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Abstract
A new type of saturable absorber (SA) based on indium tin oxide (ITO) nanocrystals for passively Q-switched erbium-doped fiber (EDF) laser has been demonstrated experimentally. High quality ITO nanocrystals were prepared by the coprecipitation method. As a degenerate oxide semiconductor, the ITO has a wide range of saturable absorption regions, strong plasmonic absorption peaks, and ultra-fast recovery time. The nonlinear absorption properties of ITO are investigated by a balanced twin-detector measurement system. The saturation intensity and modulation depths are 9.87 MW/cm2 and 0.83%, respectively. We successfully fabricated ITO SAs by depositing the ITO dispersion solution at the end of a fiber connector. By inserting the ITO SAs into an EDF fiber cavity, stable passively Q-switched pulses with the minimum pulse width of 1.15 μs under a repetition rate of 81.28 kHz were obtained. The maximum output power of 1722 μW and pulse energy of 21.19 nJ were realized when the pump power was 480 mW at the wavelength of 1530.3 nm. The results suggest that ITO nanocrystals are expected to develop into another effective SA for ultrafast photonics.
© 2017 Optical Society of America
1. Introduction
Q-switched fiber lasers have attracted great attention because of their various applications in laser processing, telecommunication, optical fiber sensing, range finding, and medicine [1–5]. Passively Q-switched technique with saturable absorbers (SAs) was the most common method to achieve Q-switched operation in fiber lasers. In the past two decades, many efforts have been payed to the study of different kinds of SAs for passively Q-switched fiber lasers, such as the ion-doped crystals [6,7], semiconductor saturable absorption mirror (SESAM) [8,9], carbon nano-tube (CNT) [10,11], graphene [12,13], transition-metal dichalcogenides (TMDs) [14–16], topological insulator (TI) [17,18] and black phosphorous (BP) [19,20]. Among them, SESAM is the most widely used because of its high flexibility and stability. But, SESAM has the drawbacks of expensive price, complex fabrication and narrowband wavelength operation range, which greatly limits its application [21]. For graphene based SAs, it attracted a great deal of interest duo to their broadband saturable absorption and ultrafast recovery time [22,23]. However, when the strong light-matter interaction is required, the low absorption coefficient and the absence of band-gap of graphene also restraint its applications. Besides, TMDs (MoS2, WS2) has unique absorption property and thickness dependent band-gap [24,25]. It has also been extensively studied and successfully applied to passively Q-switched fiber lasers as SAs. Although it is possible to introduce TMDs into infrared or mid infrared optoelectronics by introducing some suitable defects, the fabrication process is complex. TI (Bi2Se3, Bi2Te3,etc.) based SAs have also attracted much attention of researches due to their gap-less surface states and indirect band-gap of 0.35 eV [26–28]. In addition to the materials mentioned above, the BP [29,30], MoSe2 [31] and ReS2 [32] as SAs has been successfully applied to passively Q-switched fiber lasers. However, the inefficient light-matter interaction due to the intrinsic property of these materials is a bottleneck for optical nonlinearity that limits their applications in optoelectronic devices [33]. Thus, researchers are still making some efforts even now to seek for new SAs which are expected to have the ideal characteristics of ultrafast recovery time, broad saturable absorption region, strong light-matter interaction, low cost and uncomplicated preparation process. By comparing different SAs based on various nanomaterials, a stable sequence is proposed for us to select SAs: oxide plasmonic nanocrystals > graphene, TMDs > black phosphorus (BP) and other metal compounds (selenide, telluride, etc.) [34]. Strong plasmonic absorption peak with a broad bandwidth, large optical nonlinearity, efficient light-matter interaction and well-developed preparation process determines the oxide plasmonic nanocrystals as a promising SA [35,36]. Hence, it is desirable to select the oxide plasmonic nanocrystals as SAs for passively Q-switched fiber lasers.
To our knowledge, we first demonstrated that the the indium tin oxide (ITO) nanocrystals can be used as SAs for passively Q-switched fiber lasers. ITO was selected as the SAs has the following three reasons: first, by controlling the concentration of tin doping, the position of the plasmonic absorption peak of ITO can be varied from 1600 to 2200 nm [37]. This property enables ITO to have a wide range of saturable absorption regions; second, due to much lower carrier density of most conductive oxides, the plasmon frequencies of ITO are located in the NIR. This allows ITO to have a strong plasmonic absorption peak with a broad bandwidth [35]; third, a recent study shows that ITO thin films exhibit large optical nonlinearities [36]. They report that ITO can acquire an ultrafast and large intensity-dependent refractive index and has a ultrafast recovery time of about 360 fs. So, ITO is expected to become a qualified saturable absorber for passively Q-switch fiber lasers.
In our work, we use a coprecipitation method to obtained ultrafine ITO nanocrystals with an average size of 33.6 nm. The ITO SAs are constructed by direct dripping of the ITO dispersion solution onto the end of a fiber connector. The fabrication of the ITO SAs device is simple and straightforward. By inserting the ITO SAs into the erbium-doped fiber (EDF) laser, stable Q-switched pulses were obtained at 1.5μm. This suggests that ITO nanocrystals could be developed as an effective SA for ultrafast photonics.
2. Experimental
2.1 Preparation of ITO nanocrystals
The ITO nanocrystals with the In/Sn atomic percent of 91.2/8.8 were prepared by a coprecipitation method. First, the metal indium was dissolved in sulfuric acid to get the In2(SO4)3 solution. And then the precursor solutions were obtained by mixing the In2(SO4)3 solution and the SnCl4 solution at In2O3/SnO2 = 9/1 (mass ratio). The precursor solution was heated in a constant temperature water bath at a temperature of 70 °C. Next, the NaOH solution (2 mol/L) was added to the precursor solution at high speed stirring to make the pH value reach 7. In order to make the chemical reaction complete, we continued to stir the solution for 30 min. Finally, ITO nanocrystals were obtained by filtration, washing, drying, calcining and grinding the coprecipitation precursor.
2.2 Preparation of ITO SAs
Figure 1 shows the preparation process of the ITO SAs. First, 0.75 g ITO nanocrystals were mixed in the 150 ml alcohol. The mixture was placed in the ultrasonic cleaner for 3 hours to get the ITO dispersion solution. Second, the ITO dispersion solution was dripped directly onto the entire fiber end-facet. Finally, the ITO-based SA was successfully fabricated by evaporating the fiber end-facet into the oven for 30 minutes.
2.3 Apparatus and characterization
The morphologies of ITO nanocrystals were observed using a transmission electron microscopy (TEM) system (Hitachi H-800). The absorbance spectra of the ITO dispersion solution were measured using UV/VIS/NIR spectrophotometer (Jasco V-570). The energy-dispersive X-ray (EDX, Zeiss Gemini Ultra-55) spectroscopy was used for chemical composition analysis of ITO nanocrystals. The crystalline quality of ITO nanocrystals was characterized by X-ray Diffraction (XRD, Bruker D8).
3. Results and discussions
To gain information about the morphology and structural features of the ITO nanocrystals, the prepared ITO nanocrystals were investigated by using TEM system, as shown in Fig. 2(a). Spherical and elongated shaped ITO nanocrystals can be seen from the TEM image. We randomly chose one hundred ITO nanocrystals from the TEM image for statistical analysis, as presented in the inset of Fig. 2(a). The diameter of the nanocrystals is coexistent in the range of 14-60 nm and the average diameter is 33.6 nm. As can be seen from Fig. 2(b), the lattice fringe is observed from the HRTEM image, which indicates that ITO nanocrystals prepared by a coprecipitation method have high quality. We used EDX spectroscopy to characterize the elemental content of the prepared ITO nanocrystals, as shown in Fig. 2(c). The peaks associated with indium and tin are clearly observed and the atom ratio of In: Sn is 91.2: 8.8, as shown in the inset of Fig. 2(c). As shown in Fig. 2(d), the crystal structure of the ITO nanocrystals was investigated by using XRD spectroscopy. It can be seen that all the diffraction peaks are in good agreement with ITO peaks (JCPDS No. 06—0416), which indicates that ITO was successfully prepared by coprecipitation method. The optical absorption of the ITO dispersion solution was examined with UV/VIS/NIR absorption spectroscopy, as shown in Fig. 2(e). Based on the UV/VIS/NIR absorption spectrum, the ITO has a wide absorption bandwidth in the range of 1200-2000 nm and the absorption peak is about 1643 nm. This optical absorption is attributed to the localized surface plasma resonance (LSPR) of the ITO nanocrystals. The LSPR arising from the collective oscillation of conducting electrons features a strong and broad absorption band, whose bandwidth and location depends on the carrier density and geometric factors. The ITO nanocrystals have lower carrier density and nanoscale size, which allows the plasmonic resonance peak of ITO have a wide bandwidth in the NIR. Thus, the ITO nanocrystals offering new possibilities for modern photonic applications in the NIR range. We measured the nonlinear absorption properties of the ITO SAs by using a balanced twin-detector measurement system with 480 fs pulses at a wavelength of 1562.3 nm, as shown in Fig. 2(f). The saturation intensity and modulation depth were obtained by fitting the data using the following formula [21]:
where is transmission, is modulation depth, is input intensity of laser, is saturation intensity and is non-saturable absorbance. By fitting, the saturation intensity and modulation depths are 9.87 MW/cm2 and 0.83%, respectively.
Fig. 2 (a) The TEM image and particle size distribution of the ITO nanocrystals. (b) The HRTEM image of the ITO nanocrystals. (c) EDX data from the ITO nanocrystals. (d) The XRD pattern of the ITO nanocrystals. (e) The absorption spectrum from the ITO dispersion solution. (f) Measured saturable absorption data of the ITO SAs.
The Q-switched EDF laser based on ITO SAs is schematically shown in Fig. 3. A piece of 28 cm erbium-doped fiber (EDF) used as gain medium was pumped by a 974 nm laser diode, coupled through a 980/1550 nm wavelength division multiplexer (WDM). The rest of the fiber used is standard single mode fiber (SMF). An optical polarization-independent isolator (PI-ISO) was used to ensure the unidirectional light propagation. Two polarization controllers (PC) were placed on both sides of the PI-ISO to optimize the pulse stability and fine tune the birefringence of the laser cavity. A 10% output coupler was used to output the signal. The output coupler and WDM are connected by fiber flange. The ITO-based SAs were placed inside the fiber flange. The total cavity length was about 9.07 m and the overall dispersion of the laser cavity is about −0.188 ps2. The blue arrow indicates the direction of light propagation in the laser cavity.
Fig. 3 Schematic of Q-switched fiber laser: laser diode, single mode fiber (SMF) wavelength division multiplexer (WDM), erbium-doped fiber (EDF), polarization controller (PC), polarization-independent isolator (PI-ISO), output coupler and ITO SAs.
In the experiment, stable Q-switched operation from EDF laser based on ITO SAs was obtained once the pump power exceeded a threshold of 160 mW. Figure 4 summarizes the characteristics of the Q-switched pulses emitted from the fiber laser. As shown in Fig. 4(a), a typical stable Q-switched pulse train of the fiber laser at the pump power of 320 mW was obtained. The individual pulse has a nearly uniform intensity distribution, which demonstrates the excellent stability of the Q-switched pulses train. Besides, the repetition rate of the pulse train is 56.94 kHz. In order to study the pulse train in detail, the corresponding single pulse profile was measured with a narrower sweep span of 1 us/div, as presented in Fig. 4(b). The pulse has a symmetric intensity profile with a full width at half maximum (FWHM) of 1.742 μs. The corresponding output optical spectrum is shown in Fig. 4(c), where a 3 dB spectral bandwidth of 0.05 nm and the central wavelength of 1530.3 nm are observed. As shown in Fig. 4(d), the evolution of the Q-switched pulses trains with the increasing of the pump power was revealed. From a series of images, we can clearly see that as the pump power increases, the pulse interval decreases gradually. Besides, the pulse trains always maintain a uniform intensity distribution without obvious fluctuation, which is the typical feature of Q-switched operation. These experimental results demonstrate the successful performance of passively Q-switched fiber lasers based ITO SAs.
Fig. 4 (a) Typical Q-switched pulse train, (b) single pulse profile and (c) output optical spectrum from EDF laser based on ITO SAs at pump power of 320 mW. (d) Evolution of Q-switched pulse trains with increasing pump power.
The relations between the repetition rate, pulse width, output power and pulse energy of the Q-switched pulses and the different pump power are summarized in Fig. 5. Figure 5(a) shows the repetition rate and pulse width versus pump power. The repetition rate of the Q-switched pulses increased near linearly from 37.24 to 81.28 kHz with the increasing of pump power from 160 to 480 mW. However, the decrease of the pulse widths from 3.71 to 1.15 μs is nonlinear. The pulse width decreases rapidly in the low pump power range, probably due to the fast accumulation of electrons at the upper energy level. But, when the pump power reaches a relatively high range, the speed of accumulation slows down due to the over-saturation of ITO SAs, leading to the no obvious change of pulse width. Figure 5(b) demonstrates the evolution of the output power and pulse energy with the pump power. With the increasing of pump power from 160 to 480 mW, the output power linearly increases from 204 to 1722 μW, while the single pulse energy varied in the range of 5.47 to 21.19 nJ. To investigate the long-term stability of the Q-switched pulses, we measured the output power every hour at a fixed pump power of 480 mW, as shown in the Fig. 5(c). The shadow area represents the vibration range of the output power. The black line in the shaded area is the average output power. All the intensities of the output power lie within a 2.8% variation range, which reveals the excellent stability of the Q-switched pulses at pump power of 480 mW. We also tried to increase the pump power to 500 mW. At first the Q-switched pulses becomes unstable, and then the Q-switching state suddenly disappears. We cannot get the Q-switched pulses again even if reduce pump power. The reason for this may be that the ITO SAs is broken by the laser. We believe that some impurities were doped when we prepared ITO nanocrystals. When the material is irradiated by laser, these impurities leading to the impurity absorption and form localized high temperature on the surface of the material [38]. The accumulation of heat in the film causes the expansion of the material in the local area, and changes the force field of the material. When the force field exceeds the critical strength of the material rupture, the damage of the material is caused.
Fig. 5 (a) The repetition rate and the pulse width as a function of pump power, (b) The output power and the pulse energy as a function of pump power. (c) The output power was measured every hour at a fixed pump power of 480 mW.
4. Conclusion
In conclusion, a stable passively Q-switched EDF laser enabled with ITO SAs at 1.5 um has been demonstrated. The Q-switched operation was achieved for a threshold pump power of 160 mW. The highest repetition rate and shortest pulse width of the Q-switched pulses are 81.28 kHz and 1.15 μs, respectively. The maximum output power was 1722 μW at pump power of 480 mW, corresponding to a single-pulse energy of 21.19 nJ. Our experimental results clearly show that ITO nanocrystals can be used as effective SAs for Q-switched fiber laser.
Funding
National Natural Science Foundation of China (NSFC) (Grant No. 11674199, 11474187, 11405098, 61205174); Excellent Young Scholars Research Fund of Shandong Normal University; China Postdoctoral Science Foundation (2016M602177); Shandong Provincial Natural Science Foundation (ZR2016FP01).
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