mode 350.5 m-long ring cavity was demonstrated for achieving large-energy operations.
In our work, InSe-PVA film was prepared and employed as a modulator for demonstrating large-energy mode-locked Er-doped fiber laser. The saturation intensity and modulation depth of the InSe-PVA film-type modulator were 13.7 MW/cm2 and 16.5%, respectively. Stable mode-locked operation with a maximum average output power of 11.96 mW and a signal-to-noise ratio of about 45 dB was obtained. Under a pulse repetition rate of 586.3 kHz, the largest pulse energy was as high as 20.4 nJ. Our experiment results fully proved that InSe could be employed as modulation devices for demonstrating pulsed fiber laser with large pulse energy.
2. Preparation and characterization of InSe nanosheets
Previously, for preparing layered materials, different methods including mechanical exfoliation (ME), liquid-phase exfoliation (LPE), pulse laser deposition (PLD), magnetron sputtering deposition (MSD), chemical vapor deposition (CVD) and so on have been employed. In detail, ME can be used to produce high-quality single-crystal flakes from bulk materials, which is unsuitable for achieving multi-layers materials. PLD produces material films following ablation from a target, which always lead to structural alterations in the materials. CVD is an efficient method for the productions of single or few-layer 2D materials with uniform shape, meanwhile, it also exhibit the disadvantages of high-cost and complex equipment. Additionally, LPE method exhibits the advantages of simple equipment, convenient operation and low cost, which is suitable for the preparation of 2D materials with layered structures [44–45]. In our work, InSe nanosheets were also prepared by LPE method. At first, 0.1 g InSe powder was added into 50 ml 40% alcohol for 3 days to prepare InSe-alcohol mixture. The mixture was placed in a ultrasonic cleaner for 9 hours to produce InSe nanosheets. After that, the dispersion was mixed with a 6 wt% polyvinyl alcohol (PVA) solution at the volume ratio of 2:3. The prepared InSe-PVA solution was placed in the ultrasonic cleaner for another 9 hours. And then, 100 µL dispersion solution was spin coated on a sapphire substrate. The coated substrate was placed into a oven for 8 hours at 30℃. After that, a thin InSe-PVA film was obtained. Finally, a square film was cut off from the substrate and placed at the end of the fiber end for using as a modulator.
In our work, for better understanding the nonlinear absorption performance of the InSe-PVA modulator, the morphology, chemical composition and nonlinear optical characteristics of the used InSe nanosheets and the InSe-PVA film were investigated.
Firstly, the Raman spectrum of the used InSe powder was tested by a Raman spectrometer (Horiba HR Evolution). As is shown in Fig. 1(a), Three obvious Raman shifts (112.36, 173.77 and 222.37 cm−1) corresponding to the A1’, E’ and E″, and A2″ modes of the InSe are observed, respectively. Our recorded results are in agreement with the results reported before [30–36], indicating that pure InSe powder was prepared in our experiment. The crystal structure of the InSe was analyzed using X-ray diffraction (XRD). The tested power diffraction XRD spectrum from the InSe is shown in Fig. 1(b), as is shown, peaks corresponding to the (004) and (008) planes in InSe are clearly described, the obvious (004) peak indicates that InSe used in our work exhibits excellent crystallization properties.
Fig. 1. (a) The Raman spectrum of the used InSe nanosheets. (b) The X-ray Diffraction of the used InSe nanosheets
For further verifying the layered properties of materials, a scanning electron microscope (SEM) (Sigma 500) was used to test the morphology properties of the InSe nanosheets. As is shown in the insert of Fig. 2(a), the InSe used in our work exhibits obvious layer-structure. Figure 2(a) shows the EDX spectroscopy from the InSe nanosheets. The peaks associated with In and Se are clearly observed, The In:Se atomic ratio of 1.06 is almost equal to the stoichiometric ratio of 1 for InSe. In addition, for testing if single or few layer InSe nanosheet was obtained after the ultrasonic stripping, the TEM image of the dispersion solution is shown in Fig. 2(b), which was recorded by the TEM microscope (JEM-2100) with an optical resolution of 50 nm. The results, shown in Fig. 2, indicate that layered-structure InSe nanosheets were obtained in our work.
Fig. 2. (a) The EDX spectrum of the InSe powder. Insert. The SEM image of the InSe nanosheets under the resolution of 1 µm. (b) The TEM image of the InSe nanosheets under the resolution of 50 nm.
As is known, for 2D material-based SAs, its saturation intensities and modulation depths are all dependent on its layer numbers. Thus, the thickness characteristics of the prepared InSe nanosheets were tested by an atomic force microscope (Bruker Multimode 8), the recorded results are shown in Fig. 3(a) and (b), the thicknesses of the marked samples varies from about 30 to 45 nm, corresponding to the layer numbers of about 36-54.
Figure 4(a) shows the linear transmission spectrum of the InSe-SA measured by a spectrophotometer (Hitachi, U-4100 within the range of 850-1750nm. It is obvious that the SA exhibits wavelength-dependent optical transmission properties. The optical transmission of the SA at 1530 nm is about 63%. Additionally, the nonlinear optical saturable absorption response of the InSe modulator was investigated experimentally based on a commonly reported two-arm detection method [30–31]. A home-made nonlinear polarization rotation mode-locked Er-doped fiber laser was used as the pump source, the pulse width, maximum output power and pulse repetition rate were 660 fs, 12.3 mW and 18.9 MHz, respectively. The recorded results are shown in Fig. 4(b). Additionally, based on the well-known formula [30–31], the saturation intensity and modulation depth of the InSe modulator were calculated to be 13.7 MW/cm2 and 16.5%, respectively.
Fig. 4. (a) The linear transmission properties of the prepared InSe-PVA film. (b) The nonlinear absorption properties of the InSe-PVA film.
3. Design and construction of the mode-locked fiber
The construction of the large-energy mode-locked Er-doped fiber laser is shown in Fig. 5. As is shown, a 974 nm laser diode (LD) with a maximum output power of 630 mW is employed as the pump source. A 35 cm long Er-doped fiber (LIEKKI, Er-110, 4/125) is used as the laser gain medium. The Er-doped fiber exhibits a mode field diameter at 1550 nm of 6.5 ± 0.5 µm, a peak core absorption of 110 ± 10 dB/m at 1530 nm, a core numerical aperture of 0.2, and a dispersion value of −46 ps/nm/km. The pump source is injected into the Er-doped fiber through a 980/1550 wavelength division multiplexer (WDM). A polarization independent isolator (PI-ISO) guarantees the unidirectional transmission of the pulse in the ring cavity. Two polarization controllers (PCs) are used for adjusting the polarization states in the cavity. A 10:90 optical coupler (OC) is used to output the laser through its 10% port. The InSe modulator is inserted into the cavity between the PC and the OC. Finally, the total cavity length is about 350.5 m, thus the net dispersion value is calculated to be about −7.57 ps2. The output performance of the mode-locked fiber laser were recorded a fast-speed InGaAs photodetector (MR-F-3G), a digital oscilloscope (Tektronix, DPO4104), a power meter (PM100D-S122C), an optical spectrum analyzer (YOKOGAWA, AQ6370D) and a spectrum analyzer (R&S FPC1000).
4. Results and discussion
Firstly, without inserting the InSe modulator into the ring laser cavity, only continuous-wave operation was detected by adjusting the value of the pump power and the states of the PCs, no self-mode-locked or Q-switched operations were recorded in the experiment. And then, the InSe modulator was inserted into the laser cavity. When the pump power was higher than 350 mW, stable mode-locked operation was record by adjusting the state of the PCs, the phenomenon indicates that the mode-locked operation is due to the modulation effect of the InSe saturable absorber. In addition, the threshold power of our work was 350 mW, which was much higher than the results reported before [2,4,6–7,15–16]. In our opinion, the high threshold was mainly due to the loss of the long-length single-mode fiber and the large insert loss of the InSe modulator. The insert loss of the InSe modulator was measured to be 1.63 dB. In the experiment, mode-locked operations remained stable when the pump power arranged from 350 to 560 mW. However, when the pump power was higher than 560 mW, unstable phenomenon was detected in the experiment, which was mainly due to the limitation of the peak power. It should be stated that no passively Q-switched laser operation was recorded by inserting the SA into the laser cavity and adjusting the polarization states of the PCs, indicating that our demonstration was not suitable for generating passively Q-switched laser operations, which was mainly due to the relative long laser cavity length. As is reported, passively Q-switched fiber laser operations were mainly obtained within short-length laser cavities [17,20,24,46].
Under the pump power of 560 mW, the emission spectrum of the mode-locked laser was recorded by an optical spectrum analyzer with a resolution of 0.05 nm and shown in Fig. 6. As is depicted, the emission spectrum exhibits different central wavelengths and no typical Kelly side-band peaks were obtained, the spectral results proved that the mode-locked soliton obtained in our work was different from commonly reported solitons such as traditional soliton, dark soliton, dissipative soliton, noise-like soliton and so on.
The corresponding pulse trains recorded with different time scales under the pump power of 560 mW are shown in Fig. 7(a)(b)(c), respectively. The pulse train shown in Fig. 7(a) proved that the mode-locked operation exhibited good stability. As is shown in Fig. 7(b), the pulse-to-pulse time is 1.706 µs, corresponding to a pulse repetition rate of 586.3 kHz, which is in agreement with the total length of the ring cavity. For a mode-locked operation, the cavity-length-dependent pulse repetition frequency rate is one of the most typical characteristics, which will prove the laser operating at a mode-locked state [47]. Single pulse sharp was shown in Fig. 7(c), the pulse width was 389.2 ns. In the experiment, the pulse duration was detected with an optical autocorrelator (Femtochrome FR-103 XL), however, no pulse information was recorded, indicating that the pulse width of 389.2 ns measured by the digital oscilloscope was accurate. In general, the pulse width of mode-locked Er-doped fiber laser was always at the picosecond or femtosecond level. However, the pulse width of our work was 389.2 ns, the wide pulse width was particle due to the large dispersion value. Additionally, as is described in Fig. 6, the emission spectrum exhibited various central wavelength, which proved that the laser operated at a multi-wavelength state, as is known, the interaction between multiple wavelengths also can lead to a wide pulse width. The radio frequency (RF) spectrum was recorded by a spectrum analyzer (R&S FPC1000) for testing the stability of the mode-locked operation. Figure 7(d) depicts the radio frequency spectrum located at the fundamental repetition rate of 586.3 kHz with a bandwidth of 0.7 MHz and a resolution of 100 Hz, the signal-to-noise ratio is about 45 dB. The results exhibit that mode-locked pulses with high stability was obtained in our experiment.
Fig. 7. (a) and (b) Emission pulse trains of the mode-locked laser. (c) The single pulse sharp of the mode-locked generation. (d) the RF spectrum located at 586.3 kHz with a bandwidth of 0.7 MHz.
The relationship between the average output power and the pump power is shown in Fig. 8(a). The output power increases with the growth of the pump power linearly , the maximum average output power is 11.96 mW under the pump power of 560 mW, corresponding to an optical-to-optical conversion efficiency of 2.1%. In our experiment, the pulse widths under different pump powers were also recorded. The results are also depicted in Fig. 8(a), it is obvious that the pulse width also increases with the increasing of the pump power. The corresponding pulse energies and peak powers under different pump powers are provided in Fig. 8(b), the maximum pulse energy is 20.4 nJ, which is also larger than traditional-soliton mode-locked operations. Particularly, as is shown, firstly, the peak power increases to be about 60.6 mW, however, when the pump power is higher than 500 mW, the peak power decreases, which was mainly due to the peak-power-limitation mechanism.
Fig. 8. (a) Average output power and pulse width under different pump power. (b) The relationship between the pulse energy, peak power and the pump power.
Table 1 shows the comparison results of large-energy mode-locked lasers which based on different 2D materials as SAs, the SAs with different modulation depths and saturable intensities were prepared by with diverse methods [48–53]. As is shown, commonly reported materials including graphene, BP, TI and TMDs with excellent optical properties have been employed for the enhancement of the laser output characteristics. In comparison, our results are also at the same level with the reported works, indicating that our experimental design is efficient for the generation of large-energy mode-locked fiber laser. In addition, our results prove that InSe exhibit competitive excellent optical properties in comparison with the widely reported 2D materials (graphene, BP, TI, TMDs, etc).
Table 1. comparison of large-energy mode-locked lasers based on 2D SAs.
In conclusion, InSe-PVA film-type modulator was successfully prepared in our work. Nonlinear optical properties of the InSe-PVA modulator were investigated. Based on the InSe-PVA SA, a large-energy mode-locked Er-doped fiber laser was generated. Stable mode-locked operation with a maximum average output power of 11.96 mW under a pulse repetition rate of 586.3 kHz was obtained, the corresponding pulse energy was 20.4 nJ. It is obvious that InSe nanosheets with suitable bandgap value and good nonlinear saturable absorption characteristics can be used as efficient modulators for generating large-energy pulse lasers. Our results will provide useful references for demonstrating pulsed fiber lasers based on InSe devices.
Funding
National Natural Science Foundation of China (NSFC) (11604182); Natural Science Foundation of Shandong Province (ZR201702100132).
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