Abstract
1 Introduction
High-power ultrafast lasers at a central wavelength of near 1
The Yb:CALGO femtosecond oscillator was first reported in 2006, generating 47 fs pulses with 38 mW average output power[11]. Since then, there have been many reports about sub-100 fs Yb:CALGO lasers, although the output power is limited to several watts or even tens of milliwatts[12–16]. Based on Yb:CALGO crystal, the average output power of 3.5 W was generated with a pulse duration of 60 fs[17] and pulses as short as 37 fs with average output power of 0.9 W were demonstrated[18]. Furthermore, the shortest pulse duration down to 32 fs with 0.9 W has been recently demonstrated from an Yb:CALGO laser[19]. Recently, a record pulse duration of sub-50 fs with 1.7 MW of peak power was achieved from a KLM Yb:CALGO oscillator[15]. Tian et al. demonstrated a high-power KLM Yb:CaYAlO4 (Yb:CALYO) laser generating 59 fs pulses with 6.2 W average power[20]. Furthermore, the Yb:CALGO crystal has great potential for the development of thin-disk lasers. For example, Ricaud et al. demonstrated an average output power of 28 W from pulses with a duration of 300 fs and a pulse energy of 1.3
In this paper, we report on the SESAM mode-locked Yb:CALGO oscillator pumped by the diode laser. The self-starting mode-locked laser pulses as short as 247 fs have been measured with 140 nJ pulse energy at the repetition rate of 71.66 MHz, corresponding to a 10 W average power and 0.56 MW peak power, respectively.
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2 Experimental setup
The schematic experimental setup of the SESAM mode-locked Yb:CALGO laser is shown in Figure 1. In our experiment, the fiber coupled LD emitted at 976 nm is used as the pump source, which delivers the maximum power of 50 W. The fiber has a core diameter of 105
Figure 1.Schematic diagram of the experimental setup. DM, dichroic mirror; GTI1, GTI2, Gires–Tournois interferometer mirrors; LD, fiber-coupled laser diode; M1–M3, concave mirrors; M4, plane mirror; OC, output coupler; SESAM, semiconductor saturable absorber mirror.
3 Results and discussion
In preliminary experiments, a CW laser was obtained using an HR mirror as the end mirror instead of the SESAM. At the pump power of 34 W, the maximum output power in the CW regime is 15 W, corresponding to the optical-to-optical efficiency of 44.1%. As the HR mirror is replaced by the SESAM, the laser operates on Q-switched mode-locking condition because of the large spot size on the SESAM. In addition, this is also related to the 420
However, Meng et al.[23] have reported the Yb:CALGO laser delivering 93 fs pulses with maximum average output power of 4.5 W. Then, we obtained promising results with the generation of 156 fs pulses at 7 W. To further scale the output power and shorten pulse duration, the HR mirrors with low dispersion and higher-quality SESAM were employed. The spot sizes on SESAM were increased to 340
Figure 2.With a 10% transmittance of OC, average output power versus pump power for the Yb:CALGO laser at 1041 nm.
Figure 3.The output power stability of the Yb:CALGO oscillator in 3 h.
Figure 4.The output laser spectrum of Yb:CALGO femtosecond oscillator.
Figure 5.Autocorrelation trace of center wavelength of 1041 nm.
The typical pulse train of mode-locking in the time scale of 2 ns, 10 ns, and 2
Figure 6.The typical pulse train of mode-locking in different timescales: (a) 2 ns, (b) 10 ns, and (c) 2 s.
Figure 7.Typical RF spectrum of the Yb:CALGO laser with the RBW of 1 kHz. Inset: RF spectrum at 1 GHz wide-span with the RBW of 300 kHz.
The beam quality is a crucial parameter to evaluate the laser performance. In this work, the Yb:CALGO mode-locked oscillator with output power of 10 W exhibits a high beam quality factor, and M2 is measured to be 1.017 and 1.016 in the horizontal and vertical directions by a commercial M2 factor meter (Spiricon M2-200s), as shown in Figure 8.
Figure 8.The beam spatial profile of the Yb:CALGO laser at the output power of 10 W.
4 Conclusions
In conclusion, we have presented a passively mode-locked Yb:CALGO oscillator that delivers 247 fs pulse duration and 140 nJ pulse energy at the repetition rate of 71.66 MHz, corresponding to an average power 10 W and a peak power 0.56 MW. Reliable and self-starting soliton mode-locking is achieved using a SESAM. The actual time-bandwidth product of 0.342 was close to the Fourier transform limitation for a sech2 pulse shape (0.315). In particular, the beam quality factor M2 is better than 1.02. Moreover, we also obtain more than 15 W of CW power with high optical-to-optical efficiency of 44.1%.
In our experiments, power scaling in the Yb:CALGO laser is limited by a range of parameters such as the damage threshold of SESAM as well as available crystal sizes and doping concentration. Higher power and shorter pulses were expected to be obtained by increasing the spot size on the SESAM, replacing the output mirror with different transmittance, and accurately compensating for the dispersion in the resonator. We foresee that the combination of the excellent optical properties of Yb:CALGO and the use of a high-power LD pumping scheme will make mode-locked Yb:CALGO lasers very attractive in various applications, such as seeding solid-state amplifiers, the pumping of optical parametric oscillators, multi-photon imaging, and terahertz applications.
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