We demonstrated a high-repetition-rate Q-switched fiber laser with topological insulator Bi2Se3 absorber. The absorber was made into a film structure by spin-coating method using few-layer Bi2Se3 nano-platelets which had regular shape. The uniform film had a low saturable optical intensity of 11 MW/cm2, which is the lowest saturable optical intensity in the saturable absorbers made by topological insulator till now. By inserting the absorber film into an Erbium-doped fiber laser, a high-repetition Q-switched laser with the repetition rates from 459 kHz to 940 kHz was achieved. The maximum output power was 22.35 mW with the shortest pulse duration of 1.9 μs. To the best of our knowledge, both of the repetition rate and the output power were the highest values among the Q-switched fiber lasers with topological insulator absorber.
© 2014 Optical Society of America
High-repetition-rate Q-switched fiber lasers are particularly desirable in medicine, high-resolution photoacoustic microscopy, remote sensing and optical parametric oscillators [1–3]. Passively Q-switching technique with saturable absorber (SA) was the most effective method to achieve the Q-switched operation in lasers. In the past two decades, there are several kinds of SAs, such as dye SAs, semiconductor saturable absorber mirrors (SESAMs) , carbon nanotubes (CNTs) , and graphene . Although the SESAMs are very mature but still limited by narrowband wavelength operation range, complex fabrication and expensive price . The easy fabrication and cheap cost make the carbon nanotube SAs very popular, but their working wavelengths were connected with the diameter of the nanotubes . The zero-gap material graphene could be used as wideband SAs, but they have a small absorption of 2.3% at 1550 nm . In order to obtain ideal SAs with the characters of wavelength-independent, low saturable optical intensity, high damage threshold and large modulation depth, researchers are still making effort. Most recently, the topological insulator (TI) was studied and the character of the optical saturable absorption of this material was confirmed. Till now, the TI has been used as the SA for mode locking and Q-switching technology [10–16].
TI is states of quantum matter with the metallic states on the surface and has the narrow topological non-trivial energy gaps for TI: Bi2Se3 the ΔE is about 0.3eV , exhibiting the Dirac-like linear dispersion. When the narrow-bandgap TI is excited by strong light with the single-photon energy more than the TI bandgap, the saturable absorption could happen due to the Pauli blocking principle [10–12]. Recently, Zhao et al. have measured the saturable optical intensity of multilayer (>50 layer) and it is 0.49 GW/cm2 at 1550 nm . By further inserting such TI SAs, which were fabricated by dropping the dispersion onto quartz plane directly, into the fiber laser cavities, they have successfully obtained the mode-locked Er-doped fiber lasers at 1550 nm [10, 11]. Luo et al. have measured the saturable optical intensity of few-layer TI: Bi2Se3 (53 MW/cm2 at 800 nm), which was lower than the saturable optical intensity of multilayer SAs. The few-layer SA was used in the Yb-doped fiber laser by optical deposition and Q-switching operation was achieved . However, it should be noted that: 1) most good TI: Bi2Se3 nano-platelets (NPs) exhibit hexagonal morphologies with planar dimensions extending up to several micrometers, while some of them show a truncated trigonal morphology . But, in Ref. 14, the few-layer TI: Bi2Se3 NPs were irregular. In the Ref. 11, the few-layer TI: Bi2Se3 NPs were partly regular but too thick (> 50 layer). 2) The TIs were directly dropped on quartz or directly deposited on the surface of the fiber, so the SAs were not well-distributed. If few-layer regular shaped Bi2Se3 NPs were well-distributed in polyvinyl alcohol (PVA) and fabricated into film by spin-coating, its saturable optical intensity is expected to be lower and the corresponding mode-locking/Q-switching might be started more easily. In Ref. 15, PVA-based topological insulator Bi2Se3 SA was used in the Er-doped fiber laser, which pulse width was 660 fs and average output power was 1.8 mW. In Ref. 16, bulk-structured Bi2Te3 was deposited onto a side-polished fiber working as SA, and 600 fs pulse was obtained. Because of the high loss of the bulk-structured SA, the output power was 0.8 mW.
In this paper, few-layer high quality Bi2Se3 nano-platelets which had regular shape were obtained by optimizing the reaction condition. Different from the recently reported papers [10, 11, 14], the nano-platelets were well-distributed in polyvinyl alcohol solution. Then a uniform film was achieved by spin-coating method. The saturable optical intensity of the high quality film was 11MW/cm2 at 1550 nm, which was much lower than the reported result of 490 MW/cm2 . By inserting the absorber film into an Erbium-doped fiber laser, a high-repetition Q-switched laser with the repetition rates from 459 kHz to 940 kHz was achieved. The maximum output power was 22.35 mW with the shortest pulse duration of 1.9 μs. The maximum pulse energy is 23.7 nJ at center of 1565 nm.To the best of our knowledge, both of the repetition rate and the output power were the highest values among the Q-switched fiber lasers with topological insulator absorber.
2. Preparation of Bi2Se3/PVA film
Here, the Bi2Se3 NPs we used were synthesized via a polyol method . All reagents were of analytic purity and used without further purification. A 0.60 g Bi(NO3)3•5H2O, 0.3 g sodium selenite, 1.32 g polyvinyl pyrrolidone, and 60 mL ethylene glycol were added into a 250.0 mL two-neck flask containing a Teflon-coated magnetic stirring bar. The flask was connected with a reflux condenser and placed on a heating mantle. The temperature of the solution was increased to 190 °C under constant stirring. In order to achieve regular Bi2Se3 NPs, the chemical reactions were reacted with different time. Then the products were cooled, centrifuged, washed with isopropyl alcohol (IPA) several times. Figure 1 is the SEM images of the Bi2Se3 NPs with different chemical reaction time: (a) 40 min, (b) 100 min, (c) 140 min and (d) 180 min. After 40 min, the products were rod-like. After 100 min, many large NPs were exfoliated from rods, and the size of the large NPs were about 5-10 μm. After 140 min, the quantity of the small size NPs increased significantly, but there were still a lot of large size NPs. After reaction time of 180 min, the large size NPs disappeared, most of the products were small size NPs. The products with the 180 min reaction time of NPs were dried. Then put the NPs powder into 10 mL aqueous solution and ultrasonically agitated for 10 minutes. In order to move out the small and large NPs, the solution was centrifuged at 7000 r/min and 4000 r/min respectively and the normal size NPs was left. Figure 2 is the SEM image of the NPs with regular shape after centrifugation. The AFM image of the TI: Bi2Se3 NPs was measured (Fig. 3) and the thickness of the TI: Bi2Se3 NPs was about 5-7 nm. Normally the thickness of single-layer Bi2Se3 was 0.96 nm , so the TI: Bi2Se3 NPs we obtained were 5-7 layers.
Put some PVA powder into the Bi2Se3 solution and dissolved at 90 °C with ultrasonic agitation for 2 hours and then the Bi2Se3/PVA dispersion was obtained. By spin-coating the dispersion and the Bi2Se3/PVA film was reached finally. We measured the absorption spectrum of PVA film and Bi2Se3/PVA film from 1520 nm −1620 nm (Fig. 4(a)). The absorbance curve of the PVA film was almost flat, while the Bi2Se3/PVA film had a higher absorbance of 49% at the 1520 nm and lower absorbance of 32% at the 1620 nm. At the 1565 nm, the absorbance of the TI film was 43.5%. The nonlinear transmittance of the Bi2Se3/PVA film was measured using the near field Z-scan technique (Fig. 4(b)). The pump source used for near field Z-scan test was a femtosecond fiber laser with a pulse width of 100 fs, repetition rate of 150 MHz and maximum output power of 13 mW. The initial transmittance was about 59.4% and the saturable transmittance was about 63.7%, which indicated the modulation depth was 4.3%. The saturable optical intensity of the Bi2Se3/PVA film was 11 MW/cm2 at 1550 nm, which was about 45 times less than the reported one (0.49 GW/cm2) of the multilayer Bi2Se3 (>50 layer) . The low saturable optical intensity indicated that the optical property of the few layer Bi2Se3 NPs with regular shape was much advanced compared with the multilayer Bi2Se3 NPs. With such low saturable intensity, it can be predicted that the threshold of mode-locking or Q-switching operation using the Bi2Se3/PVA film could be significantly reduced. Meanwhile the repetition rate of the Q-switching could also be greatly increased.
3. Experimental setup of TI-SA passively Q-switched Er-doped fiber laser
First step was to prepare a TI-SA. Cut the Bi2Se3/PVA film into small pieces and put it between the two surfaces of a fiber connecter. Then the TI-SA was achieved. Figure 5(a) shows one surface of the fiber connecter with the Bi2Se3/PVA film. Figure 5(b) shows the picture of TI: Bi2Se3 solution with 180 min reaction time. The schematic diagram of the Q-switched fiber laser is shown in Fig. 5(c). The ring cavity consisted of a gain fiber, a wavelength division multiplexer (WDM), a polarization independent isolator (PI-ISO), optical coupler (OC), a polarization controller (PC) and a TI-SA. A 40cm long erbium doped fiber (EDF) (LIEKKI Er110-4/125) with absorption coefficient of 61dB/m at 980 nm was employed as the gain medium. The EDF was pumped by a 980 nm laser diode (LD). The PI-ISO was used to force the unidirectional operation in the fiber ring cavity. The PC was engaged to achieve different polarization orientation states. The optical coupler was used, 20% portion of the laser was coupled out from the cavity. The total of the fiber laser cavity was about 7 m.
4. Results and discussions
In the experiment, the EDF laser started the continuous-wave lasing at the pump power of 80 mW, and the Q-switched operation obtained at 96 mW. The laser threshold of 80 mW was slightly higher, which was primarily attributed to the higher cavity loss in the fiber laser. In the experiment, the gain fiber was LIEKKI Er 110-4/125 with a core diameter of 4 μm, which could not match with the other fibers (diameter: 7 μm). Thus the cavity loss in the fiber was relatively larger and the threshold of the fiber laser was slightly higher. When the pump power was increased, the repetition rates increased at the same time (Fig. 6(a)). The shortest pulse width of 1.9 μs was achieved with the pump power of 360 mW (Fig. 6(b)). When the pump power was over 360 mW, the Q-switched pulses became unstable. Increase the pump power further to 400 mW, the Q-switching stopped. However, after the pump power was decreased from 400 mW, the stable Q-switched operation was observed again. This phenomenon indicated that the TI-SA was not destroyed by the thermal accumulation. The possible reason for the unstable Q-switching maybe was the over-saturation of the SA at higher pump intensity. This kind of unstable phenomenon also was observed in some other passively Q-switched fiber lasers [14, 19, 20]. If the pump power was increased to 415 mW, the Q-switched pulse would disappear permanently, so the damage threshold of the TI-SA was 415 mW.
Figure 7(a) shows the pulse widths and repetition rates versus incident pump powers. The pulse widths decreased from 7.76 μs to 1.9 μs while the pump power increasing from 96 mW to 360 mW, and the repetition rate increased from 459 kHz to 940 kHz. The high repetition rate of the Q-switched fiber laser could be attributed to the low saturable optical intensity. Till now, the repetition rates reported in Q-switched lasers were in a range of ~200 kHz at 1.5 μm . In our experiment, the maximum repetition rate was 940 kHz, which was 4.7 times of the highest repetition rate ever reported . The high repetition rate of the Q-switched fiber laser should be owed to the lower saturable optical intensity of the TI- SA. The lower the saturable energy of the SA is, the shorter the built up time of the pulses is, so the Q-switched fiber laser could sustain with a high repetition rate. The shortest pulse width of 1.9 μs was similar to the reported passively Q-switched fiber lasers with TI-SAs, CNTs-SAs and graphene SAs [13, 14, 19–21]. The pulse width of 1.9 μs was slightly wider for the Q-switched lasers. The pulse width can be improved by shortening the laser cavity and increasing the modulation depth of the TI-SA . The average output power and single pulse energy versus incident pump power were shown in Fig. 7(b). At the pump power of 360 mW, the maximum average output power was 22.35 mW. Compare with the other Q-switched fiber lasers with TI-SAs, in which the maximum average output powers were only 145 μW and 0.46 mW, the average output power in our experiment was significant advanced [13, 14]. The maximum single pulse energy was about 23.8 nJ.
The spectra of the CW and Q-switched pulses under different pump powers were shown in Fig. 8. The spectra were multi-peak structure at the center of 1565 nm. When the pump power was increased, the wavelength curves assume blue shift. The spectrum was split into dual- or triple-wavelength and the interval of the spectrum of the laser was 0.7 nm. This may be caused by the F-P effect from one optical element because of the fixed frequency interval. In the experiment, by adjusting the polarization controller, the dual- or triple-wavelengths can operate stably. The RF spectrum of the Q-switched fiber laser was provided (Fig. 9), in which the signal-to-noise ratio of 50 dB indicated the Q-switching state was very stable.
A high-repetition-rate Q-switched fiber laser with a topological insulator Bi2Se3 absorber was demonstrated. A low saturable optical intensity TI-SA film was made by regular shaped few-layer Bi2Se3 NPs and PVA. The saturable optical intensity of the TI-SA was as low as 11 MW/cm2. With the TI-SA, a high-repetition-rate Q-switched laser was achieved. The highest repetition rate was 940 kHz The Q-switched pulses have the shortest pulse duration of 1.9 μs, and the maximum pulse energy of 23.7 nJ at center of 1565 nm. The results indicate that the TI-SA made by the regular shaped Bi2Se3 NPs had good optical property and when using it in a Q-switched fiber laser, both of the repetition rate and the output power could be increased significantly.
This work was supported by the 973 program (Grant No. 2013CB922404), the National Scientific Research Project of China (Grant No. 61177047) and the key project of the National Natural Science Foundation of China (Grant No. 61235010).
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