Passively Q-Switched Erbium-Doped Fiber Laser Based ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 1, JANUARY 1, 2015

69

Passively Q-Switched Erbium-Doped Fiber Laser Based on Few-Layer MoS2 Saturable Absorber Heping Li, Handing Xia, Changyong Lan, Chun Li, Xiaoxia Zhang, Jianfeng Li, and Yong Liu Abstract— We demonstrate a passively Q-switched erbiumdoped fiber laser (EDFL) based on few-layer MoS2 as a saturable absorber (SA). Few-layer MoS2 is prepared by the chemical vapor deposition method. The prepared MoS2 is transferred onto the end face of a fiber connector to form a fiber-compatible MoS2 -based SA. The saturation intensity and modulation depth of the MoS2 SA are measured to be 0.43 MW/cm2 and 33.2%, respectively. The Q-switched EDFL has an all-fiber linear cavity with two fiber Bragg gratings as the end mirrors. By inserting the MoS2 SA into the laser cavity, stable Q-switched operation is achieved at 1.55 µm. The laser has a pump threshold of 20.4 mW, a pulse repetition rate tunable from 10.6 to 173.1 kHz, and a minimum pulse duration of 1.66 µs. Our results show that fewlayer MoS2 is a promising SA for Q-switching laser operation. Index Terms— Fiber molybdenum sulfide.

lasers,

Q-switching, nanomaterials,

I. I NTRODUCTION HERE is growing interest in passively Q-switched fiber lasers because of their potential applications in medicine, micromachining, metrology, fiber optical sensing, and telecommunications [1], [2]. Compared with the active Q-switching technique, the passive Q-switching technique based on saturable absorbers (SAs) has significant advantages in compactness, simplicity, and flexibility of implementation [1]. To date, both artificial SAs and natural SAs have been implemented, such as nonlinear polarization rotation (NPR) [3]–[5] semiconductor saturable absorber mirrors (SESAMs) [2], and single-wall carbon nanotubes (SWNTs) [6], [7]. However, due to certain intrinsic drawbacks of these SAs, such as environmental sensitivity, complex optical alignments, complicated fabrication and expensive packaging, or limited operating bandwidth, their optical applications are restricted. Therefore, there is constantly a strong motivation to seek new high-performance SAs for pulsed laser systems. Since the first demonstration of graphene-based modelocking in fiber lasers [8], [9], graphene has been successfully exploited as a promising SA for passive Q-switching and mode-locking applications [10]–[13]. The success of graphene

T

Manuscript received June 18, 2014; revised September 21, 2014; accepted October 3, 2014. Date of publication October 8, 2014; date of current version December 8, 2014. This work was supported in part by the National Basic Research Program of China under Grant 2012CB315701 and in part by the National Natural Science Foundation of China under Grant 61378028, Grant 61106040, Grant 61377042, and Grant 61275039. The authors are with the State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [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.2014.2361899

has greatly encouraged scientific researchers to explore other graphene-like 2dimensional (2D) nanomaterials for photonic applications [14]. Recently, topological insulators (TIs), a new class of materials characterized by an insulating bulk state and a gapless Dirac-type surface/edge [15], are attracting great interest in ultrafast photonics [16]. Taking advantage of the excellent saturable absorption of TIs, Chen and co-workers successfully achieved stable Q-switched pulses in an erbium-doped fiber laser (EDFL) operating at around 1.55 μm [17]. With the broadband saturable absorption of TI:Bi2 Se3 , Luo et al. experimentally demonstrated the generation of Q-switched pulses at 1.06 μm [18] and 2.0 μm [19]. In addition to the aforementioned TI-SAs, 2D molybdenum disulfide (MoS2 ) has recently been exploited as a SA for ultrafast laser applications [20]–[25]. Wang et al. investigated the saturable absorption behavior of MoS2 nanosheets in dispersions under femtosecond laser excitation at 800 nm [20]. Zhang et al. presented an ytterbium-doped fiber laser (YDFL) mode-locked by a MoS2 SA, which emitted dissipative solitons at 1.054 μm [21]. Using a MoS2 -polymer composite, Woodward et al. demonstrated the generation of Q-switched pulses in an YDFL at 1.068 μm [22]. By using broadband few-layer MoS2 SAs containing suitable defects, Wang et al. reported passively Q-switched solid-state lasers operating at wavelengths of 1.06 μm, 1.42 μm and 2.1 μm [23]. Very recently, we demonstrated an EDFL passively mode-locked by a few-layer MoS2 SA operating at 1.56 μm [24]. Liu et al. achieved femtosecond pulses in an EDFL mode-locked with a MoS2 -polymer composite SA [25]. These reported results indicate that few-layer MoS2 is a promising broadband SA for pulsed lasers. However, to date, a MoS2 Q-switched fiber laser operating at 1.55 μm has not yet been reported. In this letter, we demonstrate a passively Q-switched EDFL using few-layer MoS2 as a SA. The few-layer MoS2 was prepared by the chemical vapor deposition (CVD) method [26] and sandwiched between two fiber connectors (FCs) with a fiber adapter to form a fiber-compatible MoS2 SA. The linear laser cavity was constructed by using two fiber Bragg gratings (FBGs) as the two end mirrors. By inserting the MoS2 SA into the laser cavity, stable Q-switched pulses were obtained at 1.55 μm. The Q-switched EDFL exhibited a pump threshold of 20.4 mW, a pulse repetition rate tunable from 10.6 to 173.1 kHz and a minimum pulse duration of 1.66 μs. II. P REPARATION AND C HARACTERIZATION OF M O S2 -BASED SA A. Preparation and Characterization of Few-Layer MoS2 SA The few-layer MoS2 used in this experiment was synthesized in a tube furnace by the CVD method. The material

1041-1135 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

70


Fig. 3.

Fig. 1. Raman spectra of typical as-grown few-layer MoS2 and bulk MoS2 .

Fig. 2.

Measured saturable absorption data of the few-layer MoS2 SA.

was grown on a Si substrate capped with a 300-nm thermal SiO2 layer [26]. Through a transfer procedure [24], the grown MoS2 film was transferred and attached to the end face of a FC to form a MoS2 SA. Details regarding the preparation and characterization of MoS2 films were reported in Ref. [24] The Raman spectra of the as-grown MoS2 film and bulk MoS2 were measured using a Renishaw’s RA100 Raman analyzer with an excitation wavelength of 514 nm and laser power of 2 mW, as shown in Fig. 1. The as-grown MoS2 film exhibited two Raman characteristic bands at 404.0 cm−1 and 379.8 cm−1 [27], corresponding to the A1g and E12g modes, respectively. It should be noted that the peak frequency difference () between the A1g and E12g modes can be used to identify the number of MoS2 layers [27]. The value of the as-grown sample was 24.2 cm−1 , corresponding to a layer number of 4∼5 [27].

Schematic configuration of MoS2 -based Q-switched fiber laser.

When irradiated by photons with an energy greater than the band gap, few-layer MoS2 can be excited by absorbing one photon and exhibits saturable absorption at high excitation intensities due to the Pauli blocking effect [20]. Recently, Wang et al. reported that the band gap of few-layer MoS2 could be reduced to 0.08 eV (λ = 15.4 μm) by introducing suitable S atomic defects [23]. This finding indicates that fewlayer MoS2 with S atomic defects is a broadband saturable absorber. Our experimental results show that the few-layer MoS2 fabricated in this letter exhibits large saturable absorption at 1550 nm. Further study is needed to identify the mechanisms involved in the saturable absorption observed. III. E XPERIMENTAL S ETUP OF THE M O S2 Q-S WITCHED F IBER L ASER The experimental setup of our proposed passively Q-switched EDFL is schematically shown in Fig. 3. The laser has a linear cavity, which is pumped by a 980-nm laser diode (LD) through a 980-/1550-nm wavelength division multiplexer (WDM). A piece of 0.5-m heavily-doped EDF (LIEKKI Er80-8/125) was used as the gain medium. A standard single mode fiber (SMF) was also used in the cavity, and the total cavity length was approximately 1.95 m. The prepared MoS2 SA acted as a passive Q-switcher. Two fiber Bragg gratings (FBG1 and FBG2) with reflectivities of 85.87% and 87.41%, respectively, were used as the two end mirrors of the laser cavity. The gratings had the same central wavelength of 1549.86 nm and a 3-dB bandwidth of 0.21 nm. The laser output from the WDM was simultaneously monitored using a 500-MHz oscilloscope (Tektronix TDS3052C) together with a 2-GHz photodetector, a radio-frequency (RF) spectrum analyzer (Advantest R3267), and an optical spectrum analyzer (Yokogawa AQ 6370C).

B. Nonlinear Absorption Properties of MoS2 SA The nonlinear absorption properties of the MoS2 SA were measured using a balanced twin-detector measurement system (similar to that reported in Ref. [9]) with 250-fs pulses at a wavelength of 1550 nm. The optical transmittance of the MoS2 SA at different input powers was recorded, as shown in Fig. 2. The nonlinear transmission T (I ) data gathered for the SA were then fitted according to a simple SA model [17]: T (I ) = 1 − T exp(−I /Isat ) − Tns , where T is the modulation depth, I is the input intensity, Isat is the saturation intensity, and Tns is the non-saturable absorbance. The SA parameters of modulation depth and saturation intensity were determined to be 33.2% and 0.43 MW/cm2 , respectively. It should be noted that monolayer MoS2 is a direct semiconductor with a band gap of ∼1.8 eV, whereas few-layer MoS2 is an indirect semiconductor with a band gap of ∼1.2 eV [28].

IV. E XPERIMENTAL R ESULTS AND D ISCUSSION Continuous wave operation was begun at a pump power of 16.6 mW, and stable Q-switched pulses were achieved when the pump power was increased to 20.4 mW. Figure 4 shows typical oscilloscope traces of the Q-switched pulse trains at pump powers of 25.5 mW, 85.2 mW, 139.5 mW and 185.6 mW. Unlike the fixed repetition rate of a modelocked fiber laser [29] the pulse repetition rate in our laser increased with the pump power from 19.4 to 145.2 kHz, which is a typical feature of passive Q-switching operation [12]. To verify that the passive Q-switching was attributed to the MoS2 SA, the FC coated with MoS2 was replaced with a common clean FC. In this case, no Q-switched pulses were observed on the oscilloscope even when the pump power was adjusted over a wide range. This finding confirmed that

LI et al.: PASSIVELY Q -SWITCHED ERBIUM-DOPED FIBER LASER

71

Fig. 6. Pulse repetition rate and pulse duration versus incident pump power.

Fig. 4. Oscilloscope traces of the Q-switched pulses at different pump powers Pp. (a) Pp = 25.5 mW, (b) Pp = 85.2 mW, (c) P p = 139.5 mW, (d) Pp = 185.6 mW.

Fig. 7. Average output power and single-pulse energy versus incident pump power.

Fig. 5. Typical characteristics of the Q-switched pulses emitted from the fiber laser at a pump power of 210 mW. (a) Optical spectrum, (b) oscilloscope trace of the Q-switched pulse train, (c) single-pulse profile, and (d) RF spectrum at the fundamental frequency.

the MoS2 SA was responsible for the passively Q-switched operation of the laser. Typical characteristics of the Q-switched pulses emitted from the fiber laser at a pump power of 210 mW are presented in Fig. 5. The optical spectrum of the Q-switched pulses is centered at 1549.91 nm, and the 3-dB spectral width is approximately 0.06 nm, as shown in Fig. 5(a). Figure 5(b) displays a recorded oscilloscope trace of the Q-switched pulse train, which shows a pulse period of 6.2 μs. Figure 5(c) shows a zoom-in single-pulse profile. The pulse has a symmetric intensity profile with a pulse duration (FWHM) of 1.72 μs. Figure 5(d) displays the corresponding RF spectrum with a 30-Hz resolution bandwidth. The pulse repetition rate was 161 kHz, matching the pulse period of 6.2 μs. The signal-tonoise ratio (SNR) was ∼42.5 dB, indicating good Q-switching stability. Figure 6 shows the pulse repetition rate and pulse duration of the Q-switched fiber laser as functions of the incident pump power. By increasing the pump power from 20.4 to 227 mW,

the pulse repetition rate could be varied over a wide range of frequencies, from 10.6 to 173.1 kHz. At every pump power and pulse repetition rate, the Q-switched pulse output was stable and no amplitude modulations in these pulse trains were observed, which indicates that there was no self-mode locking (SML) effect during the Q-switching operation. On the other hand, the pulse duration decreased from 6.11 μs near the pump threshold to 1.66 μs at a pump power of 227 mW. At lower pump powers (198 mW), the pulse duration remained nearly unchanged, which clearly indicated that the SA was saturated. For a passively Q-switched laser, the pulse duration τ can be evaluated using the following equation [30]: τ = 3.52TR /T ,

(1)

where TR is the cavity round-trip time and T is the modulation depth of the SA. Therefore, the minimum pulse duration obtained in our experiment could be further narrowed by shortening the cavity length and improving the modulation depth of the few-layer MoS2 . Figure 7 shows the dependence of the average output power of the Q-switched fiber laser versus the pump power. Based on the measured average output power and the repetition rate, the pulse energy was calculated, which is also displayed in Fig. 7. When the pump power exceeded the threshold, the average output power increased almost linearly with the pump power. The maximum average output power was 4.71 mW at a pump power of 227 mW, corresponding to a pulse energy of 27.2 nJ. Above a pump power of 227 mW, the Q-switched pulse train became unstable or even disappeared, as typically observed in certain passively Q-switched fiber lasers [6], [17], [18]. Subsequently, when the pump power was adjusted back to a level below 227 mW, the

72


stable Q-switched pulse train appeared again. Hence, unstable Q-switched operation at high pump power may be attributed to over-saturation of the MoS2 SA rather than to thermal damage [18]. The pump power limit of 227 mW for stable Q-switching operation is comparable to that of graphene [12] and TI:Bi2 Te3 Q-switched fiber lasers [17]. Compared to that of the MoS2 Q-switched YDFL [22], our pulse energy is 4 times greater because of the different cavity design. We believe that the performance of Q-switched pulses produced by the laser could be further improved by optimizing the SA parameters of few-layer MoS2 and the cavity design.

[10] Z. Sun et al., “Graphene mode-locked ultrafast laser,” ACS Nano, vol. 4, no. 2, pp. 803–810, Jan. 2010. [11] H. Zhang, D. Tang, R. J. Knize, L. Zhao, Q. Bao, and K. P. Loh, “Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser,” Appl. Phys. Lett., vol. 96, no. 11, p. 111112, Mar. 2010. [12] D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, and A. C. Ferrari, “Graphene Q-switched, tunable fiber laser,” Appl. Phys. Lett., vol. 98, no. 7, p. 073106, Feb. 2011. [13] Y. Cui and X. Liu, “Graphene and nanotube mode-locked fiber laser emitting dissipative and conventional solitons,” Opt. Exp., vol. 21, no. 16, pp. 18969–18974, Aug. 2013. [14] F. Bonaccorso and Z. Sun, “Solution processing of graphene, topological insulators and other 2d crystals for ultrafast photonics,” Opt. Mater. Exp., vol. 4, no. 1, pp. 63–78, Jan. 2014. [15] H. Zhang, C.-X. Liu, X.-L. Qi, X. Dai, Z. Fang, and S.-C. Zhang, “Topological insulators in Bi2 Se3 , Bi2 Te3 and Sb2 Te3 with a single Dirac cone on the surface,” Nature Phys., vol. 5, no. 6, pp. 438–442, Jun. 2009. [16] H. Yu et al., “Topological insulator as an optical modulator for pulsed solid-state lasers,” Laser Photon. Rev., vol. 7, no. 6, pp. L77–L83, Nov. 2013. [17] Y. Chen et al., “Large energy, wavelength widely tunable, topological insulator Q-switched erbium-doped fiber laser,” IEEE J. Sel. Topics Quantum Electron., vol. 20, no. 5, Sep./Oct. 2014, Art. ID 900508. [18] Z. Luo et al., “1.06 μm Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi2 Se3 as a saturable absorber,” Opt. Exp., vol. 21, no. 24, pp. 29516–29522, Nov. 2013. [19] Z. Luo et al., “Topological-insulator passively Q-switched double-clad fiber laser at 2 μm wavelength,” IEEE J. Sel. Topics Quantum Electron., vol. 20, no. 5, Sep./Oct. 2014, Art. ID 902708. [20] K. Wang et al., “Ultrafast saturable absorption of two-dimensional MoS2 nanosheets,” ACS Nano, vol. 7, no. 10, pp. 9260–9267, Oct. 2013. [21] H. Zhang et al., “Molybdenum disulfide (MoS2 ) as a broadband saturable absorber for ultra-fast photonics,” Opt. Exp., vol. 22, no. 6, pp. 7249–7260, Mar. 2014. [22] R. I. Woodward et al., “Q-switched fiber laser with MoS2 saturable absorber,” in Proc. CLEO, Sci. Innov., 2D Other Novel Mater., Opt. Soc. Amer., Jun. 2014, p. SM3H.6. [23] S. Wang et al., “Broadband few-layer MoS2 saturable absorbers,” Adv. Mater., vol. 26, no. 21, pp. 3538–3544, Jun. 2014. [24] H. Xia et al., “Ultrafast erbium-doped fiber laser mode-locked by a CVD-grown molybdenum disulfide (MoS2 ) saturable absorber,” Opt. Exp., vol. 22, no. 14, pp. 17341–17348, Jul. 2014. [25] H. Liu et al., “Femtosecond pulse erbium-doped fiber laser by a few-layer MoS2 saturable absorber,” Opt. Lett., vol. 39, no. 15, pp. 4591–4594, Aug. 2014. [26] S. Najmaei et al., “Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers,” Nature Mater., vol. 12, no. 8, pp. 754–759, Jun. 2013. [27] H. Li et al., “From bulk to monolayer MoS2 : Evolution of Raman scattering,” Adv. Funct. Mater., vol. 22, no. 7, pp. 1385–1390, Apr. 2012. [28] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS2 : A new direct-gap semiconductor,” Phys. Rev. Lett., vol. 105, no. 13, p. 136805, Sep. 2010. [29] D. Mao, X. Liu, and H. Lu, “Observation of pulse trapping in a nearzero dispersion regime,” Opt. Lett., vol. 37, no. 13, pp. 2619–2621, Jun. 2012. [30] J. J. Zayhowski and P. L. Kelley, “Optimization of Q-switched lasers,” IEEE J. Quantum Electron., vol. 27, no. 9, pp. 2220–2225, Sep. 1991.

V. C ONCLUSION We experimentally demonstrated a passively Q-switched linear-cavity EDFL based on a few-layer MoS2 SA. The fewlayer MoS2 was prepared by the CVD method and sandwiched between two FCs with a fiber adapter to form a fiber-compatible MoS2 -based SA. Stable Q-switched pulses at 1549.91 nm were successfully obtained. The laser showed a pump threshold of 20.4 mW and a minimum pulse duration of 1.66 μs. The pulse repetition rate could be varied over a wide range of frequencies, from 10.6 to 173.1 kHz, by adjusting the pump power. Our experimental results suggest that few-layer MoS2 is a promising material for pulsed laser applications. R EFERENCES [1] R. Paschotta et al., “Passively Q-switched 0.1-mJ fiber laser system at 1.53 μm,” Opt. Lett., vol. 24, no. 6, pp. 388–390, Mar. 1999. [2] T. Hakulinen and O. G. Okhotnikov, “8 ns fiber laser Q switched by the resonant saturable absorber mirror,” Opt. Lett., vol. 32, no. 18, pp. 2677–2679, Sep. 2007. [3] Z.-C. Luo, J.-R. Liu, H.-Y. Wang, A.-P. Luo, and W.-C. Xu, “Wideband tunable passively Q-switched all-fiber ring laser based on nonlinear polarization rotation technique,” Laser Phys., vol. 22, no. 1, pp. 203–206, Oct. 2012. [4] X. Liu, “Soliton formation and evolution in passively-mode-locked lasers with ultralong anomalous-dispersion fibers,” Phys. Rev. A, vol. 84, no. 2, p. 023835, Aug. 2011. [5] X. Liu, “Interaction and motion of solitons in passively-mode-locked fiber lasers,” Phys. Rev. A, vol. 84, no. 5, p. 053828, Nov. 2011. [6] D.-P. Zhou, L. Wei, B. Dong, and W.-K. Liu, “Tunable passively Q-switched erbium-doped fiber laser with carbon nanotubes as a saturable absorber,” IEEE Photon. Technol. Lett., vol. 22, no. 1, pp. 9–11, Jan. 1, 2010. [7] X. Liu et al., “Versatile multi-wavelength ultrafast fiber laser modelocked by carbon nanotubes,” Sci. Rep., vol. 3, p. 2718, Sep. 2013. [8] T. Hasan et al., “Nanotube–polymer composites for ultrafast photonics,” Adv. Mater., vol. 21, nos. 38–39, pp. 3874–3899, Oct. 2009. [9] 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, Oct. 2009.