Doped ZBLAN Fiber Lasers Q-Switched by Topological ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 22, NOVEMBER 15, 2015

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635-nm Visible Pr3+-Doped ZBLAN Fiber Lasers Q-Switched by Topological Insulators SAs Duanduan Wu, Zhiping Cai, Yile Zhong, Jian Peng, Jian Weng, Zhengqian Luo, Member, IEEE, Nan Chen, and Huiying Xu Abstract— Passive Q-switching of praseodymium (Pr3+ )-doped fiber lasers at 635 nm with topological insulators (TIs) saturable absorbers (SAs) was experimentally demonstrated for the first time. TIs (Bi2 Se3 , Bi2 Te3 ) nanosheets were embedded in polyvinyl alcohol for film-forming and incorporated into a visible Pr3+ -doped fiber laser cavity for Q-switching operation, respectively. The Bi2 Se3 -based Q-switched fiber laser has a wide range pulse-repetition-rate from 191.6 to 454.5 kHz, and the Bi2 Te3 one can be continuously tuned from 164.5 to 403.2 kHz. Both the Q-switching lasers have narrow pulse duration of nanosecond (ns)-level. Our results reveal that the TIs-based SAs are available for pulsed operation in the visible spectral range. Index Terms— Q-switched fiber laser, visible laser, topological insulator, pulse-repetition-rate.

I. I NTRODUCTION

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USLED lasers are of great interest because of their intensive applications in biomedical, optical sensing, fine processing, underwater communications, and spectroscopy. Compared to active pulsed lasers, passive ones with saturable absorbers (SAs) possess the attractive advantages of compactness, simplicity, and flexibility in design. To date, various SAs, such as semiconducting saturable absorber mirror (SESAM) [1], [2], single-walled carbon nanotube (SWCNT) [3], [4], and two-dimensional (2D) materials (e.g. graphene, topological insulators (TIs)) [5]–[17] have been used to realize pulsed operation in lasers. Nevertheless, the currently reported lasers generally operated in infrared region (e.g. near-infrared and mid-infrared region) [5]–[17]. There exists a less employed domain in visible region. Motivated by the applications in display technology, much effort was recently directed to visible pulsed lasers [18], [19]. Towards the low-cost, compact, and high-efficient visible pulsed lasers with SAs, SESAM suffers drawbacks of complex fabrication and narrow bandwidth, and crystals have low-compatibility (such as with fibers). In contrast, 2D materials have been Manuscript received June 28, 2015; revised July 27, 2015; accepted August 7, 2015. Date of publication August 27, 2015; date of current version September 30, 2015. This work was supported in part by the Specialized Research Fund for the Doctoral Program of Higher Education under Grant 20120121110034 and in part by the National Natural Science Foundation of China under Grant 61275050. D. Wu, Z. Cai, Y. Zhong, Z. Luo, N. Chen, and H. Xu are with the Department of Electronic Engineering, Xiamen University, Xiamen 361005, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). J. Peng and J. Weng are with the Department of Biomaterials, Xiamen University, Xiamen 361005, China (e-mail: pj13545639809@ 126.com; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2015.2466473

considered as an ideal candidate due to the good advantages of low saturation intensity, high damage threshold, ultrafast recovery time, and especially the ultra-wideband response ranging from ultraviolet (UV) to terahertz [20]. Among the various kinds of 2D materials, TIs (e.g. Bi2 Se3 , Bi2 Te3 ) show an impressively ultra-high absolute modulation depth, and thus could be preferred. Besides the need of visible-available SA, efficient and compact fiber laser is also a key element to realize high-performance visible passive Q-switching/mode-locking operation. Unlike near-infrared light, visible light could be depleted in silica fibers transmission. Fortunately, Pr3+ -doped ZrF4 -BaF2 -LaF3 -AlF3 -NaF (ZBLAN) fibers, have attracted great attention as gain mediums in visible fiber lasers because of the low maximum phonon energy (< 600 cm−1 ) and low loss (< 0.1 dB/m) in the visible waveband as well as abundant visible transitions (e.g. blue, green, orange and red) [21]. Meanwhile, down-conversion of Pr3+ ions pumped by a blue InGaN laser diode (LD) could enhance the laser efficiency [22]. However, Pr3+ -doped ZBLAN fiber is difficult to splice with other fibers which largely limits the stable all-fiber configuration. If a fiber-compatible visible reflection mirror (e.g. dielectric film coated fiber-end mirror) is obtained, compact visible-wavelength Pr3+ -doped ZBLAN all-fiber lasers would be easily constructed [23]. Furthermore, incorporating the filmy TI SA into the visible Pr3+ -doped ZBLAN all-fiber laser, passive Q-switching or mode-locking in the visible wavelengths could thus realize with high-efficiency and comparative good environmental durability. In this letter, we experimentally demonstrated passively Q-switched 635 nm Pr3+ -doped all-fiber lasers with TIs (Bi2 Se3 and Bi2 Te3 ) SAs. The pulse-repetition-rate of Bi2 Se3 SA-based Q-switching red fiber laser can be widely tuned from 191 to 454 kHz and Bi2 Te3 one is from 164 to 403 kHz. The short pulse duration of Bi2 Se3 and Bi2 Te3 are 244 ns and 327 ns, respectively. The results indicate that TIs are available SAs for red-light pulsed laser, revealing their potential for pulse generation in the visible region. II. 635 nm VISIBLE Pr3+ -D OPED ZBLAN F IBER L ASER Fig.1(a) shows the proposed 635 nm Pr3+ -doped ZBLAN fiber laser. A piece of 98.5 cm Pr3+ -doped ZBLAN fiber (6/125, NA of 0.15, Pr concentration 1000 ppm) was pumped by a 444 nm/2 W GaN LD. Two planoconvex lenses (PL) and a micro-objective lens (MOL) were used to collimate and focus the pump light to the flat cleaved end of the Pr3+ -doped ZBLAN fiber. The coupling efficiency was

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Fig. 1. (a) Schematic of the 635 nm Pr3+ -doped red fiber laser. (b) Transmittance and inset with image of the FPM. (c) CW output power as a function of the incident pump power, and inset with CW traces.

measured to be ∼17.4%. The fiber pigtail mirror (FPM) and the 4% Fresnel reflection of the Pr3+ -doped fiber-end facet formed the laser resonant cavity, which is a compact all-fiber configuration. The FPM by coating high-quality dielectric films has a high reflectivity in the range from 580 to 713 nm, and an image in the inset of Fig.1(b) shows the appearance of the FPM. From the transmission spectrum (Fig.1(b)) of the FPM, we can see that it transmits (T) only ∼0.06% (i.e. reflects (R) ∼99.94%) red light at 635 nm, which can be used as a very efficient red-light all-fiber reflector. A red mirror (RM, T∼84%@444 nm and R∼99%@635 nm) was angled before the MOL to couple out the intracavity light. The laser operation without any SA was performed. From Fig.1(c), the threshold of the red laser is ∼83 mW, and the maximum output power is 165 mW with a slope efficiency of 59.4%. By monitoring with the oscilloscope, we confirmed that no regular pulse train (inset of Fig.1(c)) was observed, indicating the continuous-wave (CW) operation. III. Bi2 Se3 -BASED PASSIVE Q-S WITCHING 635 nm F IBER L ASER The few-layer Bi2 Se3 used in our experiment was prepared by the liquid-phase exfoliation method as follows: 1) synthesizing the Bi2 Se3 crystal from bismuth oxide powder (Bi2 O3 ) and selenium power (Se) through the hydrothermal method and 2) adding the as-synthesized Bi2 Se3 into the N-methyl2-pyrrolidone (NMP) solution and sonicate for 24 hours to produce the Bi2 Se3 solution. Fig.2 shows the characterization of the as-prepared Bi2 Se3 . Both the bulk Bi2 Se3 and the fewlayer Bi2 Se3 are characterized by X-ray diffraction (XRD) in Fig.2(a). All the labeled peaks of the bulk Bi2 Se3 can be easily indexed to rhombohedral Bi2 Se3 (JCPDs NO. 89-2008). The XRD pattern of the few-layer Bi2 Se3 shows a high [006] orientation and some characteristic peaks disappeared, indicating the bulk Bi2 Se3 has been successfully exfoliated. The Raman spectroscopy is also shown in Fig.2(b). The characteristic peaks of the bulk Bi2 Se3 are calibrated at 72, 128, and 172 cm−1 . Compared with the bulk Bi2 Se3 , few-layer Bi2 Se3 sample shows an obvious red shift of peak due to the thickness decreasing. Furthermore, atomic force microscopy (AFM)

Fig. 2. (a) The XRD of bulk Bi2 Se3 and few-layer Bi2 Se3 . (b) Raman spectra of bulk Bi2 Se3 and few-layer Bi2 Se3 . (c) AFM image of few-layer Bi2 Se3 . (d) The height profile diagram of the few-layer Bi2 Se3 .

Fig. 3. Picture of the proposed Bi2 Se3 -based SA passively Q-switched Pr3+ - doped red fiber laser, inset with the Pr3+ -doped fiber-end covered by filmy Bi2 Se3 .

image was also registered for characterizing the thickness of the as-prepared few-layer Bi2 Se3 (see Fig.2(c)). The height profile diagram (see Fig.2(d)) shows the Bi2 Se3 nanosheets are around 3.5 nm, which indicates the Bi2 Se3 nanosheets are about 3 to 4 layers since the thickness of single layer is 0.96 nm [24]. For the practical usage, the Bi2 Se3 nanosheets would be preferred to be made into polymer-composite structure. Thus, we dispersed the as-prepared few-layer Bi2 Se3 suspension into the polyving akohol (PVA), which is helpful for film-forming. Although the saturable absorption property of the few-layer Bi2 Se3 in visible region is important, we could not measure it due to the lack of ultrafast visible laser source in our lab. An as-prepared few-layer Bi2 Se3 with the insertion loss of ∼1.8 dB was sandwiched between the FPM and the other end of the Pr fiber to construct the fiber-compatible SA, as shown in inset of Fig.3. At an incident pump power of 126.7 mW, CW operation occurred. Q-switching operation started with increasing the incident pump power to 132.3 mW, and stable pulse trains appeared on the oscilloscope screen, which can be obviously seen in Fig.3. The output optical spectrum, radio-frequency (RF) spectrum, the average output power and the Q-switching pulse trains were monitored and measured by an optical spectrum analyzer (OSA),

WU et al.: 635-nm VISIBLE Pr3+ -DOPED ZBLAN FIBER LASERS

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Fig. 5. (a) XRD and (b) Raman spectra of the bulk Bi2 Te3 and few-layer Bi2 Te3 samples. (c) AFM image and (d) height profile of an as-prepared few-layer Bi2 Te3 sample.

of Bi2 Se3 -based SA under high pump intensity, and could be improved by further optimizing the cavity parameters (e.g. cavity designs, Bi2 Se3 SA’s performance). Fig. 4. (a) Typical oscilloscope traces of the Q-switched pulse trains under different pump powers: 141.2, 160.5, 175.0 and 194.3 mW. (b) RF output spectrum. (c) Output optical spectrum. (d) Repetition-rate and pulse duration as a function of the incident pump power. (e) The narrow pulse duration of 244 ns.

a RF spectrum analyzer together with a photodetector and a power meter, respectively. Under the different pump power, we give the typical oscilloscope traces of the Q-switched pulse trains in Fig.4(a). When gradually increases the incident pump power, the pulserepetition-rate monotonously increases, exhibiting the typical feature of passive Q-switching. The Q-switched pulse trains were stable. As shown in Fig.4(b), the RF signal-to-noise ratio (SNR) of fundamental frequency peak at 284.1 kHz is ∼43 dB. At the pump power of 160.5 mW, the output optical spectrum was also measured. From Fig.4(c), the Bi2 Se3 -based Q-switched fiber laser shows a dual-wavelength simultaneous oscillation at visible 635.5 and 635.7 nm. Fig.4(d) shows the repetition-rate and the pulse duration as a function of the incident pump power. By varying the pump power from 136.4 to 194.3 mW, the pulse-repetition-rate of the Q-switched red laser can monotonously increase from 191.6 kHz to 454.5 kHz. Meanwhile, the pulse duration decreases from 761 ns to 244 ns (in Fig.4(e)). The narrow pulse duration mainly benefits from the short cavity length. The output power linearly increases at first, and then becomes saturated at higher pump power. To exclude the thermal damage of the Bi2 Se3 SA, we decreased the incident pump power and the output power can recover again, indicating no damaged of the Bi2 Se3 SA. At an incident pump power of 189.4 mW, the maximum output power was measured to be 7.6 mW. The maximum pulse energy was calculated to be 22.3 nJ. The output power and pulse energy was limited due to the over-saturation and thermal-accumulation

IV. Bi2 Te3 -BASED PASSIVE Q-S WITCHING 635 nm F IBER L ASER The few-layer TI Bi2 Te3 was also prepared by the liquid-phase exfoliation method as follows: 1) adding the purchased Bi2 Te3 (325 mesh power, aladdin) into the NMP solution and sonicate for 20 hours to produce the few-layer Bi2 Te3 suspension; 2) centrifuging the few-layer Bi2 Te3 suspension for 30 minutes at 2000 rpm to remove bulk Bi2 Te3 ; 3) decanting the supernatant to another centrifuge tube and centrifuging at 13000 rpm for 30 minutes to remove free NMP. Fig.5 shows the characterization of the TI Bi2 Te3 . The bulk Bi2 Te3 and the as-prepared few-layer Bi2 Te3 are both characterized by XRD in Fig.5(a). All the labeled peaks of the bulk Bi2 Te3 can be easily indexed to rhombohedral Bi2 Te3 (JCPDs NO. 15-0863). And the few-layer Bi2 Te3 shows some characteristic peaks disappeared, indicating the bulk Bi2 Te3 has been successfully exfoliated. The Raman spectroscopy is shown in Fig.5(b). The characteristic peaks of the bulk Bi2 Te3 are calibrated at 60.2, 100.8, and 133.2 cm−1 . Compared with the bulk Bi2 Te3 , few-layer Bi2 Te3 sample shows a new peak at 115 cm−1 , indicating the successful preparation of fewlayer Bi2 Te3 [25]. Furthermore, AFM image was registered for characterizing the thickness of the as-prepared few-layer Bi2 Te3 (see Fig.5(c)). The height profile diagram (see Fig.5(d)) shows the TI Bi2 Te3 nanosheets are around 4.1 nm, which indicates the TI Bi2 Te3 nanosheets are about 4 layers since the thickness of single layer is about 1 nm [26]. The as-prepared few-layer Bi2 Te3 suspension was also dispersed into PVA for film-forming. A Bi2 Te3 -based SA with the insertion loss of ∼2.3 dB was also inserted into the red laser cavity to achieve passive Q-switching. The laser reached the threshold at 145.5 mW and the Q-switching was initiated at 156.2 mW. Under the different pump power, we give the typical oscilloscope traces of the

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Fig. 6. (a) Typical oscilloscope traces of the Q-switched pulse trains under different pump powers: 156.2, 185.1, and 204.4 mW. (b) RF output spectrum. (c) Output optical spectrum. (d) Pulse-repetition-rate and pulse duration as a function of the incident pump power.

Q-switched pulse trains in Fig.6(a). The pulse-repetitionrate of the Q-switched red laser can be widely tuned from 164.5 kHz to 403.2 kHz by varying the pump power from 156.2 to 204.4 mW. In Fig.6(b), the RF SNR of fundamental frequency peak at 284.1 kHz is ∼43 dB, indicating the Qswitched pulse trains were stable. The output optical spectrum was also measured in Fig.6(c)), which locates at the visible wavelength of 635.2 nm. Fig.6(d) shows the pulse- repetitionrate and the pulse duration as a function of the incident pump power. When gradually increases the incident pump power, the pulse-repetition-rate monotonously increases and the pulse duration becomes narrower and narrower. The narrowest pulse duration of our red Q-switching laser is 327 ns which mainly benefits from the short cavity length. The output power and pulse energy were also measured and calculated. The maximum output power was 5.1 mW and the maximum pulse energy was 14.3 nJ. The performance of the Bi2 Te3 -based passive Q-switching red fiber laser could also be improved by further optimizing the cavity parameters (e.g. cavity designs, Bi2 Te3 -based SA’s performance). V. C ONCLUSION In summary, TIs (Bi2 Se3 , Bi2 Te3 )-based SAs have been successfully used to realize 635 nm visible passive Q-switching in Pr3+ -doped ZBLAN down-conversion all-fiber lasers. The lasers have wide pulse-repetition-rate range of >200 kHz and narrow pulse duration of ns-level. Our work suggests that TIs could be promising SAs for pulse generation in the visible spectral region. And dielectric film coated fiber-end facet mirror could be used to simplify the special fiber (e.g. ZBLAN fiber, chalcogenide fiber) lasers. R EFERENCES [1] U. Keller et al., “Semiconductor saturable absorber mirrors (SESAM’s) for femtosecond to nanosecond pulse generation in solid-state lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 2, no. 3, pp. 435–453, Sep. 1996. [2] J. F. Li et al., “Semiconductor saturable absorber mirror passively Q-switched 2.97 µm fluoride fiber laser,” Laser Phys. Lett., vol. 11, no. 6, p. 065102, 2014.

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