1ps passively mode-locked laser operation of Na,Yb:CaF2 crystal Juan Du, Xiaoyan Liang State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
[email protected] [email protected]
Yonggang Wang Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Liangbi Su, Weiwei Feng, Enwen Dai, Zhizhan Xu and Jun Xu State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
Abstract: Diode-pumped passively mode-locked laser operation of Yb3+,Na+:CaF2 single crystal has been demonstrated for the first time. By using a SESAM (semiconductor saturable mirror), simultaneous transformlimited 1-ps passively mode-locked pulses, with the repetition rate of 183MHz, were obtained under the self-Q-switched envelope induced by the laser medium. The average output power of 360mW was attained at 1047nm for 3.34W of absorbed power at 976nm, and the corresponding pulse peak power arrived at 27kW, indicating the promising application of Yb3+,Na+codoped CaF2 crystals in achieving ultra-short pulses and high pulse peak power. © 2005 Optical Society of America OCIS codes: (140.3380) Laser materials; (140.3480) Lasers, diode-pumped; (140.4050) Modelocked lasers; (140.5680) Rare earth and transition metal solid-state lasers;
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(C) 2005 OSA
Received 22 July 2005; revised 6 September 2005; accepted 20 September 2005
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1. Introduction Increasing attention has been focused on Yb3+-based laser systems since the rapid development of high power and high brightness laser diodes emitting at 900−980-nm, which have been expected to be the most potential alternatives to the Nd3+-doped ones in the near-IR spectral range. Compared to their Nd3+ counterparts, Yb3+-doped crystals have broader absorption and emission spectra than Nd3+-doped ones owing to the strong electron-phonon coupling [1]. In addition, Yb3+ has a much simpler energy level scheme and hence a low intrinsic quantum defect (10%), which leading to a weak thermal load, an absence of luminescence quenching, and an enhanced laser action. Laser action near 1μm has been demonstrated in a number of Yb3+-doped materials [2-7], and it is obvious that hosts possessing higher thermal conductivity are favorable to exhibit the excellent laser performance of Yb3+. As a fluoride single crystal, CaF2 possesses higher transparency in a broad wavelength range, lower refractive-index-limiting nonlinear effect, and lower phonon-energy-reducing nonradiative relaxation between adjacent energy levels. In addition, compared with other fluoride single crystals, CaF2 is more popular owing to its lower phonon frequency, higher thermal conductivity, and easily being grown with a large diameter. Based on the advantages mentioned above, we choose CaF2 as our host. Currently, some researches have been focused on Yb:CaF2 crystal [6,7], and a series of approving results were achieved. Recently, we codoped Yb3+ with Na+ as a charge compensator with the purpose of enhancing quantum efficiency and suppressing the formation of Yb2+ ions [8]. It exhibited more excellent performance in direct diode-pumped laser operation than Yb:CaF2 crystal as described in Ref. 9. In this paper, we report for the first time the passively mode-locked performance of this novel Na,Yb:CaF2 single crystal. Its self-Q-switching performance with the highest conversion efficiency ever reported is also mentioned here. 2. Experiments The Yb3+,Na+:CaF2 single crystal used in our study was grown by the temperature gradient technique (TGT) in an Ar and PbF2 atmosphere. The 5×6×6-mm3 Na,Yb:CaF2 crystal (polished with parallel end faces, uncoated) was wrapped with indium foil and mounted in a water-cooled copper block, and the water temperature was maintained at 17ºC. The concentration of Na is 3.0-at.%, and the ratio of Na:Yb was 1.5:1. Before study of the mode-locking property of our Yb3+, Na+:CaF2 single crystal, we operated the laser in self-Q-switching once more to optimize its lasing performance. In this paper, we selected a fiber-coupled laser diode with a 200-μm fiber core diameter and a numerical aperture of 0.22, emitting at the wavelength range of 975−978-nm as our pump source. The self-Q-switching operation resonator was a stable three-mirror folded cavity similar with that used in [9], which was designed to permit TEM00 oscillation only by keeping the laser mode matching with the pump beam. With the output coupler of 3%, we obtained the maximal self-Q-switching output power of 495mW centered at 1051nm without any tuning device (Fig. 1, and the inset shows a single self-Q-switching pulse at a certain output power of 400mW). And the maximum slope was 30% near the maximum pump power, which implied that more excellent laser performance was feasible when enhancing the pump level. It turned out that the pump beam with radius of 100-μm in the gain medium matched more easily and #8218 - $15.00 USD
(C) 2005 OSA
Received 22 July 2005; revised 6 September 2005; accepted 20 September 2005
3 October 2005 / Vol. 13, No. 20 / OPTICS EXPRESS 7971
much better with the laser beam than that with small radius. Therefore, we adopted this diode for our further mode-locking investigation.
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Fig. 1. Dependence of the average output power on the absorbed pump power in self-Qswitched and mode-locked operation, respectively. The inset is a single self-Q-switched pulse at output power of 400mW.
With the same Yb3+,Na+:CaF2 single crystal, the passive mode-locking operation cavity consisted of two high reflector (at 1050 nm) mirrors, M1 and M2; one output coupler (OC) (T=3% at 1050 nm) giving a total output coupling of ~6% for two output beam; and a SESAM device, as shown in Fig. 2. The curvature radii of M2 and OC were 300 and 100-mm, respectively. Distances between each cavity mirror were designed for better mode matching with the pump beam and to provide the proper spot size of 40~60μm in diameter on the SESAM. The SESAM was mounted on a heat sink, but no active cooling was applied. Its saturation pulse energy was estimated to be about 60μJ/cm2. The modulation depth, nonsaturable losses, and absorption recovery time of the SESAM were 1.0%,
