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Abstract
We reported on a passively Q-switched double-clad thulium-doped fiber laser by using oxygen vacancies (OVs) enriched with MoO3-x as a saturable absorber (SA). The MoO3-x was successfully embedded in polyvinyl alcohol and acted as the SA for a stable Q-switching operation in the 2 µm wavelength region. Stable Q-switched pulses were obtained at a central wavelength of ∼1992.1 nm with a maximum output power of ∼62.7 mW, maximum pulse energy of ∼1.63 µJ and a minimum pulse duration of ∼2 µs. The repetition rate of the pulses can be tuned from ∼15.5 kHz to ∼38.5 kHz with increasing the pump power. Our experiment was the first demonstrated use of MoO3-x as a SA for pulse laser generation, in which OVs were considered as the original reason for the optical saturable absorption. Moreover, it provides a novel idea for optical SA preparation by controlling the vacancy defects in such materials.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Q-switched fiber lasers with typical pulse duration range from several microseconds to nanoseconds have attracted tremendous attentions in the past decades, since the high energy Q-switched pulse can be widely used in materials processing, medical application, remote sensing, laser cleaning and scientific research [1–3]. Comparable to the actively Q-switched fiber lasers, the passive ones have received more attention. The main reason can be attributed to two aspects. One is that passively Q-switched fiber laser can be operated at stable Q-switching state by easily integrating a saturable absorber (SA, a kind of material with intensity dependent transmission) into the laser cavity, which is much simpler and more flexible than the active Q-switched laser. Another motivation is the rapid development of different kinds of SAs in recent years.
As well known, the SA is the key component for passively Q-switched fiber laser. Thus, a lot of research efforts associated with passively Q-switched fiber lasers are focused on the SAs. With the great efforts of researchers, different SAs, including “artificial” ones (such as nonlinear polarization rotation [4] and Kerr effect [5]) and “real” ones (such as semiconductor saturable absorber mirrors (SESAMs) [6,7], carbon nanotubes (CNTs) [8–10], graphene [11–13], topological insulators (TIs) [14,15], transition metal dichalcogenides (TMDs) [16–20], black phosphorus [21,22], metal nanoparticles [23], titanium(IV) oxide [24], even alcohol and pure water [25–27]), have been utilized for passively Q-switched operation. In previous works, we note that most of the TMDs have bandgap values larger than 1 eV, e.g., WS2 (indirect, ∼1.4 eV; direct: ∼2.1 eV), MoS2 (indirect: ∼1.29 eV; direct: ∼1.8 eV), MoSe2 (indirect: ∼1.1 eV; direct: ∼1.55 eV) [28], which means the materials cannot absorb the incident photons with energy lower than bandgap (such as photons at ∼1.5 µm or 2 µm wavelength) [29–31]. However, the property of optical absorption had been verified in few-layer TMDs at ∼1.5 µm and ∼2 µm wavelengths by many experimental results [32,33]. Subsequently, the sub-band saturable absorption was attributed to the material defects, including the edges defects (e.g., LPE-processed TMD flakes) and vacancy defects (e.g., CVD-processed TMD flakes) [17]. To the best of our knowledge, there is only few works about directly constructing defects in materials to act as optical SAs for pulse laser generation [17].
Molybdenum trioxide (MoO3) is a low cost and non-toxic n-type semiconductor with high stability and wide bandgap(∼3.1 eV), which was widely investigated in catalysis, gas sensing, capacitor, and lithium-ion battery [34–36]. Generally, MoO3 contains three kinds of polymorphs (the orthorhombic phase, α-MoO3; the hexagonal phase, h-MoO3; and the monoclinic phase, β-MoO3). Among these phases, the α-MoO3 has a better thermodynamical stablility and more stable layered structure, which was widely studied in recent years [37,38]. Inspired by the oxygen deficient structure of other transition metal oxides (e.g., WO3, TiO2, ZnO, etc), which exhibit better properties in charge transfer and narrowing bandgap in catalytic reactions, the oxygen vacancies (OVs) enriched MoO3-x have recently also been investigated to improve the electrical conductivity [39] and control the catalytic properties [40,41]. Nevertheless, the optical properties (especially the sub-bandgap saturable absorption) have received much less attention [42]. In fact, the vacancy defects also can result in abundant defect energy levels which could increase the absorption bandwidth and absorptivity.
In this paper, we firstly utilized OVs enriched α-MoO3-x as a SA in thulium-doped fiber laser. After integrating the MoO3-x-PVA film into the cavity, stable Q-switched pulses with shortest pulse duration of ∼2 µs and pulse energy of ∼1.67 µJ were obtained. Our experiment results are qualitative evidence of vacancy defects which can make the material behave the property of optical saturable absorption. Also, it provides a novel idea for optical SA preparation.
2. Preparation and characterization of MoO3-x SA
In our experiment, the MoO3-x was synthesized by a solvothermal method. Firstly, 1 mmol of Mo powder (Aladdin, 99.9%), 1.5 mL of H2O2 (Aladdin, AR, 30wt%) and 28.5 ml of ethanol solution (Aladdin, AR) were added to a 50 ml Teflon vessel in sequence. Then the mixed solution was magnetically stirred for ∼30 min, and a transparent yellow solution was obtained. After that, the Teflon vessel was sealed in a stainless steel autoclave, heated and maintained at 160°C for 12 h. After being cooled to room temperature naturally, the MoO3-x power was collected by centrifugation at 10,000 rpm for 30 min, rinsed with ethanol three times and dried under vacuum. The MoO3 sample was obtained by calcining the MoO3-x at 550 °C in air for 2.5 h to recover the OVs. The characterizations of the prepared MoO3-x were shown in Fig. 1. Different with the off-white powder of MoO3, the color of MoO3-x powder exhibits dark blue, as shown in the insert of Fig. 1(a). For better understand the morphology of the MoO3-x, the Scanning electron microscope (SEM) was introduced and shown in Fig. 1(a). The MoO3-x has a complicated structure which is clustered by many nano-layer structures. In order to verify the existence of OVs in MoO3-x, we measured the Raman and X-ray photoelectron spectroscopy (XPS) spectra of MoO3-x and MoO3, respectively, as shown in Fig. 1(b) and 1(c). In comparison between Raman spectra of MoO3-x and MoO3, we found that the intensity ratio between the two wagging modes of the terminal Mo = O1 group at 283 cm−1 (B2g) and 290 cm−1 (B3g) for MoO3-x is much lower than that for MoO3. The reduction of the intensity ratio reveals the broken symmetry which is induced by the OVs [43]. With the XPS spectra, the composition and oxidation state of MoO3-x was studied. For a more detailed Mo 3d spectrum, the overall curve can be fitted into two peak groups, which are attributed to two different oxidation states of Mo ions. The first peak groups with binding energies of 235.8 eV and 232.7 eV were ascribed to Mo6+, and the peak groups with lower binding energies of 234.4 eV and 231.3 eV were corresponded to the oxidation state Mo5+[44]. The result of mixed-valent Mo ions in MoO3-x further proved the existence of enriched OVs. After separately dissolving ∼15 mg materials (MoO3-x/MoO3) in ∼20 ml ethanol, we fabricated the corresponding films with the method of suction filtration and measured the relative absorption properties. Due to the OVs, the optical absorptions of MoO3-x, both in intensity and wavelength, have increased, as plotted in Fig. 1(d). With the limitation of the light source, a wider range had not been measured.
Fig. 1. (a) SEM image of MoO3-x (insert figures are powders of MoO3-x (dark blue one) and MoO3 (off-white one), respectively), (b) Raman spectra of MoO3-x and MoO3, (c) XPS spectra of MoO3-x and MoO3, (d) the relative absorption curves of MoO3-x and MoO3. Note that part of the results of MoO3-x samples are taken from previous work for comparison [41].
In Fig. 2, we illustrated the fabrication procedure of MoO3-x-PVA film. Firstly, the MoO3-x ethanol solution was prepared with concentration of ∼1 mg/mL after ∼2 hours ultrasonic agitating. Following, the dispersed MoO3-x ethanol solution mixed with Polyvinyl Alcohol (PVA) aqueous solution (∼12 wt%). After the mechanical and ultrasonic dispersion, the well dispersed mixed-solution was poured into a polystyrene cell and dried in a 50 °C vacuum oven for longer than 24 hours. Finally, the MoO3-x-PVA film with surface area of several square centimeters and thickness of ∼55 micrometers was prepared, as shown in Fig. 2. It should be noted that the thickness of the film can be easily tuned by controlling the concentration and ratio of PVA solution. With the consideration of the insert loss and the convenience of integrating, a moderate thickness was utilized in our experiment.
For the convenience of integrating the film into fiber laser, we further cut it into smaller pieces (∼1×1 mm2). Relying on the self-made two-arm structure experiment setup, the nonlinear optical absorption of the MoO3-x-PVA was measured. The 2 µm probe light source was a home-made mode-locked thulium-doped fiber laser, operating at ∼1958 nm with pulse duration and repetition rate of ∼725 fs and ∼55 MHz, respectively. Figure 3 gives the detail nonlinear absorption curve of the MoO3-x-PVA film. It was clear that the transmittance of the film was increasing with the peak intensity of the pulse and then reaching a stable value. The modulation depth, saturable intensity and non-saturable absorption were ∼15.7%, ∼14 MW/cm2 and ∼52.9%, respectively.
3. Experimental setup
The schematic diagram of the Q-switched thulium-doped fiber laser is presented at Fig. 4. A section of ∼3 m double-clad TDF (SM-TDF-10P/130-HE, Nufern) was utilized as the gain fiber, which was pumped by a ∼793 nm diode laser (LD) via a (2 + 1)×1 combiner. At the end of the TDF, a home-made cladding-power-stripper (CPS) was used for leaking out the residual pump light. Whereafter, a polarization independent isolator (PI-ISO) was placed to ensure the unidirectional operation of the fiber laser. About 45% laser energy was extracted from the laser cavity by the following fiber fused coupler. The MoO3-x-PVA film was sandwiched between two optical fiber ferrules as a Q-switcher, while two polarization controllers (PCs) were placed at both sides of the SA to ensure the proper polarization states and optimize the Q-switching pulse performance. Thus, a simple ring cavity TDF laser was implemented with total cavity length of ∼13 m.
4. Results and discussion
At the beginning, the MoO3-x film was not inserted into the laser cavity, and no stable pulse was observed with properly increasing the pump power and fully manipulating the PCs. The fiber oscillator can only operate at continuous-wave state, although there are some chaotic noise-pulses can be observed, which are resulted from the self-pulsing operation [45]. Nevertheless, when the MoO3-x film was sandwiched between the two fiber ferrules, noticeable and regular Q-switched pulses were initialized at pump power of ∼2.17 W. Further increasing the pump power, the Q-switched pulses became more stable. Figure 5 exhibits the typical recorded pulse trains by an oscilloscope (Tektronix, DPO 7104C) under different pump powers of ∼2.26 W, ∼2.55 W, ∼2.83 W, and ∼2.97 W, respectively. The pulse repetition rate and intensity are increased with the pump power, which coincides with the characteristics of the passively Q-switched operation, since the larger pump power will lead to faster build-up progress of Q-switched pulse and higher pulse energy.
Fig. 5. Q-switched pulse trains at different pump power, (a) ∼2.26 W; (b) ∼2.55 W; (c) ∼2.83 W; (d) ∼2.97 W.
The single pulse profile with pulse duration of ∼2 µs at maximum pump power of ∼2.97 W is shown in Fig. 6(a). In our experiment, the mainly reason for the large pulse fluctuation is considered as follows. In the laser resonator, the double-clad Tm-doped fiber was used as gain fiber, which was cladding-pumped by a 793 nm LD with a high pump power. The relative poor stability of cladding-pumped and power fluctuation of pump source will increase the fluctuation of the pulse. Moreover, we noted that the double-clad Tm-doped fiber was more sensitive to the polarization than the single-clad fiber, which will be easy influenced by the environment disturbance. Similar experimental results with double-clad Q-switched Tm-doped fiber laser were shown in [13,46]. The corresponding output spectrum was measured by an optical spectrum analyzer (Bruker, Tensor 27) with resolution of ∼0.2 nm, as shown in Fig. 6(b). The Q-switched fiber laser oscillated at central wavelength of ∼1992.1 nm with 3dB bandwidth of ∼1 nm. It should be noted that, in the experiment, we further increasing the pump power larger than ∼2.97 W, the Q-switched pulses would be unstable and vanished eventually, which indicates the pump power approached to the damage threshold of the SA. For better illustrated the stability of the passively Q-switched operation, we measured the radio-frequency (RF) spectra of the Q-switched pulses by a spectrum analyzer (Agilent, E 4407B) at pump power of ∼2.97 W, as given in Fig. 6(c). The signal-noise ratio of the fundamental repetition rate of ∼38.5 kHz is about ∼60 dB, which means a relatively good stability of the passively Q-switched operation. Moreover, a larger scale harmonic RF spectrum with resolution of ∼1 kHz is shown in the inset of the Fig. 6(c).
Fig. 6. (a) Profile of Q-switched pulse at pump power ∼2.97 W; (b) Corresponding output spectrum; (c) Output radio-frequency spectra
The output characteristics of the passively Q-switched TDF laser are shown in Fig. 7. Figure 7(a) depicted the variation of pulse duration and pulse repetition rate with the pump power. The pulse duration shorten from ∼7.2 µs to ∼2 µs, while the pulse repetition rate increased from ∼15.5 kHz to ∼38.5 kHz with pump power increasing from the threshold of ∼2.17 W to the maximum value of ∼2.97 W. Also, the average output power of the Q-switched TDF laser was measured at the same range of pump power, as illustrated in Fig. 7(b). The Q-switched fiber laser has maximum output power of ∼62.7 mW with a moderate output slope efficiency of ∼7.5%. According to the output powers, the corresponding pulse energies with maximum value of ∼1.63 µJ were calculated, as also can be seen in Fig. 7(b) blue line.
Fig. 7. (a) Pulse duration and repetition rate versus pump power; (b) Average output power and pulse energy variation with pump power.
In order to clearly exhibit the Q-switched performances with the proposed thin-film SA, Table 1 summarized the representative output performances of all fiberized passively Q-switched Tm-doped ring-cavity fiber lasers by other thin-film SAs. As shown in Table 1, our Q-switched pulses have the largest average output power and pulse energy. Moreover, considering the pulse duration of the Q-switched pulse has the positive correlation with the cavity length, the pulse duration in our experiment could be shortened by optimizing the cavity length(cavity length of this experiment is ∼13 m).
Table 1. Comparison of our work with the typical passively Q-switched Tm-doped ring-cavity fiber lasers by other thin-film SAs
Furthermore, to verify the saturable absorption property of the MoO3-x was caused by the OVs, we also have tested the OVs-recovered MoO3 (off-white powder, as shown in Fig. 1(a)). With the same parameters and procedure, depicted in Fig. 2, we prepared the MoO3-PVA film, and replaced the MoO3-x-PVA film in laser cavity. Nevertheless, the fiber laser just could operate at continuous-wave state, similar with experiment results without SA except a higher laser threshold. This result will further prove the saturable absorption property of the MoO3-x originated from the OVs. In fact, the saturable absorption property caused by vacancy defects has been guessed in many kinds of TMDs [28], since most of these materials exhibited the sub-bandgap absorption and the vacancy defects could result in complex defect bandgaps. Considering the amount of OVs can be controlled in the synthetic progress, and it might have influence on modulation depth and absorption bandwidth, the potential application of mode-locked fiber laser or other wavelength pulse lasers with MoO3-x could be expected, as well as other vacancy deficient structure materials.
5. Summary
In a summary, we have demonstrated a passively Q-switched TDF laser with oxygen deficient structure MoO3-x-PVA film. The TDF laser can deliver Q-switched pulse with pulse duration of ∼2 µs, maximum output power of ∼62.7 mW and pulse energy of ∼1.63 µJ, at central wavelength of ∼1992.1 nm. To the best of our knowledge, it is the first report of utilizing MoO3-x as a SA for 2 µm wavelength pulse laser generating. The experiment results are qualitative evidence of vacancy defects can cause the material to behave the property of optical saturable absorption. Also, it will provide a novel way for wide-band optical saturable absorber preparation. Moreover, the wideband saturable absorption properties of the MoO3-x in 1.5 µm range, even 3 µm range could be investigated.
Funding
National Natural Science Foundation of China (61575129); Science and Technology Projects of Shenzhen City (JCYJ20160328144942069).
Disclosures
The authors declare no conflicts of interest.
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