Diode-pumped high-power sub-100 fs Kerr- lens mode-locked Yb:CaYAlO4 laser with 1.85 MW peak power

We demonstrated a diode-pumped high-power Kerr-lens mode-locked Yb:CaYAlO4 (Yb:CALYO) laser with a dual-confocal cavity, directly generating 59-fs pulses with 6.2 W average power, which is the highest average power from any sub-60 fs Yb-doped solid-state lasers. With the repetition rate of 50 MHz, the corresponding single pulse energy was 124 nJ and the peak power was 1.85 MW, which to the best of our knowledge is the highest peak power delivered directly from a sub-100 fs Yb-based bulk lasers ever. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

promising for reported in 20 Yb:CALYO l uncompressed emission spec pumped by si tunability of 4 from a KLM graphene in 2 achieved from low-brightnes power in abov level, which i Generally fs pulses is m the Kerr effec using multimo as well as a st one of the po pulse duratio method [26]. power of sing and Kerr med the Kerr-lens previous work laser based on which also ind Here we f at 50 MHz in investigated t couplers (OC) with 15% out 108 nJ and 1.2 pulses with u corresponding To the best o that has ever b  The schematic of the experimental setup is shown in Fig. 1. The pump laser is a multi-mode fiber coupled diode laser emitting at 976 nm with the maximum output power of 50 W, whose core diameter is 105 µm. In order to realize high-power Kerr-lens mode-locking, a Kerr medium at the Brewster's angle was inserted in the cavity which is similar with the setup in our previous work [8]. However, several improvements are implemented here: firstly, a 6-mm long c-cut 5 at. % doped Yb:CALYO crystal with anti-reflection coating was used to replace the original 2-mm long, 8 at.% doped, uncoated laser crystal. The pump absorption efficiency of this crystal without lasing is 91%. To move the heat accumulation and maintain a stable temperature, the gain crystal was wrapped in indium foil and mounted on heat sink kept at 13 °C. Secondly, we optimized the mode-matching between the pump and laser, so that no hard aperture was needed to realize stable KLM operation. Last but not least, different kinds of Kerr medium not only quartz but also CaF 2 were explored. In addition, three different output couplers (OCs) (2.5%, 5%, 15%) were used in our experiment, which was mounted on a translation stage to start up mode-locking. The dispersion compensation was achieved by using different combinations of several Gires-Tournois interferometer mirrors (GTIs) inside the cavity, with total group delay dispersion (GDD) varies from -1850 fs 2 to -3300 fs 2 . And no extra-cavity pulse compression was employed. DM was a dichroic mirror with high reflective for 1 μm laser and high transmittance for pump. The total length of the cavity was about 3 m which corresponds to a repetition rate of 50 MHz.

Results and discussion
We firstly characterized the output performances with different output couplers (OCs) using a 2-mm thick quartz as the Kerr medium. With placing the quartz at the Brewster's angle near the center between the M3 and M4, continuous-wave (CW) oscillation started by aligning the laser cavity. For different OCs of 2.5%, 5% and 15%, the maximum CW output powers were 3 W, 6 W and 7.8 W, respectively. The corresponding slope efficiencies were 11.1%, 17.6% and 22.9%, respectively. Then, after finely tuning the position of M4 and the quartz in the cavity, KLM operation was easily realized by fast moving the translation stage of the OC. For each OC, the pulse duration was optimized by fine tuning of the positions of the quartz to adjust the intra-cavity nonlinear phase shift and by managing the negative intra-cavity GDD.
The corresponding spectra as well as auto-correlation traces are shown in Figs. 2(a) and 2(b), which were measured by commercial optical spectrum analyzer (AvaSpec-ULS2048, Avantes) and intensity auto-correlator (APE PulseCheck USB), respectively. For 2.5% OC, the central wavelength at CW operation was at 1060 nm and shifted to 1041 nm once mode locked. With a total amount of −3300 fs 2 GDD introduced in the cavity by GTIs, 1.5-W KLM operation was realized. The spectral bandwidth of the spectrum was 11.8 nm which supported a Fourier-limited pulse duration of 96 fs. The FWHM bandwidth of the autocorrelation trace was about 153 fs, corresponding to 99-fs pulse duration if a sech 2 -pulse shape was assumed. This corresponds to a pulse energy of 30 nJ and a peak power of 270 kW. It is worth to notice that the mode-locking has a Q-switch envelope, which can also tell from the spikes on the spectrum and background on the auto-correlation trace. When using 5% OC, the central wavelength at CW operation was at 1054 nm and shifted to 1052 nm with bandwidth of 18.2 nm at the optimal KLM operation with −2900 fs 2 GDD from GTIs in the cavity. The corresponding auto-correlation trace indicates a pulse duration of 70 fs if a sech 2 pulse shape assumed. The average power in this case was 2.5 W, corresponding to the single pulse energy of 50 nJ and a peak power of 0.63 MW. For 15% OC, the optimal KLM operation was obtained with the GTIs providing −2600 fs 2 GDD. The central wavelength was 1047 nm with a FWHM bandwidth of 15.4 nm, which corresponding to 75-fs transform limited pulse duration. The corresponding auto-correlation trace had a duration at half maximum of 121 fs, results in a pulse duration of 79 fs assuming a sech 2 pulse shape, which is very close to the transform limited pulse duration. The average power in this case was up to 5.4 W, corresponding to the single pulse energy of 108 nJ and the peak power of 1.2 MW, respectively. For the mode-locking with 5% and 15% OCs, they were getting much more stable. As shown in Fig. 2(c), the pulse train of KLM operation with 5% OC was recorded with an oscilloscope with 500 MHz bandwidth, no evidences of Q-switch was observed and the auto-correlation trace in 50 ps time span also shows only one single peak, meaning no multi-pulses operation. Besides using quartz which has high nonlinear refractive index as Kerr medium, we also investigated the behavior of using a lower nonlinearity Kerr medium of CaF 2 with 2-mm thickness. For this case, the maximum CW output power was about 8 W with central wavelength of 1060 nm using 15% OC. By optimizing the position of the CaF 2 in the cavity and utilizing a suitable value of round-trip dispersion of −2900 fs 2 , as short as 59 fs pulses with as high as 6.2 W average power were directly generated from the oscillator. The modelocked optical spectrum was centered at 1047 nm with a bandwidth of 17 nm as shown in Fig.  3(a). Via Fourier transform with zero chirp, the transform-limited pulse duration is 50 fs. The corresponding intensity autocorrelation trace was shown in Fig. 3(b). The FWHM bandwidth of the autocorrelation trace was about 91 fs, corresponding to 59 fs pulse duration if a sech 2 pulse shape was assumed, which was close to the Fourier limited pulse duration. The nearfield beam profile of the KLM pulses as shown inserted in Fig. 3(a) was a little elliptical because of the astigmatism origin from the four folded concave mirrors. The beam radius along its major and minor axises were 0.63 mm and 0.61 mm, respectively.
To verify the stability of the mode-locking operation, we measured the radio frequency (RF) spectrum using a commercial RF spectrum analyzer (Agilent 4407B). The signal was recorded in a frequency window of 4 MHz with 1 kHz resolution bandwidth (RBW) and 0.6 GHz frequency span with 100 kHz RBW, respectively, as described in Figs. 4(a) and 4(b). The RF spectrum of the fundamental harmonic at 50 MHz had a signal-to-noise ratio of about 83 dBc. No obvious side peaks of the higher orders harmonics were observed, which indicates that the KLM operation was running stably without Q switch instability. The fluctuation of the harmonics peaks in Fig. 4(b) might be originated from the limited sampling rate of the RF spectrum analyzer. The results in this work were summarized in Fig. 5. As Kerr medium, CaF 2 has a better performance than quartz in both pulse duration and average output power during our experiment, which is due to the less nonlinear phase. The KLM operations were easily started and able to last hours once been realized. No evidences of saturation effects or thermal degradation were observed. Our results are compared with other Yb-doped lasers generating sub-100 fs pulses with >5 W output power with both bulk and thin-disk geometries in Fig. 6, which was previously only achieved from Yb:Lu 2 O 3 [28,29], Yb:CALGO [1,30] and Yb:LuScO 3 [31] crystals. It can be seen that there is no previous work reporting on generating pulses with >6 W average power as well as <60 fs pulse duration. In addition, for the region with >100 nJ pulse energy or >1 MW peak power with sub-100 fs pulse duration, there were only results based on Yb:CALGO bulk laser as well as Yb:Lu 2 O 3 thin-disk laser before. Although, the TDL has the advantage in thermal management, it has also more complex pumping system. In this work, we reported comparable results from the Yb:CALYO crystal, with up to 1.85-MW peak power, which shows the advantages in high-power sub-100 fs pulses generation of such Yb-doped calcium aluminate crystals. It will push people to develop and investigate new materials belongs to this family for better performances in ultra-fast, ultra-intense lasers.

Conclusion
In conclusion, we have studied on the output performances from a diode-pumped high power KLM Yb:CALYO laser with different Kerr-medium. When using a 2-mm thick quartz, the KLM Yb:CALYO laser directly produced 79 fs pulses with 5.4 W of average output power with 15% OC and 70 fs pulses with 2.5 W of average power with 5% OC. When using a CaF 2 crystal as the Kerr medium, as short as 59 fs pulses with as high as high as 6.2 W average power were directly generated from the oscillator, which is the highest average power for the sub-60 fs Yb-doped solid-state lasers. The repetition rate of the KLM Yb:CALYO laser was 50 MHz, resulting in the maximum of 124-nJ single pulse energy and 1.85-MW peak power, which is also the highest peak power ever achieved from any sub-100 fs Yb-doped bulk oscillators. Taking into consideration of high nonlinear reflective index of Yb:CALYO crystal, diode-pumped Kerr-lens mode-locked oscillator with dual-crystal and dual-confocal cavity aiming higher output power with sub-100-fs pulse duration is under implement. Pulse energy (nJ)