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 17°C. 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 much better with the laser beam than that with small radius. Therefore, we adopted this diode for our further mode-locking investigation.

 

Fig. 1. Dependence of the average output power on the absorbed pump power in self-Q-switched and mode-locked operation, respectively. The inset is a single self-Q-switched pulse at output power of 400mW.

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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%, <0.2% and ∼20-picosecond, respectively.

 

Fig. 2. Configuration of passively mode-locked operation with a SESAM device.

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3. Results and discussions

The lasing threshold was 1.6W, and near lasing threshold the output was effectively self-Q-switching as we demonstrated above; slightly increasing the pump power to 1.7W, simultaneous mode-locked pulses under self-Q-switched envelope was emerged. When the pump power was increased further, the self-Q-switched envelope became regular both in the features of pulse duration and pulse repetition rate. Figure 1 illustrated the dependence of the average mode-locking output power on the absorbed pump power. At the maximum absorbed pump power of 3.34W, total average output power of 360mW was achieved. The maximum slope of the power curve reached 27.2%. Fig. 3(a) and (b) show the regular sequence of the self-Q-switched pulses and a single Q-switched modulation pulse about 8μs sampling from the train at absorbed power of 3W, respectively. The pulse-to-pulse amplitude fluctuation of the self-Q-switched pulse train is found to be less than ±5%. As shown in Fig. 3(c), simultaneous mode-locked pulse trains inside the self-Q-switched pulse induced by Na,Yb:CaF₂ crystal are achieved with a repetition rate of ∼183MHz. The measured autocorrelation trace is shown in Fig. 4(a). The full width at half maximum (FWHM) of the autocorrelation trace is about 1.4-ps, assuming a Gaussian pulse profile, and the pulse width of mode-locked pulses is then estimated to be 1-ps. The narrow pulse width should be attributed to the broad gain bandwidth of the Yb³⁺, Na⁺:CaF₂ crystal. Also, it can be calculated that the peak power of a single pulse near the maximum of the self-Q-switched envelope reached 27kW approximately. To our knowledge, this is the first demonstration of the passively mode-locked operating for the Na,Yb:CaF₂ crystal laser.

 

Fig. 3. (a) Self-Q-switched pulse train of a mode-locked Yb³⁺, Na⁺:CaF₂ laser, (b) a single self-Q-switched pulse sampling from the train and (c) a pulse train of mode-locked pulses under the self-Q-switched envelope.

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The Na,Yb:CaF₂ crystal exhibits self-Q-switching characteristic, other than cw laser characteristic. Therefore, Su et al have put forward a tentative assumption to explain the mechanism for the self-Q-switching operating of the Yb³⁺, Na⁺-codoped CaF₂ crystal [9]. From the point of view of absorption (as shown in Fig. 4 in Ref. 9), we construed the self-Q-switching phenomenon as the well-known ${{\mathrm{F}}_2}^{-}$ centers (pairs of anion vacancies with three electrons) [10], which are effective passive Q-switcher in irradiated LiF crystals [11–13]. The ${{\mathrm{F}}_2}^{-}$ centers could be formed in Yb³⁺, Na⁺:CaF₂ crystal during growth process, owing to the excessive electrons from Yb³⁺ ions substituting Ca²⁺. The absorption band of ${{\mathrm{F}}_2}^{-}$ centers could overlap with that of Yb³⁺ in Yb³⁺, Na⁺:CaF₂. An additional absorption band peaking at 1066nm was observed, which probably be attributed to the ${{\mathrm{F}}_2}^{-}$ centers, might be a convincing proof of this explanation. The explanation of self-Q switching mechanism was just qualitative, and more accurate could be obtained by resolving the population equation.

There was an interesting new phenomenon to note that, unlike traditional Q-switched mode-locking where the Q-switched envelope was induced by the incomplete saturation of saturable absorber [14], here the Q-switching envelope was due to the self-Q-switching ability of the Yb³⁺, Na⁺:CaF₂ crystal. This could be confirmed simply by replacing the SESAM with a high reflector mirror. We found that the repetition rate of the self-Q-switched pulses coincided with before of ∼5-kHz, as shown in Fig. 3(a). Furthermore, Q-switched pulses resulted from SESAM typically repeated faster. In a Q-switched laser, it was convinced that the pulse duration generally decreased with shorter cavities and with higher pump power (increased small-signal gain). And that was why in this paper the self-Q-switched pulse duration was longer than that mentioned in [9] on the similar pump level. On the other hand, the laser in our experiment was in self-Q-switching operation, then the power density in the laser cavity was enhanced times than in the case of cw laser operation. Consequently, the SESAM was more easily to be saturated strongly at lower average laser power than common laser crystals.

 

Fig. 4. Autocorrelation of 1-ps pulses at 300mW output power is shown in the left inset. The dots indicate the experiment data and the solid line indicates the Gaussian fit data. The right inset shows the corresponding optical spectrum.

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To investigate the quality of the mode-locked pulses further, we also measured the laser spectrum. The corresponding optical spectrum measured using an optical spectrum analyzer (InSpectrum, Acton) is shown in Fig. 4(b). Under a scanning resolution of 0.02 nm, the bandwidth (FWHM) was measured to be about 1.8nm corresponding to Δv=493GHz, centered at 1047nm. Then the time-bandwidth product is about 0.493, suggesting that the output pulse is unchirped.

4. Conclusions

We have reported on the passively mode-locked laser performance of the Yb³⁺, Na⁺:CaF₂ crystal for what is to our knowledge the first time. Transform-limited 1-ps passively mode-locked pulses, with the pulse peak power of 27kW corresponding to merely 180mW average output power of single beam, were obtained under the self-Q-switched envelope induced by the laser medium. Experiment results convince that the novel crystal is a promising gain medium in achieving ultra-short pulses and high pulse peak power. Currently we are working on the coated, different doping concentration crystals, the improved SESAM and optimized output couplers, and it is believed that more considerable interest for laser applications will be made.