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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 3, FEBRUARY 1, 2015
Topological Insulator Solution Filled in Photonic Crystal Fiber for Passive Mode-Locked Fiber Laser Peiguang Yan, Rongyong Lin, Hao Chen, Han Zhang, Aijiang Liu, Haipeng Yang, and Shuangchen Ruan
Abstract— We first reported that the topological insulator (TI) nanosheets solution filled in photonic crystal fiber can operate as an effective saturable absorber (SA) with the merits of low-insertion loss (∼0.42 dB), long interaction length (>10 cm), and high-power tolerance. This SA device exhibited a saturable intensity of 14.9 MW/cm2 , modulation depth of 19.1%, and nonsaturable loss of 25% at 1060 nm. Upo employing, this device rendered us to establish an ytterbium-doped all-fiber laser oscillator, where stable evanescent wave mode-locking operation has been achieved. This letter provided a new way of utilizing the unique nonlinear optical property of TI. Index Terms— Topological insulator, mode-locked fiber laser, evanescent wave mode-locking.
I. I NTRODUCTION
N
OVEL saturable absorbers (SAs) based on two-dimentional (2D) nano-materials hold great promise for the ultrafast photonics and have been an attractive research topic in recent years. Despite the semiconductor saturable absorber mirrors (SESAMs) have been applied in the commercial laser system so far, they often have relatively narrow operation bandwidth and complicated manufacturing technology [1]. To seek for new and high-performance SAs, different kinds of nanomaterials like: carbon nanotubes (CNTs), and graphene are intensively investigated and still attracting considerable attention currently. The CNTs SAs are wavelength dependent on their diameter [2], [3] usually introducing strong non-saturable losses in laser cavity if for a broadband operation wavelength. In comparison, graphene SAs [3]–[7] exhibit a spectrally uniform saturable absorption over a broad bandwidth and allow reaching ultimately short pulse durations [8]. Most recently, a new
Manuscript received August 7, 2014; revised September 28, 2014; accepted October 3, 2014. Date of publication October 8, 2014; date of current version January 19, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61275144 and Grant 61378024, in part by the Natural Science Foundation of Guangdong Province under Grant S2013010012235, in part by the Foundation for Scientific and Technical Innovation in Higher Education of Guangdong Province under Grant 2013KJCX0161. P. Yan, R. Lin, H. Chen, A. Liu, and S. Ruan are with the Shenzhen Key Laboratory of Laser Engineering, Key Laboratory of Advanced Optical Precision Manufacturing Technology of Guangdong Higher Education Institutes, College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China (e-mail:
[email protected]; 417377815@ qq.com;
[email protected];
[email protected];
[email protected]). H. Zhang is with the Key Laboratory of Optoelectronic Devices and Systems of Guangdong Province, Ministry of Education, Shenzhen University, Shenzhen 518060, China (e-mail:
[email protected]). H. Yang is with the College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China (e-mail:
[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.2014.2361915
class of Dirac-materials called topological insulators (TIs) arises as a new kind of SA characterized with a small band gap in their bulk state and a gapless metallic state in their edge/surface. Their surface state in three-dimention shows a graphene-like band structure, but the directions of spin and momentum are locked together, which provides the basis for a source of highly spin-polarized electrons with tunable polarization direction [9], [10]. TIs also exhibit broadband saturable absorption features at both optical and microwave band [11] and giant third order nonlinear optical property (∼10−14 m2 /W) [12]. Compared with graphene, TIs have been proved exhibiting ultra large modulation depth (∼98%) [13] and saturable intensity (∼10 GW/cm2 ) [12], hence can operate as an effective SA for passive mode locking of lasers. TIs can be fabricated by the MBE growth, the vapor-liquid-solid growth, the mechanical exfoliated method and the filmy method in polyvinyl alcohol [14]–[18]. Usually the TIs are deposited onto the fiber ferrule [19]. Owing to large material losses, the interaction length of the TI with light is limited. Another disadvantage is low power tolerance because the laser light is directly transmitted through the nanomaterial. The SA device based on the evanescent wave interaction is another effective approach to achieve mode-locked fiber laser [20]–[24]. Nevertheless, the microfiber-based SAs are very frangible with the disadvantage of high insertion loss (IL, 5.23 dB) and short interaction length (1.2 mm) [24], the side-polished-fiber-based SAs usually need the index match oil to induce evanescent wave interaction [22], [23], and exhibit polarization dependent loss due to centrosymmetric structure. Photonic crystal fiber (PCF) [24], characterized with regular air holes along fiber length, has been utilized as platform of SA by filling the sample into cladding holes or hollow core region [25]–[29]. Compared with microfiberbased and side-polished-fiber-based SAs, the distinguished merits in this scheme can be easily predicted as follows: 1) Stronger interaction effect, the SA sample can interact strongly with the evanescent wave from the small core of PCF (few micrometers). 2) Longer interaction length, determined by the SA sample injected into how long PCF, is favorable for sufficient interaction of SA sample with light. 3) Larger nonlinear effect, introduced not only by the PCF but by the giant third order nonlinear optical property of SA sample, can benefit for the generation of harmonic mode locking (HML) pulses [24], [30], [31]. 4) Robust and compact. PCF here, acting as the platform of SA sample, is totally undamaged in laser system, and can be easily spliced with the single mode fiber. However, all the former results with graphene or
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YAN et al.: TI SOLUTION FILLED IN PHOTONIC CRYSTAL FIBER
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Fig. 1. Experimental setup. Inset (a,b) SEM of PCF and Splicing point of TI filled PCF and SMF-28 fiber.
graphene-oxide SA in PCF focused on passive mode-locked Erbium-doped fiber laser operating in anomalous dispersion regime, and the insertion loss remained very large (4.1 dB [26], ∼6 dB [28]). In this letter, we first reported that the PCF filled with TI:Bi2 Te3 nanosheets solution can act as an effective SA with the merits of low IL (0.42 dB), longer interaction length (>10 cm), high power tolerance and robust against environment perturbation. An Ytterbium-doped fiber (YDF) laser oscillator was constructed based on this TI: Bi2 Te3 filled PCF (TI-PCF) SA device, and stable evanescent wave mode-locking operation was achieved. II. E XPERIMENTAL S ETUP Our fiber laser was schematically shown in Fig.1. The laser was pumped by a 976 nm pump laser through a wavelength division multiplexer (WDM). A segment of ∼3.4 m YDF with absorption of 250 dB/m@975 nm was used as the gain medium. A polarization independent isolator (ISO) was used to ensure the unidirectional operation of the ring laser cavity, and the output of the laser was coupled through a 10% fiber optical coupler (OC). The TI-PCF SA was incorporated into cavity after the OC. A three-spool polarization controller (PC) was employed to adjust the cavity birefringence. A bandwidth filter with a central wavelength of 1064 nm and 3 dB bandwidth of ∼5 nm was inserted into the cavity. A total length of ∼180 m HI-1060 Flex SMF was also incorporated to the cavity. The total cavity length was ∼185 m. The monitoring of the output temporal pulse trains and optical spectrum were performed using a 20-GHz mixed signal oscilloscope (Tektronix MSO72004C) with a 45-GHz photo-detector (Newport 1014) and an optical spectrum analyzer (OSA, AQ6370B) with a minimum resolution of 0.02 nm, respectively. The radio frequency (RF) spectrum of the fundamental mode-locking was measured by a Mixed Domain Oscilloscope (Tektronix MDO4034B-3). The PCF had a core/cladding diameter of 9.5/127 µm, as shown in the inset (a) of Fig.1. The air holes in cladding had average diameter/pitch of 5.62/8.3 µm, respectively. The mode field diameter (MFD) was measured ∼8.7 µm at 1310 nm, very close to standard single mode fiber (SMF-28). As a result, the single splicing loss between PCF and SMF-28 fiber could easily optimized to ∼0.1 dB. The attenuation and the dispersion parameter at 1060 nm were measured to be 2.2 dB/km and ∼20.5 ps/km/nm, respectively.
Fig. 2. (a,b) SEM of TI:Bi2 Te3 nanosheets; (c,d) XRD and transmission spectrum of the solution.
Considering the dispersion properties of other fibers in cavity, our laser operated in normal dispersion regime. The inset (b) clearly showed the enlarged region between the TI-solutionfilled PCF and SMF-28 fiber after splicing, where the red marked region was the evidence of the liquid breakpoints in air channels of PCF. During splicing, obvious movement of the breakpoints could be observed. The TI:Bi2 Te3 nanosheets were synthesized based on the hydrothermal intercalation/exfoliation method. For measuring the scanning electron microscope (SEM), and X-ray diffraction (XRD), the TI: Bi2 Te3 nanosheets solution was dripped onto a glass substrate, and had an uniform distribution by a low-speed spincoating, then dried in an oven. The Fig. 2 (a,b) showed the SEM of TI:Bi2 Te3 nanosheets. It could be seen that the nanosheets had regular hexagonal shape. The average edgeto-edge size of the nanosheets was ∼450 nm. The average sample thickness was ∼20 nm. The XRD pattern of TI: Bi2 Te3 nanosheets, as shown in Fig. 2(c), exhibited a high [006] orientation and some characteristic peaks [015 and 0015] with the bulk Bi2 Te3 . For preparing the TI solution filled in PCF, we dispersed 0.15 mg sample into 3 mL deionized water, and homogenized with 1.5-hour ultrasonication. The solution here could be homogeneous without subsidence for several days. The transmission spectrum of the solution was measured using an optical spectrometer (Perkinelmer Lambda 7500), and the linear-transmission ratio was ∼47.5% at 1060 nm as shown in Fig.2 (d). Low IL could effectively reduce the oscillation threshold and improve the overall laser performance. In the experiment, the TI-PCF SA had a length of 12 cm. Both ends of PCF were spliced with 50-cm-long SMF-28 fiber to form a compact SA device as shown in the inset of Fig. 3(a). Fig. 3(a) showed the measured IL of the SA device under launched power of the 974 nm CW LD source from 20 mW to 430 mW. It could be seen that the IL of this SA device had an average IL as low as 0.42 dB. In comparison with those IL results reported in ref. [24], [26], and [28], the IL was reduced about one magnitude. It was worth noting that the low IL value here was not conflicted with the linear-transmission ratio in Fig. 2(d).
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Fig. 3. (a) Measured IL. (b) Measured nonlinear saturable absorption curve of the TI-PCF SA device.
Fig. 4. (a) Fundamental mode-locking pulse train. Inset: single pulse width, (b) corresponding spectrum, (c) fundamental frequency signal, (d) RF spectrum.
The saturable absorption property of the TI-PCF SA was measured using the same experimental setup as in ref. [32], which had been shown in Fig. 3(b). The results gave a saturable intensity of 14.9 MW/cm2 , a modulation depth of 19.1%, and a non-saturable loss of 25%. It should be noted that the saturable intensity of the TI-PCF SA was about two-orders of magnitude less than the reported 0.48 GW/cm2 of TI: Bi2 Te3 deposited the nanosheets onto a thick quartz plate [19], but at the same level with those reported 53 MW/cm2 of TI: Bi2 Se3 deposited onto fiber ferrule [33] and 12 MW/cm2 of TI: Bi2 Se3 embedded into PVA film [18]. The difference of reported saturable intensity results might arise from the inherent material property, the SA fabrication method or the measuring condition. With such low saturable intensity, it could be predicted that the threshold of mode-locking operation using the TI-PCF SA device could be significantly reduced. III. E XPERIMENTAL R ESULTS AND D ISCUSSION The pump power threshold of continuous wave laser operation was ∼74 mW. Under pump power of ∼115 mW (the output power is 1.2 mW), the single-pulse train at fundamental cavity frequency ∼1.11 MHz could be easily observed through selecting appropriate polarization state of the PC, and the pulse train had a cavity round-trip time of ∼905.6 ns. The output pulse was depicted in the inset of Fig. 4(a).
It exhibited quite symmetrical shape, with the full width at half maximum (FWHM) of the single pulse of ∼960 ps. The corresponding optical spectrum was given in Fig. 4(b). The central wavelength was ∼1064.47 nm, and the spectral bandwidth at half-maximum was ∼1.11 nm. It was recorded with 2.50 kHz resolution bandwidth (RBW) [Fig. 4(c)], and the signal-to-noise ratio (SNR) was 60 dB. The RF spectrum for the 100 MHz scanning range with an RBW of 2.50 kHz was shown in Fig. 4(d). For testing the power tolerance of TI-PCF SA, we gradually increased the pump power to maximum 430 mW (limited by the LD), and then decreased the pump power to 115 mW, the stable mode-locking operation was observed again, indicating that the TI-PCF SA was not damaged. When the filled fiber was spliced into the laser cavity, it was really observed relatively stable operation in few hours in lab condition. At the same time, no obvious changes were observed in optical parameters a few days after the PCF fiber was filled. In contrast with the reported cases of TI-film between the fiber ferrules, the high power tolerance ability of TI-PCF SA was attributed to the interaction of evanescent wave and TI nanomaterials. The modulation effect of TI on light was along the whole length of PCF involved, resulting in an inherent ability of high power tolerance. This property could find the application in large pulse energy laser system. This ability could be further enhanced by optimizing the TI nanomaterials with multi-layers and high modulation depth [13], by utilizing the PCF with multicore structure, and by taking the solution with different density or the TI materials in completely dry state. To verify whether the TI-PCF SA contributed to the passive mode-locking, we removed it from the ring laser system. In this case, even we carefully adjusted the PC orientation and pump power, the mode-locking state could not be observed. This result testified that the TI-PCF SA was indeed contributing to the mode-locking operation. In operation, the TI nanosheets would be captured onto the core surface by evanescent wave as has done by micro-fiber based SA in ref. [24], which would work as the real SA. In the all-normal-dispersion cavity, spectral filter bandwidth, and group-velocity dispersion as the key parameters that determine the behavior and properties of these lasers [34]. Spectral filtering could provide strong pulse-shaping of a highly-chirped pulse [35]. The in-line band-pass filter could limit the bandwidth and pulse diffusion, which could be conducive to generate stable pulses in the all-normal-dispersion mode-locked fiber lasers. In order to emit the stable and narrow pulses in our laser, we inserted a band-pass filter with the FWHM of 5 nm. However, the mode-locked results here were not the best, which reflected in the spectrum with the FWHM of 1.11 nm didn’t cover the whole spectrum range of band-pass filter and the output pulses were large. Because the spectrum didn’t cover the whole spectrum range of band-pass filter, the laser here could also operate correctly without filter. The corresponding reason was that the self phase modulation here was not strong enough. We could take some measures in a further performance optimization. For example, we could shorten the whole cavity length. Or we could further improve the saturable absorption properties of topological Insulator (TI)
YAN et al.: TI SOLUTION FILLED IN PHOTONIC CRYSTAL FIBER
nanosheets solution to increase the pulse shaping ability and reduce the non-saturable absorption loss. IV. C ONCLUSION As a summary, a TI-PCF SA device was firstly demonstrated with some unique advantages, such as low IL, longer interaction length, high power tolerance, nonlinearity enhancement. An ytterbium-doped all-fiber laser has been constructed based on this device. The evanescent wave mode-locking operation was achieved and the output pulse centered at 1064.47 nm with a pulse width of 960 ps and a SNR of 60 dB. This experimental result clearly evidenced that the TI-PCF SA device possesses the desired optical properties for laser mode locking, paving the way for ultra-fast photonics. Moreover, by embedding the 2D materials such as graphene, MoS2 [36] and other TIs into PCFs, it was expected to create novel multi-functional devices by combining their unique optical property together, which might motivate new research interest in novel fiber lasers [37], [38] sensors and optical communications. ACKNOWLEDGMENT The authors would like to thank YOFC.com for providing the PCF. 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] S. Yamashita et al., “Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates and fibers and their application to mode-locked fiber lasers,” Opt. Lett., vol. 29, no. 14, pp. 1581–1583, 2004. [3] T. Hasan et al., “Nanotube-polymer composites for ultrafast photonics,” Adv. Mater., vol. 21, nos. 38–39, pp. 3874–3899, 2009. [4] Q. Bao et al., “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater., vol. 19, no. 19, pp. 3077–3083, 2009. [5] H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, and K. P. Loh, “Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene,” Opt. Exp., vol. 17, no. 20, pp. 17630–17635, 2009. [6] S. Jaroslaw, S. Grzegorz, P. Iwona, K. Aleksandra, S. Wlodek, and M. A. Krzysztof, “Simultaneous mode-locking at 1565 nm and 1944 nm in fiber laser based on common graphene saturable absorber,” Opt. Exp., vol. 21, no. 16, pp. 18994–19002, 2013. [7] S. Jaroslaw et al., “Passive synchronization of erbium and thulium doped fiber mode-locked lasers enhanced by common graphene saturable absorber,” Opt. Exp., vol. 22, no. 5, pp. 5536–5543, 2014. [8] N. Tolstik, E. Sorokin, and I. T. Sorokina, “Graphene mode-locked Cr:ZnS laser with 41 fs pulse duration,” Opt. Exp., vol. 22, no. 5, pp. 5564–5571, 2014. [9] B. A. Bernevig, T. L. Hughes, and S. C. Zhang, “Quantum spin Hall effect and topological phase transition in HgTe quantum wells,” Science, vol. 314, no. 5806, pp. 1757–1761, 2006. [10] C. Jozwiak et al., “Photoelectron spin-flipping and texture manipulation in a topological insulator,” Nature Phys., vol. 9, pp. 293–298, Mar. 2013. [11] S. Q. Chen et al., “Broadband optical and microwave nonlinear response in topological insulator,” Opt. Mater. Exp., vol. 4, no. 4, pp. 587–596, 2014. [12] S. B. Lu et al., “Third order nonlinear optical property of Bi2 Se3 ,” Opt. Exp., vol. 21, no. 2, pp. 2072–2082, 2013. [13] C. J. Zhao et al., “Wavelength-tunable picosecond soliton fiber laser with topological insulator: Bi2 Se3 as a mode locker,” Opt. Exp., vol. 20, no. 25, pp. 27888–27895, 2012.
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