We demonstrate a high peak power mode-locked Yb:CaF2 oscillator pumped by a single-mode laser diode. The laser operated in hybrid Kerr-lens and SESAM mode-locked regime. Its performance was optimized by varying the output coupler ratio. Pulses as short as 65 fs were generated with 0.4% transmission. Employing 5% output coupler enabled generation of 77 fs pulses with 46 kW of peak power (262 mW of average output power). We believe that such high peak powers can open a way to practical applications of single-mode diode-pumped ultrafast ytterbium lasers.
© 2017 Optical Society of America
Since the advent of femtosecond lasers based on Yb-ion-doped active media in late-nineties such sources have been actively developed [1–10]. Mode-locked oscillators operating at 1 µm are indispensable for several industrial as well as scientific applications, which demand high peak power in this spectral range [2,3]. While the most common setup of a mode-locked laser is based on multimode (MM) diode pumping with semiconductor saturable absorber mirror (SESAM ) as the mode locking mechanism, other laser configurations can present certain benefits. For example, although MM diode pumping can offer a route to high output powers, another interesting alternative is single-mode (SM) pumping which has several advantages such as better mode-matching for higher efficiency, lower thermal effects in the gain medium to simplify the setup and increase long term stability, and shorter pulse durations that benefit from lower intracavity nonlinearities. However, the advantages of low pumping power also translate into the main problem of the SM diode-pumped lasers – the output average and peak powers are typically limited to below 50 mW and 5 kW [4–10], respectively, which severely limits their practical use in real-world applications. Therefore, development of high peak power femtosecond lasers with simple configuration design is very attractive for many fields.
In this work we demonstrate a solution of this dilemma by turning to a combination of Kerr-lens (KLM) and SESAM mode locking techniques as well as employing a higher power SM pump source with a popular Yb:CaF2 gain medium. This crystal is very promising in terms of ultrashort pulse generation  and has been previously investigated in solid-state lasers (including thin-disk geometry) pumped by MM [12–14] as well as SM diodes . At the same time, the combined KLM and SESAM mode locking approach is gaining popularity owing to its ability to produce short pulses with high output average and peak powers [15–22]. Since KLM is a truly ultrafast absorber, in the mode-locked regime it can typically produce shorter pulses . Moreover, it discriminates against the continuous wave (CW) background radiation more effectively than SESAM thus allowing high power operation. On the other hand, SESAM facilitates a start of mode locking as well as operation away from the stability edge which also leads to higher output power. Thus, when used together, one can get high peak power ultrashort pulses.
In this paper we report on a high peak power Kerr-lens and SESAM mode-locked Yb:CaF2 oscillator pumped by a 1 W SM laser diode. The performance of the laser was optimized in terms of pulse duration, average and peak power. This enabled generation of 77 fs pulses with 46 kW of peak power and 262 mW of average output power. This is a remarkable improvement regarding the previously demonstrated SM diode-pumped Yb-ion-based solid-state lasers which enables implementation of such devices in various practical applications. The laser also produced pulses as short as 65 fs with a lower peak power of 7.3 kW.
2. Experimental setup
Figure 1 presents a scheme of the employed Z-shaped cavity. As a gain medium we used an antireflection coated 4-mm-long flat/flat Yb:CaF2 crystal with 5 at.% doping level. It was longitudinally pumped by a fiber-coupled SM laser diode delivering up to 1080 mW of polarized radiation at 980 nm (3SP Group, 2000CHP). The diffraction-limited pumping allowed for high intensity within the crystal which is crucial for Kerr-lensing and generation of broadband spectrum. The radius of the pump beam at the focal plane was measured to be 19 μm. The length of the cavity was approximately 230 cm which corresponds to a repetition rate of 65 MHz. One arm of the cavity was terminated with an output coupler (OC) mirror. The performance of the oscillator was optimized by investigating OC mirrors with various transmission ranging from 0.4% to 10%. The anomalous group delay dispersion (GDD) necessary for balancing the effect of material dispersion and induced self-phase modulation was implemented using three GTI mirrors with total GDD = −1400 fs2 (Layertec).
Unlike the double tungstates , the Yb:CaF2 crystal exhibits relatively low nonlinear refractive index n2 ≈1.9∙10−16 cm2/W , which is not beneficial for KLM operation. However this can be offset by using expensive high brightness fiber laser pumping . Therefore, employing a low power SM diode pumping is much more challenging. In our experiment it was possible by tight focusing of the pump within the active medium. The cavity mode waist was estimated to be 24 μm x 25 μm (calculated with ABCD formalism). Moreover, in order to facilitate the start of the mode-locked operation we employed a SESAM (BATOP), which was mounted on a translation stage. By changing the position of the SESAM we can precisely tune the size of the cavity mode. The modulation depth, non-saturable losses, saturation fluence and relaxation time of the saturable absorber amounted to 2.6%, 1.4%, 50 μJ/cm2 and 500 fs, respectively. The beam size on the SESAM that was used as one of the cavity end mirrors (Fig. 1) was calculated to be 51 μm x 54 μm, enabling for strong absorber saturation. No active cooling of the SESAM or gain medium was necessary.
Initially, we have tried to obtain pure SESAM mode locking. Several SESAMs with different modulation depths (0.3%, 1.2% and 2.6%) and focusing conditions (mode size on the absorber down to 23 μm x 25 μm) to lower the mode locking threshold were tried. Unfortunately, owing to the relatively long fluorescence time (τf = 2.4 ms) of Yb:CaF2 , the mode-locked regime could not be obtained due to the SESAM damage caused by the appearance of the Q-switched mode-locked pulses . Alternatively, we have also tried to achieve pure KLM with SESAM replaced with a highly reflective (HR) flat mirror. We have exploited a soft-aperture KLM where the size of the cavity mode within the gain medium had to be larger than the pump beam. The needed mode size was achieved by translating the HR mirror with respect to the focusing mirror M3. It should be noted that pure KLM operation is challenging to obtain owing to the low power pumping, low nonlinear refractive index of Yb:CaF2 and its long upper state lifetime. Indeed, in fiber laser pumped KLM Yb:CaF2 oscillator the mode locking threshold amounted to 7 W . In our case obtaining KLM pulsing was feasible only with maximum pumping power. However, it was very difficult to obtain and the stability of the mode-locked operation was poor. Therefore, the laser was operated in hybrid Kerr-lens and SESAM mode-locked regime. The use of the SESAM in the KLM cavity greatly facilitated the start and the stabilization of the mode locking. Thus, both KLM and SESAM contributed to the generation of stable ultrashort pulses in our setup. The best performance was observed with SESAM that had a modulation depth of 2.6%. SESAMs with lower modulation depths of 0.3% and 1.2% also resulted in mode-locked pulse generation, albeit with much lower stability and poor performance in terms of output power and pulse duration. Moreover, for higher pumping levels we observed an occurrence of a parasitic CW spectral component. Therefore, all of the following results were obtained with 2.6% modulation depth SESAM. The optimal value of the introduced round-trip negative GDD in terms of the pulse duration was experimentally found to be −1400 fs2. The pulsed operation was not self-starting and it was always initiated by mechanical perturbation of one of the end mirrors, which is typical for ultrafast KLM lasers. It is worth noting that Q-switched mode locking regime and damage of the SESAM was not observed in the Kerr-lens and SESAM mode-locked oscillator because of the relaxed cavity beam focusing requirements on the SESAM and effective suppression of the CW background radiation growth by the soft-aperture Kerr-lensing.
We have investigated the pulsed performance of the laser with various OC ratios ranging from 0.4% up to 10%. Figure 2 demonstrates the optical spectra and the autocorrelation traces with corresponding sech2 fit profiles obtained for the 0.4%, 5% and 7.5% OCs. As expected from the gain in quasi-three-level systems , lowering the level of output coupling was followed by the central wavelength redshift and the spectrum broadening. Consequently, the shortest pulse duration of 65 fs was obtained for the lowest OC (0.4%) which corresponds to the lowest and fairly flat gain . As expected, the average output power was the lowest in this case (35 mW) and it grew with increased OC transmission up to the maximum of 291 mW (OC = 7.5%, τ = 100 fs). Further increase of OC transmission up to 10% caused deterioration of the output power. Therefore, we did not investigate higher OC transmission values. The highest peak power of 46 kW was obtained for OC = 5%. In this case the pulse duration amounted to 77 fs, which corresponded to the 15.3 nm broad spectrum with the central wavelength of 1048.4 nm (Pav = 262 mW). The weak spectral component visible in the red part of the spectrum around 1085 nm can be attributed to a dispersive wave generation . This originates from a non-perfect compensation of the GDD across the entire spectrum and usually has a character of the CW background radiation . Reduction of OC transmission induced growth of intracavity peak power, which was responsible for the gradual amplification of dispersive wave component (see Fig. 2). In all of the cases the pulses were nearly Fourier-transform-limited with the time-bandwidth product approaching 0.32. Once the mode locking was initiated by mechanical perturbation of the SESAM mirror mount it was always stable up to the maximum available pump power level. No degradation of the pulse train quality, e.g. multi-pulsing, was observed. The output performance of the oscillator for all investigated OC mirrors is presented in Table 1.
The peak power obtained for 5% OC is unprecedented for such a laser architecture. Therefore we further investigated the behavior of the oscillator in this case. Figure 3(a) depicts the average laser output power vs pump power dependence.
The slope efficiency for the mode-locked operation was 26%. When the pump power was reduced from its maximum, the output power linearly decreased until the laser turned back to the CW regime (at Pth ≈750 mW). Since the SESAM in CW regime is not saturated, this was accompanied by an abrupt drop of the output power and a change of the slope efficiency to 22%. Figure 3(b) presents the fundamental RF beat note at 65.03 MHz with a 72 dB signal-to-noise ratio as well as wide-span RF spectrum for the maximum pump power demonstrating the stability of the laser operation. In the mode-locked regime the pulse duration decreased with the growth of the pump power owing to the increased contribution of the Kerr-lensing and self-phase modulation which is necessary to ensure broadband ultrashort pulse generation . As was mentioned before, the mode locking was stable up to the maximum available pump power. Consequently, we expect that by employing a more powerful, diffraction-limited source we will be able to further shorten the pulse duration as well as increase the average and peak powers. Unfortunately, the used pump module is currently the most powerful SM fiber coupled laser diode that is commercially available. Therefore, further power scaling would require pumping of the crystal from both sides, the use of a polarization-coupled source or a high brightness fiber laser .
Figure 4 presents a comparison of the performance of previously reported femtosecond Yb-doped bulk oscillators pumped by the SM laser diodes in terms of peak power, average output power and pulse duration [5–10,15,16,27]. Our laser exhibits almost two-fold increase in peak power (for OC = 5%) with regard to the previously demonstrated best result of 24.5 kW with Yb:CALGO . Interestingly, this work (as well as ) also used the combined action of KLM and SESAM as the mode locking mechanism. At the same time, when comparing to the SESAM mode-locked lasers we have achieved almost an order of magnitude higher peak power [5–10].
We have demonstrated a passively mode-locked high peak power Yb:CaF2 oscillator operating in the sub-80 fs regime. The employed diffraction-limited SM diode pumping enabled high intensity within the gain medium and enhanced Kerr-lensing effect. This allowed us to develop and present the first Kerr-lens and SESAM mode-locked Yb:CaF2 oscillator with such a pumping scheme. The shortest produced pulses of 65 fs were obtained with 0.4% OC value and 35 mW of average output power. Optimization of the OC transmission led to the generation of the most powerful ultrashort pulses. When 5% OC mirror was employed the laser generated 77 fs pulses with 262 mW of average output power. This corresponded to 46 kW of peak power – an exceptional value concerning the Yb-ion-based bulk lasers pumped by the SM laser diodes. The oscillator was stable in the whole range of pumping power owing to the combined action of Kerr-lens and SESAM mode locking and its performance was limited only by the available pump power. Consequently, we believe that further optimization of the design parameters can lead to the generation of even shorter and more powerful pulses from such a simple laser setup.
National Science Centre (NCN, Poland) (UMO-2015/18/E/ST7/00296); Natural Sciences and Engineering Research Council (NSERC, Canada) (Discovery grant).
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