Abstract
1 Introduction
All-solid-state femtosecond lasers that deliver high peak power and extremely narrow pulse width are of great interest in many fields such as scientific research, industry, medical treatment, and high speed communication. In particular, ytterbium-based solid-state lasers have attracted increasing attention because of their excellent optical and thermal properties. Recently, a great number of Yb-doped materials including Yb:CaF2[
In this letter, we demonstrated the single-mode fiber-laser-pumped high-power Yb:CALYO lasers passively mode-locked by semiconductor saturable absorption mirror (SESAM) and pure Kerr-lens mode-locking, respectively. 103-fs pulses with up to 3.1-W average power are obtained from passive mode-locking, corresponding to the optical-to-optical efficiency of 31%. In addition, as short as 36-fs pulses with up to 2-W average power are obtained from the pure KLM with extra-cavity dispersion compensation. With the repetition rate of 84 MHz, the pulse energies and peak powers for SESAM-based passive mode-locking and KLM are 37 nJ, 0.31 MW and 24 nJ, 0.58 MW, respectively.
2 Experimental setup
A schematic diagram of the experimental setup is shown in Figure
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3 Results and discussion
At first, the cavity was optimized in a continuous-wave (CW) operation, which delivers 4.2 or 3.6-W output powers with 15% or 10% OCs. The absorption efficiency of the crystal for pump laser is up to 99.5% under lasing. Then, to investigate the performance of a passive mode-locking operation, a commercial SESAM (SAM-1040-0.7-1ps-x from BATOP GmbH) with 0.4% modulation depth at 1040 nm was utilized. With 10% transmittance OC, a stable mode-locking operation with up to 3.1-W average power was achieved when the total group delay dispersion (GDD) per round trip introduced by the three GTIs was
For pure Kerr-lens mode-locking operation, the SESAM was replaced by a fourth GTI mirror. When the dispersion provided by GTI4 per bounce was
For a given position of GTI4 (
To reduce the residual chirp of the femtosecond pulses, extra-cavity compression was implemented by using a pair of GTI mirrors with
To claim the status of the KLM operation, the radio frequency (RF) spectra were measured via a photodetector (PD) with bandwidth of 1 GHz and a commercial RF spectrum analyzer (Agilent E4407B). As described in Figure
4 Conclusion
In conclusion, regarding the excellent optical property of the Yb:CALYO crystal, we have studied the KLM Yb:CALYO laser pumped by a single-mode fiber laser. Using an output coupler of 15% transmittance, different states have been realized by detuning the cavity. As the cavity was getting closer to the edge of stable region, the Kerr strength increased so that the modulation depth was rising accordingly. As a result, broader spectra as well as shorter pulses were obtained. The broadest spectrum was able to cover the wavelength from 980 to 1090 nm, which supports 34-fs pulses with zero chirp. By extra-cavity chirp compensation, near transform-limited pulses of 36 fs have been demonstrated. The average power after compression was 2 W, meaning that the single pulse energy and peak power were up to 24 nJ and 0.67 MW, respectively. An overview of the state-of-the-art performance of ultrafast Yb solid-state oscillators generating sub-40-fs pulses is given in Table
Crystal | Geometry | Energy | Reference | ||
---|---|---|---|---|---|
Yb:CALGO | Bulk | 40 fs | 15 mW | — | [ |
Yb:CALGO | Bulk | 37 fs | 1.5 W | [ | |
Yb:CALGO | Bulk | 32 fs | 90 mW | [ | |
Yb:CALGO | Bulk | 39 fs | 800 mW | 19 nJ | [ |
Yb:CALGO | TDL | 30 fs | 150 mW | 1.2 nJ | [ |
Yb:CALYO | Bulk | 33 fs | 36 mW | 0.3 nJ | [ |
Yb:CALYO | Bulk | 30 fs | 26 mW | [ | |
Yb:YAG | Bulk | 35 fs | 107 mW | [ | |
Yb:YCOB | Bulk | 35 fs | 36 mW | [ | |
Yb:Lu2O3 | TDL | 35 fs | 1.6 W | 30 nJ | [ |
Yb:CALYO | Bulk | 36 fs | 2 W | 24 nJ | This work |
Table 1. Overview of the state-of-the-art performance of ultrafast Yb solid-state oscillators generating sub-40-fs pulses.
Compared to the previous works in sub-40-fs generation from Yb solid-state lasers, we have presented, to the best of our knowledge, the highest average power as well as the largest single pulse energy within bulk geometry. Since no saturation in average power and multi-pulse operation was observed, sub-50-fs pulses with higher average power are believed to produce from such a single-mode fiber-laser-pumped Yb:CALYO laser with higher pump power or longer crystal. It is noticed that the shorter wavelength (980 nm–1040 nm) of the spectrum has relatively lower intensity due to the re-absorption. Another possibility is using a gain-matched output coupler[
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