Open Access
Add to BibSonomyAbstract
A mid-infrared Er doped CaF2 crystal was successfully grown by the bridgeman method. Efficiently continuous wave and Q-switched laser operations were demonstrated at 2.8 μm with a 4 at.% Er doped CaF2 crystal end-pumped by a fiber-coupled 974 nm diode laser. The continuous wave output power of 295 mW was obtained in a compact linear cavity. A stable 2.8 μm passively Q-switched Er:CaF2 laser was also demonstrated with a graphene saturable absorber. Under an absorbed pump power of 2.353 W, an average output power of 172 mW was generated with a pulse duration of 1.324 μs and a repetition rate of 62.70 kHz, corresponding to the single pulse energy of 2.74 μJ and the peak power of 2.07 W, respectively. The high-quality Er:CaF2 crystal and the monolayer graphene are an ideal combination to directly obtain a near 3 μm mid-infrared region pulse laser.
© 2016 Optical Society of America
1. Introduction
2.8 μm mid-infrared region (MIR) lasers are near the absorption peak of water due to OH-vibration in water vapour and liquid water, which makes them widely use in bio-medical field for a precise laser surgery, like tissue cutting and stitching [1,2 ]. Meanwhile, due to the heavily reducing water vapor content in space, they can be directly used in space scientific research. For generating MIR lasers, a most common means is nonlinear wavelength conversion in optical parametric oscillator (OPO). However, the laser systems, including OPO, are very complex and hard controllable due to added nonlinear crystal, phase matching, temperature factors, and so on. So, it is desired to seek a laser system which can directly generate MIR laser with compact structure, good stability and high efficiency.
To acquire this excellent laser system, an important method is to search for high quality laser materials to directly produce the 2.8 μm MIR laser. The Er:CaF2 crystals emerge as the times require and have strong competitive advantages. Due to the up-conversion, the 4I11/2 → 4I13/2 transition obtains the target laser [3]. The strong and broad absorption bands are from 960 nm to 990 nm, so that they can be pumped by commercialized and low-cost laser diode (LD). And the fluorides, as host materials, have the low phonon energy that can effectively improve the up-conversion performance of the 2.8 μm [4,5 ]. Fluorides also own many unique advantages like better thermal conductivity, low refractive index in MIR, high damage threshold and large size growth. As early as 1974 year, the Er:CaF2 laser, around 2.8 μm, had been successfully acquired by S. Kh. Batygov [6]. Afterwards, they have been extensively studied as laser material [7–9 ]. But the Er:CaF2 pulse lasers were rarely explored at 2.8 μm MIR. And, nowadays, directly diode-pumped MIR pulse lasers are one of the hottest and most challenging topics.
The passive Q-switching is an effective and concise means to acquire the pulse laser, and saturable absorber is an important component of that laser. Especially for MIR laser, no admirable saturable absorber immediately limits the laser development. Fortunately, a channel is provided by the two dimensional (2D) materials saturable absorber. The 2D materials possess the broadband absorption over a wide range, which is able to effectively act on MIR laser.
The family of 2D materials saturable absorbers mainly have the graphene, the topological insulator (TI, as Bi2Se3), the insulating hexagonal boron nitride (hBN), the transition metal dichalcogenides (TMDCs, like MoS2 and WS2) and the black phosphorus (BP) [10–22 ]. They all have outstanding physical and chemical properties and each of them possesses some inherent advantages for specific applications. Among graphene is an atomic layer arranged in a two dimensional hexagonal lattice, in which charge carriers move at ultrafast speed [23,24 ]. Saturable absorption is achieved owing to Pauli blocking of the electrons and holes [25,26 ]. And, as saturable absorber, its remarkable characteristic is zero-bandgap which makes it have broadband optical reaction. Simultaneously, it possesses ultrashort recovery time, high damage threshold, easy fabrication, and good environmental stability [27–30 ]. So, graphene is a superb saturable absorber for MIR pulse laser [31,32 ].
In this paper, pumped by LD, the 4 at.% Er:CaF2 crystal was placed in a a compact concave-plane cavity to directly acquire near 3 μm laser. When the absorbed pump power was 2.353 W, the maximum output power of continuous wave (CW) laser was 295 mW. By inserting the efficient graphene saturable absorber into the cavity, a stable 2.8 μm Q-switched Er:CaF2 laser operation was realized. The maximum output power was 172 mW, corresponding to a slope efficiency of 10.37%. The pulse repetition rate was 62.70 kHz with a pulse duration of 1.324 μs. The single pulse energy and the peak power were calculated to be 2.74 μJ and 2.07 W, respectively.
2. The spectral properties of Er:CaF2 crystal and the characteristics of graphene
The 4 at.% Er:CaF2 crystals were grown by the traditional and mature bridgeman method. The room-temperature absorption spectra is shown in Fig. 1(a) , which displays that the strongest absorption bands are 967 nm and 980 nm. Figure 1(b) is the room-temperature emission spectra under the 980 nm pumping. The crystals have a broad emission band ranging from 2600 to 2850 nm in favour of the generation of the ultra-short laser pulses. More information about the crystals was reported in [33].
Fig. 1 The room-temperature spectra of 4 at.%Er:CaF2. (a) The absorption spectra. (b) The emission spectra.
The adopted graphene was monolayer and fabricated by chemical vapor deposition (CVD) technology. Using this graphene, our team had successfully achieved Yb:Sc2SiO5 mode-locked laser at 1 μm [34]. In [34], the corresponding parameters had been given and the transmission was measured to be 81% at this test laser wavelength. The losses were from graphene film, quartz substrate, environmental factors, and so on.
3. Passively Q-switched laser based on graphene
The passively Q-switched Er:CaF2 laser was researched by a compact concave-plane cavity whose length was 40 mm as Fig. 2 . The input mirror M1 was a concave mirror having radius of 50 mm with high transmission at 974 nm and high reflection at 2.9 μm. The M2 was a plane mirror with output transmittance of 3% at 2.7-2.95 μm. The uncoated 4 at.% Er:CaF2 crystal, with the dimension of 3 mm × 3 mm × 10 mm, was mounted in a Cu holder whose temperature was stabilized at 14 °C by cooling water, in order to remove the heat and reduce the thermal effect. The pump power was provided by a fiber coupled LD (a fiber core diameter of 105 μm and numerical aperture of 0.22) which had the central wavelength and the line width were 974 nm and 1.6 nm, respectively. At the same time, the pump laser was expanded by a optics coupling system of 1:2.
In the first experiment, we investigated the CW output performance of the laser. The maximum CW output power was obtained for the output coupling of 3% transmittance. After obtaining the CW laser, the graphene saturable absorber was inserted into the cavity close to the M2 (as Fig. 2). Carefully adjusting the position and the angle of graphene, the stably Q-switched laser was obtained.
Figure 3 shows the output characteristics of Er:CaF2 laser for CW and Q-switching. To CW laser, the laser threshold absorbed pump power was 724 mW, the experiment obtained 26 mW average output power. When the absorbed pump power was increased to be 2.353 W, the maximum average output power and the slope efficiency were 295 mW and 16.74%, respectively. The average output power was measured by the power meter (30A-SH-V1, Israel). The CW spectrum centered at 2793.8 nm was measured by an optical spectrum analyzer (Zolix-Omni-λ300, China). After, accurately adjusting graphene, the Q-switched laser was obtained with a good stability. The absorbed pump power was in the same variational scope from 724 mW to 2.353 W. The average output power was from 6 mW linearly increased to be 172 mW. And the linear function had a high slope efficiency of 10.37%.
In order to protect the crystal from destroying, the incident pump power was no further increased. Moreover, if the crystal was coated for antireflection at the laser wavelength and the pump wavelength, the average output power and the slope efficiency were expected to be enhanced.
Figure 4 shows that, with the added absorbed pump power, the pulse width was reducing from 3.815 μs and the repetition rate was increasing from 28.14 kHz. When the absorbed pump power was reached 2.353 W, the experiment acquired the shortest pulse width of 1.324 μs and the highest repetition rate of 62.70 kHz. In combination with the average output power of 172 mW, the single pulse energy and the pulse peak power were calculated to be 2.74 µJ and 2.07 W.
A recorded typical oscilloscope pulse train is showed in Fig. 5 , which was measured by a fast infrared detector with a response time of 3 ns (VIGO System S.A. PVM-2TE-10.6/MIPAC250M, Poland) and a 500 MHz digital oscilloscope (Tektronix DPO4054, USA). The stably Q-switched laser could last for more than two hours, with time jitter less than 20%.
4. Conclusions
In conclusion, the practical, efficient, compact and stable pulse laser was successfully obtained at near 3 μm MIR. The Er:CaF2 crystal was grown with a broad emission band ranging from 2600 to 2850 nm. The high quality monolayer graphene had admirable saturable absorption characteristic in 2.8 μm MIR. In this paper, the Q-switched pulses were demonstrated with 1.324 μs pulse width and 62.70 kHz repetition rate. The high slope efficiency of 10.37% and the average output power of 172 mW were obtained. Synchronously, the single pulse energy and the pulse peak power were 2.74 µJ and 2.07 W. The results proved that Er:CaF2 crystal was a kind of potential efficiency laser materials for generating 2.8 μm MIR lasers and the graphenes as saturable absorber were a breakthrough for MIR pulse laser. The better results will be obtained after optimizing the crystal growth and the parameter of graphene.
Acknowledgments
The authors acknowledge support from the National Natural Science Foundation of China (Nos. 61475089, 61575088, 61422511 and 51432007) and Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (No. 15KJB430013).
References and links
1. T. Li, K. Beil, C. Kränkel, C. Brandt, and G. Huber, “Laser performance of highly doped Er:Lu2O3 at 2.8 µm,” in Advanced Solid-State Photonics, OSA Technical Digest Series (Optical Society of America, 2012), paper AW5A.6.
2. C. Ziolek, H. Ernst, G. F. Will, H. Lubatschowski, H. Welling, and W. Ertmer, “High-repetition-rate, high-average-power, diode-pumped 2.94-microm Er:YAG laser,” Opt. Lett. 26(9), 599–601 (2001). [CrossRef] [PubMed]
3. T. T. Basiev, Y. V. Orlovskii, M. V. Polyachenkova, P. P. Fedorov, S. V. Kuznetsov, V. A. Konyushkin, V. V. Osiko, O. K. Alimov, and A. Y. Dergachev, “Continuously tunable cw lasing near 2.75 μm in diode-pumped Er3+:SrF2 and Er3+:CaF2 crystals,” Quantum Electron. 36(7), 591–594 (2006). [CrossRef]
4. C. Labbe, J. L. Doualan, P. Camy, R. Moncorgé, and M. Thuau, “The 2.8 μm laser properties of Er3+ doped CaF2 crystals,” Opt. Commun. 209(1–3), 193–199 (2002). [CrossRef]
5. W. W. Ma, L. B. Su, J. Y. Wang, D. P. Jiang, X. D. Xu, and J. Xu, “The concentration effect on spectroscopic properties of Er:CaF2 crystals,” in Advanced Solid State Lasers, OSA Technical Digest Series (Optical Society of America, 2015), paper AM5A.17.
6. S. K. Batygov, L. A. Kulevskii, A. M. Prokhorov, V. V. Osiko, A. D. Savel’ev, and V. V. Smirnov, “Erbium-doped CaF2 crystal laser operating at room temperature,” Sov. J. Quantum Electron. 4(12), 1469–1470 (1975). [CrossRef]
7. G. V. Gomelauri, L. A. Kulevskii, V. V. Osiko, A. D. Savel’ev, and V. V. Smirnov, “Single-mode Q-switched CaF2:Er3+ laser,” Sov. J. Quantum Electron. 6(3), 341–342 (1976). [CrossRef]
8. L. N. Gnatyuk, M. L. Gurari, V. N. Zhiganov, L. A. Kulevskii, S. N. Marchenko, A. M. Prokhorov, V. V. Smirnov, and B. M. Stepanov, “Formation of holograms at the wavelength of 2.76 μ and measurement of the width of the emission line of a CaF2:Er3+ crystal laser by a holographic method,” Sov. J. Quantum Electron. 8(1), 94–95 (1978). [CrossRef]
9. P. Xie and S. C. Rand, “Continuous-wave, pair-pumped laser,” Opt. Lett. 15(15), 848–850 (1990). [CrossRef] [PubMed]
10. H. Zhang, Q. L. Bao, D. Y. Tang, L. M. Zhao, and K. P. Loh, “Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker,” Appl. Phys. Lett. 95(14), 141103 (2009). [CrossRef]
11. Y. G. Wang, Z. S. Qu, J. Liu, and Y. H. Tsang, “Graphene oxide absorbers for watt-level high-power passive mode-locked Nd:GdVO4 laser operating at 1 μm,” J. Lightwave Technol. 30(20), 3259–3262 (2012). [CrossRef]
12. X. L. Qi and S. C. Zhang, “Topological insulators and superconductors,” Rev. Mod. Phys. 83(4), 1057–1110 (2011). [CrossRef]
13. S. Lu, C. Zhao, Y. Zou, S. Chen, Y. Chen, Y. Li, H. Zhang, S. Wen, and D. Tang, “Third order nonlinear optical property of Bi₂Se₃,” Opt. Express 21(2), 2072–2082 (2013). [CrossRef] [PubMed]
14. H. Liu, A. P. Luo, F. Z. Wang, R. Tang, M. Liu, Z. C. Luo, W. C. Xu, C. J. Zhao, and H. Zhang, “Femtosecond pulse erbium-doped fiber laser by a few-layer MoS(2) saturable absorber,” Opt. Lett. 39(15), 4591–4594 (2014). [CrossRef] [PubMed]
15. H. Zhang, S. B. Lu, J. Zheng, J. Du, S. C. Wen, D. Y. Tang, and K. P. Loh, “Molybdenum disulfide (MoS₂) as a broadband saturable absorber for ultra-fast photonics,” Opt. Express 22(6), 7249–7260 (2014). [CrossRef] [PubMed]
16. M. Q. Fan, T. Li, S. Z. Zhao, G. Q. Li, H. Y. Ma, X. C. Gao, C. Kränkel, and G. Huber, “Watt-level passively Q-switched Er:Lu2O3 laser at 2.84 μm using MoS2,” Opt. Lett. 41(3), 540–543 (2016). [CrossRef] [PubMed]
17. X. Zheng, R. Chen, G. Shi, J. Zhang, Z. Xu, X. Cheng, and T. Jiang, “Characterization of nonlinear properties of black phosphorus nanoplatelets with femtosecond pulsed Z-scan measurements,” Opt. Lett. 40(15), 3480–3483 (2015). [CrossRef] [PubMed]
18. B. Zhang, F. Lou, R. Zhao, J. He, J. Li, X. Su, J. Ning, and K. Yang, “Exfoliated layers of black phosphorus as saturable absorber for ultrafast solid-state laser,” Opt. Lett. 40(16), 3691–3694 (2015). [CrossRef] [PubMed]
19. Z. Qin, G. Xie, H. Zhang, C. Zhao, P. Yuan, S. Wen, and L. Qian, “Black phosphorus as saturable absorber for the Q-switched Er:ZBLAN fiber laser at 2.8 μm,” Opt. Express 23(19), 24713–24718 (2015). [CrossRef] [PubMed]
20. Z. Qin, G. Xie, C. Zhao, S. Wen, P. Yuan, and L. Qian, “Mid-infrared mode-locked pulse generation with multilayer black phosphorus as saturable absorber,” Opt. Lett. 41(1), 56–59 (2016). [CrossRef] [PubMed]
21. H. Yu, X. Zheng, K. Yin, X. A. Cheng, and T. Jiang, “Nanosecond passively Q-switched thulium/holmium-doped fiber laser based on black phosphorus nanoplatelets,” Opt. Mater. Express 6(2), 603–609 (2016). [CrossRef]
22. Z. Wang, R. Zhao, J. He, B. Zhang, J. Ning, Y. Wang, X. Su, J. Hou, F. Lou, K. Yang, Y. Fan, J. Bian, and J. Nie, “Multi-layered black phosphorus as saturable absorber for pulsed Cr:ZnSe laser at 2.4 μm,” Opt. Express 24(2), 1598–1603 (2016). [CrossRef] [PubMed]
23. Q. L. Bao, H. Zhang, Z. H. Ni, Y. Wang, L. Polavarapu, Z. X. Shen, Q. H. Xu, D. Y. Tang, and K. P. Loh, “Monolayer graphene as a saturable absorber in a mode-locked laser,” Nano Res. 4(3), 297–307 (2011). [CrossRef]
24. J. M. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Measurement of ultrafast carrier dynamics in epitaxial graphene,” Appl. Phys. Lett. 92(4), 042116 (2008). [CrossRef]
25. 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. Express 17(20), 17630–17635 (2009). [CrossRef] [PubMed]
26. Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
27. G. W. Zhu, X. S. Zhu, K. Balakrishnan, R. A. Norwood, and N. Peyghambarian, “Fe2+:ZnSe and graphene Q-switched singly Ho3+-doped ZBLAN fiber lasers at 3 μm,” Opt. Mater. Express 3(9), 1365–1377 (2013). [CrossRef]
28. G. Q. Xie, J. Ma, P. Lv, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, J. Y. Wang, and D. Y. Tang, “Graphene saturable absorber for Q-switching and mode locking at 2 µm wavelength,” Opt. Mater. Express 2(6), 878–883 (2012). [CrossRef]
29. P. L. Huang, S. C. Lin, C. Y. Yeh, H. H. Kuo, S. H. Huang, G. R. Lin, L. J. Li, C. Y. Su, and W. H. Cheng, “Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber,” Opt. Express 20(3), 2460–2465 (2012). [CrossRef] [PubMed]
30. G. Sobon, J. Sotor, I. Pasternak, A. Krajewska, W. Strupinski, and K. M. Abramski, “Multilayer graphene-based saturable absorbers with scalable modulation depth for mode-locked Er-and Tm-doped fiber lasers,” Opt. Mater. Express 5(12), 2884–2894 (2015). [CrossRef]
31. M. N. Cizmeciyan, J. W. Kim, S. Bae, B. H. Hong, F. Rotermund, and A. Sennaroglu, “Graphene mode-locked femtosecond Cr:ZnSe laser at 2500 nm,” Opt. Lett. 38(3), 341–343 (2013). [CrossRef] [PubMed]
32. G. W. Zhu, X. S. Zhu, F. Q. Wang, S. Xu, Y. Li, X. L. Guo, K. Balakrishnan, R. A. Norwood, and N. Peyghambarian, “Graphene mode-locked fiber laser at 2.8 μm,” IEEE Photonic. Tech. L. 20(1), 7–10 (2016). [CrossRef]
33. W. W. Ma, L. B. Su, X. D. Xu, J. Y. Wang, D. P. Jiang, L. H. Zheng, X. W. Fan, C. Li, J. Liu, and J. Xu, “Effect of erbium concentration on spectroscopic properties and 2.79 μm laser performance of Er:CaF2 crystals,” Opt. Mater. Express 6(2), 409–415 (2016). [CrossRef]
34. W. Cai, S. Z. Jiang, S. C. Xu, Y. Q. Li, J. Liu, C. Li, L. H. Zheng, L. B. Su, and J. Xu, “Graphene saturable absorber for diode pumped Yb:Sc2SiO5 mode-locked laser,” Opt. Laser Technol. 65, 1–4 (2015). [CrossRef]
