Behaviors of multiple pulsing in high power saturable-absorber mode-locked Yb:KGW laser

Femtosecond solid-state lasers (SSLs) based on various laser gain crystals, such as Ti:sapphire, Nd:YAG, Cr:forsterite, and Tm:glass, produce ultrashort pulses over broad wavelength ranges, leading to wide applications in the fields of material processing, optical communication, and ultrafast nonlinear optics [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Since the passive mode-locking operation was demonstrated in the lasers based on Yb:YAG [3], Yb:glass [11], and Yb:KGW [8], femtosecond Yb-doped lasers operating near 1030 nm are gaining growing interest. In comparison to other femtosecond lasers, Yb-doped lasers have advantages in the direct diode laser pumping and the high pump energy conversion efficiency. Femtosecond pulses as short as 5.4 fs have been generated from a Ti:sapphire laser, and in the case of Yb-doped gain materials, a 17.8 fs Yb:CALGO laser with an average power of 26 mW has been reported [12], [13]. With regard to high power, Yb:KGW and Yb:CALGO lasers with around 10 W output power have been shown to deliver relatively long pulses [14], [15], [16], [17].

In mode-locked laser applications, high pulse energy with a narrow pulse duration is desirable for practical use. For a soliton-like pulse formation mode locked with a slow or fast saturable-absorber (SA), the pulse duration is known to be inversely proportional to the pulse energy [18], [19], [20], [21], [22]. Thus, power scaling leads to a pulse shortening up to a certain level, over which a single pulse tends to break into double or multiple pulses with deteriorating positive effects [23], [24], [25]. This multiple pulsing phenomenon according to changes in the group-velocity dispersion (GVD) as well as to variations of the small-signal gain has been observed in both fiber-type and solid-state femtosecond lasers [23], [26]. Moreover, the conditions for pulse breakup, which depend on the laser type and mode-locking method, have been numerically analyzed with model calculations [18], [24], [27]. Because output power scaling with pump power inevitably leads to pulse splitting beyond a critical point, the behavior and condition of multiple pulsing is important and must be considered especially in high power femtosecond oscillators.

In this paper, we systematically characterize the multiple pulsing of a high power SA mode-locked Yb:KGW laser pumped by a diode laser at 981 nm. In our laser configuration, the single pulse operation is maintained up to an output power of 6.3 W with a pulse duration of 140 fs. As the pump power is further increased, though the average power scales linearly, the pulse sequentially breaks into two and three pulses per round trip. We find that the systematic change of the pulse duration in inverse proportion to the pulse energy, a clear signature of a soliton pulse, is extendable to the multiple pulse regime. This characteristic feature proves the validity of the soliton-like pulse formation and splitting in 10 W level high power mode-locked lasers.

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