Passively Q-Switched Er-Doped Fiber Lasers Using ... - OSA Publishing

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 23, DECEMBER 1, 2015

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Passively Q-Switched Er-Doped Fiber Lasers Using Alcohol Zhiqiang Wang, Li Zhan, Minglei Qin, Jian Wu, Liang Zhang, Zhixin Zou, and Kai Qian

Abstract—A passively Q-switched Er-doped fiber laser using a simple and cost-effective saturable absorber (SA) of alcohol has been demonstrated. The SA is made through filling anhydrous alcohol in the gap between the end-facets of two optical patch cords, which are inserted into a cannula. The modulation depth of the SA with 820-μm alcohol layer is measured to be 18%. Based on such an SA, the Q-switching operation in the Er-doped fiber laser has been achieved. The average output power of the Q-switched fiber laser is 30.3 mW with the pulse repetition rate of 81.9 kHz, and thus, the pulse energy is up to 348.6 nJ. To the best of our knowledge, it is the first demonstration of the passively Q-switched laser based on an alcohol SA. This provides the further evidences indicating the possibility of organic liquid as effective SAs for pulsed lasers. Index Terms—Fiber lasers, Q-switched lasers, saturable absorbers.

I. INTRODUCTION HORT pulse fiber lasers, because of their short duration and high peak power, are of great interest in various applications for remote sensing, medicine, laser machining and others [1]. Mode locking [2] and Q-switching [3] are the two widely used methods to obtain short pulses. In mode-locked fiber lasers, femtosecond pulses can be generated by exploiting different mechanisms [4]–[6]. However, the pulse energy is limited to nano-Joule level, and the pulse repetition rate is fixed due to its operation mechanism. For many applications, such as laser marking and machining, laser ranging and optical time domain reflectometry, short-pulse lasers with high energy are required. Q-switching is an effective method to generate high energy short pulses in fiber lasers, and it can be realized either in an active or a passive way [7], [8]. The active Q-switching is easy to control the pulse repetition rate as well as the pulse width, whereas the use of an optical modulator for active Q-switching in the laser cavity is disadvantageous because of its cost and complex operation. Compared with the active operation, the passive Q-switching owns the unique advantage of simple structure in all-fiber designing. Usually, a saturable absorber (SA) is adopted

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Manuscript received August 12, 2015; revised September 20, 2015; accepted October 13, 2015. Date of publication October 25, 2015; date of current version November 1, 2015. This work was supported in part by the National Natural Science Foundation of China under Grants 61178014 and 11274231. (Corresponding author: Li Zhan.) Z. Wang, L. Zhan, M. Qin, J. Wu, L. Zhang, and Z. Zou are with the Department of Physics and Astronomy, Key Laboratory for Laser Plasmas (Ministry of Education), State Key Lab of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). K. Qian is with the School of Information Engineering, Hubei University for Nationalities, Enshi 445000, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2015.2490804

in operations of all passive Q-switching. Unlike SAs for modelocking, on one hand, the SA response time need not be shorter than the cavity round-trip time but should be shorter than the life time of irons in upper levels of gain media, since the pulse duration is determined by the time of the gain depletion after the saturation [9]. On the other hand, the requirement of the modulation depth of SAs for Q-switching is larger than that of SAs for mode locking. It is to be noted that the modulation depth should be large enough to change the cavity loss in Q-switching process and the SAs should support the high pulse energy operation. Therefore, an ideal SA for Q-switching operation needs to own a large modulation depth, high damage threshold, and good thermal stability, so as to maintain stable pulse operation with high pulse energy. Many kinds of SAs for Q-switching have been reported, such as semiconductor saturable absorber mirrors (SESAMs) [10], carbon nanotubes (CNTs) [11], [12], graphene [13], [14] and topological insulators (TIs) [15]–[17]. SESAM has a narrow wavelength tuning range (tens of nanometers), and its modulation depth is typically less than 10% [18]. The CNTs and graphene are ideal SAs for Q-switching because of their low saturation intensity, low cost and broadband wavelength operation [19], [20]. TIs [15], [16], as new-type SAs, have also been extensively researched in the recent years. However, the modulation depth of above mentioned SAs is generally low [21]. It is not sufficient to guarantee a safe and robust operation for higher pulse energy [22], since the Q-switched pulse energy is normally up to several hundreds of nano Joule, which has the possibility to make SAs damaged. Numerous new materials and novel fabrication methods have been created to increase the modulation depth of SAs. For example, a nanoscale p-type Bi2 Te3 power-based SA has a modulation depth of 27%, which has been used to conduct a mode-locked fiber laser [23]. By directly scribing a PVA film on the end-facet of a fiber connector, and then imprinting it with graphene nanoparticle, an graphene nanoparticle based SA can be formed, and its modulation depth can be enhanced from 11% to 20% [24]. Recently, we proposed and experimentally demonstrated alcohol to perform a liquid SA for mode-locking operation in fiber lasers [25]. The experiment results present that the modulation depth of alcohol-SAs can be flexibly modified to meet the different requirements by varying the amount of the alcohol. What’s more, it is a liquid SA with advantages of large liquidity, self-healing feature, high damage threshold, and good thermal diffusivity. Hence, it is most likely a valid way to expect the Q-switching operation in fiber lasers based on alcohol-SAs. In this paper, we present what we believe to be the first demonstration of an alcohol-based Q-switched Er-doped fiber laser (EDFL). The nonlinear optical property of alcohol-SA has been

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Fig. 2. A balanced twin-detector measurement system to characterize the nonlinear response of the alcohol-SA. Fig. 1. Experimental setup of Q-switched EDFL. The inset shows the schematic image of the SA.

characterized for realizing the Q-switching operation. Our experiment shows that the alcohol is a really good choice for Q-switched lasers, indicating the possibility of organic liquid as effective SAs for short pulse generation. II. EXPERIMENTAL SETUP AND OPERATION PRINCIPLE Fig. 1 depicts the experimental setup of the passively Qswitched EDFL using an alcohol-SA. A 1.2 m long Er-doped fiber (EDF) was used as a gain medium, which was forward pumped by a 980 nm laser with a maximum pump power of 534 mW. The EDF length is optimized according to the absorption ratio and the pump power. The EDF has an 80 dB/m absorption ratio at 1530 nm and more than 11 dB/m absorption ratio at 980 nm, respectively. The 1.2 m EDF is suitable to maximize the output power when the pump power runs at its maximum value. A polarization controller (PC) was utilized to optimize the laser output, and also served to characterize the polarization statuses of the Q-switched pulses. A polarization-independent isolator (ISO) was inserted to force the laser unidirectional circulation in the cavity. A 20:80 optical coupler (OC) extracted 20% of the generated laser for measurements. The total cavity length was 9.6 m. The output spectrum was measured with an optical spectrum analyzer (Yokogawa AQ6370), and the pulse train from the laser was monitored with a 20 GHz photodetector and was visualized on an oscilloscope (RTO 1002, 2 GHz). The alcohol-SA for Q-switched fiber laser was fabricated by inserting the end-facets of two optical patch cords into a cannula (hollow tube) as shown in Fig. 1. The diameter of the outer cladding and the inner core of fiber end-facets are 125 and 9 μm, respectively. The inner core diameter of the cannula is 125 μm, which matches well with the diameter of the outer cladding of fiber patch cords. The gap between end-facets of the cords in the cannula was filled with anhydrous alcohol to form a SA. The thickness of the alcohol layer could be easily changed, so as to obtain various SAs with different modulation depths, which is a unique advantage of this kind of SAs. The alcohol exhibits a broadband absorption ranging from 1500 to 2400 nm owing to the stretching vibration and flexural vibration of carbon hydrogen bonds (C–H), carbon–oxygen bonds (C–O) and oxygen hydrogen bonds (O–H) [25]. Consideration of wide absorption range and nonlinear saturable absorption of alcohol together, it is possible to implement the Q-switching operation in

Fig. 3.

The nonlinear characteristic of the alcohol-SA.

fiber lasers with alcohol-SAs. In our experiment, an alcohol-SA with the thickness of 820 μm, was utilized to conduct a passively Q-switched EDFL. The small gaps between the cannula and the optical patch cords are sealed with paraffin to avoid the evaporation of the sealed alcohol. First, we employed a balanced twin-detector measurement system to characterize the nonlinear optical response of the alcohol-SA as shown in Fig. 2. The probe light is a homemade mode-locked EDFL with the pulse duration of 185 fs at 1600.2 nm and the repetition rate of 33.5 MHz. Fig. 3 presents the measured transmission rate of the SA. The typical saturable absorption is observed, and the modulation depth was measured to be 18%, which can be well fitted with the saturable absorption formula [26]. According to the modulation depth and the absorption coefficient of alcohol of 2.2/cm at 1600 nm [25], the calculated thickness of the alcohol layer is 818.2 μm, which is well consistent with the measured thickness. Since the optical absorption of alcohol at this band originates from the electronics transition within the vibrational energy levels of alcohol molecules, the relaxation time should be in the order of picosecond [27]. Thus, the response time of the SA is much shorter than the life time of Er3+ ions on the metastable level (∼10 ms), and therefore, this alcohol-SA provides an adequate property for Q-switching in fiber lasers. III. EXPERIMENTAL RESULTS AND DISCUSSION Self-Q-switching of the fiber laser was excluded first. By increasing the pump power from the threshold to the maximum power and/or randomly choosing different polarization statuses

WANG et al.: PASSIVELY Q-SWITCHED ER-DOPED FIBER LASERS USING ALCOHOL

Fig. 4.

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Various pulse trains under different pump powers.

in a full range through adjusting the PC, only continuous-wave operation was observed when the laser cavity contains no SA. However, after inserting the SA into the laser structure, the remarkable passive Q-switching operation occurred at the pump power of 359.7 mW. Fig. 4 shows the oscilloscope traces of the Q-switched pulse trains under different pump power. In the process of varying the pump power, the pulse output remains stable and the repetition rates are in dozens of kilohertz range. The threshold for initiating the Q-switching is higher, suffering from the insertion loss of the SA and the absorption in the alcohol layer. In our experiment, the Q-switching operation can maintain at least several hours for whole test process. It needs not refill alcohol after each operation if the alcohol is well sealed in the cannula with paraffin. Fig. 5 shows a typical Q-switched pulse train and the corresponding optical spectrum at the pump power of 502.2 mW. The pulse repetition rate is 84.25 kHz, consisting with a time interval of 11.9 μs as shown in Fig. 5(a). The profile of single pulse is also displayed in Fig. 5(a) with the pulse duration of 692.6 ns. The central wavelength of the optical spectrum locates at 1559.1 nm with a 3 dB bandwidth of 2.14 nm. The varying of polarization states in the cavity exhibits a very weak effect on the spectrum and pulses, indicating that the Q-switching operation is polarization-insensitive. The output power and the pulse energy as a function of the pump power are shown in Fig. 6(a), and the pulse duration and the pulse repetition rate versus the pump power is presented in Fig. 6(b). By increasing the pump power, the output power varies from 21.7 to 30.3 mW and the repetition rate changes from 75.6 to 87 kHz. Hence, the single pulse energy scales from

Fig. 5. (a) The profile of Q-switched pulse at the pump power of 502.22 mW. Inset: the pulse train. (b) The corresponding Q-switched optical spectrum.

286.8 up to 348.6 nJ. The pulse duration is gradually decreasing from 1.42 μs to 670.9 ns during the pump power increasing process. Unlike the repetition rate in mode-locked operation, the repetition rate of Q-switched pulses is dependent on the pump power, not determined by the cavity length, since the pulse generation relies on the saturation of the SA. As increasing pump power, more gain is provided to saturate the SA, leading to the increase of the repetition rate. The maximum output power is 30.3 mW for 87 kHz repetition rate, and thus the single pulse energy is 348.6 nJ. It is possible to obtain pulses with higher energy, if the pump source with a higher power is used. The potential ability for this kind of SAs to gain higher pulse energies comes from the intrinsic advantages of the liquid with excellent self-healing feature, good thermal diffusivity and high damage threshold. The relationship between the Q-switched optical spectrum and the pump power has also been displayed in Fig. 7. The central wavelength is blue-shifted 5 nm with the increase in the pump power, whereas the spectral width is nearly a constant of 2 nm. The wavelength shift can be attributed to that the level of the population inversion augments with the pump power increasing. Beyond the advantages of the simple fabricating process and the low costs, the large modulation depth is another merit of the alcohol-SA for Q-switching operation. The fabricating method facilitates us to achieving SAs with different modulation depth

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mode-locking operation in fiber lasers. The organic liquid SAs can operated in a broad wavelength range, as reported in Ref. [25]. Therefore, it is likely to employ them in Tm-doped fiber lasers around 1800∼2200 nm, making them an alternative SA in various applications. Up to now, alcohol-SAs with different modulation depth have been successfully applied in EDFLs for mode locking and Q switching operations. Wider applications of this kind of SAs can be expected in other kinds of lasers, as long as the working wavelength is in the absorption band of organic liquid. For example, the alcohol-SAs with a much larger modulation depth can be also used to conduct single-frequency fiber lasers [28], [29]. Meanwhile, it is suitable to fabricate all-fiber-based SAs with liquid-filled photonic crystal fiber [20], [30], [31], which is helpful to support short pulse operation in all-fiber lasers. Certainly, the alcohol-based SAs can be applied for mode-locking or Q-switching in solid-state lasers. Conveniently, the users can design transparent cells filled with alcohol as SAs and insert them into the cavity of solid-state lasers. However, this kind of SAs can not be applied to achieve multiwavelength fiber lasers, because such SAs cannot be expected to perform anti-saturable absorption [32], [33]. IV. CONCLUSION

Fig. 6. (a) Average output power and the single pulse energy versus the pump power. (b) Pulse duration and pulse repetition rate as a function of the pump power.

In conclusion, an alcohol-SA has been demonstrated and exploited experimentally for passive Q-switching operation of an EDFL. The modulation depth of the alcohol-SA corresponding to a thickness of 820 μm is measured to be 18%. This alcohol-SA based Q-switched EDFL can deliver the average output power of 30.3 mW with the pulse repetition rate of 87 kHz, corresponding to the single pulse energy of 348.6 nJ. To the best of our knowledge, it is the first demonstration of the passively Qswitched lasers using alcohol. Considering the broad absorption range of alcohol, Q-switched fiber lasers based on alcohol-SAs working in 2 μm range can be imaged. REFERENCES

Fig. 7. Characteristics of Q-switched optical spectrum in alcohol-SA based Q-switched fiber laser versus the pump power.

by changing the thickness of the alcohol layer. However, the threshold value to initiate the Q-switching is a little high in our experiment. But we can employ collimators to improve the coupling efficiency of the optical cords, leading to the output power increasing. The alcohol-SA is a real liquid SA, so as to be with good thermal diffusion and high damage threshold, which are the benefits over the graphene, CNTs and TIs-SAs. Certainly, other organic liquid, such as methyl alcohol and acetone, can be anticipated to fabricate the SAs for Q-switching or

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Zhiqiang Wang received the B.S. degree from the Department of the Basic Science, East China Jiao Tong University, Nanchang, China, in 2010, and the M.S. degree from China Ji Liang University, Hangzhou, China, in 2013. He is currently working toward the Ph.D. degree at the Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China. His research interests include mode-locked fiber lasers, nonlinear fiber optics, and fiber-optic devices and sensors.

Li Zhan received the B.S. and M.S. degrees from Shanghai Jiao Tong University, Shanghai, China, and the Ph.D. degree from the City University of Hong Kong, Hong Kong. He is currently a Professor at the Department of Physics and Astronomy, Shanghai Jiao Tong University. He is an Author or Coauthor of more than 100 journal papers. His research interests include fiber lasers, fast/slow light, nonlinear optics, and plasmonics.

Minglei Qin received the B.Sc. degree from the East China University of Science and Technology, Shanghai, China, in 2013. He is currently working toward the M.Sc. degree at the Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai. His research interests include slow and fast light, fiber optic gyroscope and fiber lasers. His current research focuses on mode-locked fiber lasers, nonlinear fiber optics, and optical communications.

Jian Wu received the B.S. degree in physics from Huazhong (Central China) Normal University, Wuhan, China, in 2006, and the M.S. degree from Shanghai Jiao Tong University, Shanghai, China, in 2008, where he is currently working toward the Ph.D. degree in optics engineering. His current research interests include analytical chemistry, optics, and plasma physics.

Liang Zhang received the B.S. degree from the Department of Physics, Shanghai Jiao Tong University, Shanghai, China, in 2009, where he is currently working toward the Ph.D. degree in optical engineering. His research interests include fiber lasers and nonlinear fiber optics.

Zhixin Zou received the B.S. degree from the Institute of Nuclear Science and Technology, University of South China, Hunan, China, in 2012. He is currently working toward the Master’s degree in optical engineering at Shanghai Jiao Tong University, Shanghai, China. His current research interests include slow fast light and microwave photonic filters, Brillouin fiber lasers, and optical communications.

Kai Qian received the M.S. degree from Anhui University, Hefei, China, in 2007, and the Ph.D. degree from Shanghai Jiaotong University, Shanghai, China, in 2011, both in optics. From 2012 to 2013, he was a Postdoctoral Fellow with the CORIA (CNRS-UMR6614), Saint Etienne du Rouvray, France. He is currently a Teacher at the School of Information Engineering, Hubei University for Nationalities, Enshi, China. His current research interests include slow and fast light, ultrafast fiber lasers, and fiber sensors.