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We report on the first demonstration of a passively (SESAM) mode-locked Yb:CaF2 thin-disk laser operating at a repetition rate of 35 MHz with close to diffraction-limited beam quality (M2 ≈1.1) at an average output power of up to 6.6 W. The optical efficiency was 15.3%. Nearly transform limited pulses with a duration of 445 fs and a spectral width of 2.6 nm at full width half maximum (FWHM) were obtained at the maximum output power. This corresponds to a pulse-energy of approximately 0.19 μJ and a peak-power of 0.4 MW.
© 2014 Optical Society of America
1. Introduction
The interest in ultra-short pulses for many applications has caused a fast development of pulsed laser sources in the last decade. Especially for material processing, high average powers are needed to manufacture parts within reasonable production time. The thin-disk laser (TDL) concept is well-known for its good power-scaling capabilities [1]. Today, the most established laser crystals used for TDL are Yb:YAG and Yb:LuAG. Yb:YAG has shown excellent performances in continuous-wave (cw) operation with up to 4 kW of output power at close to diffraction limited (M2 < 1.4) beam quality from a single disk [2]. Soliton mode-locking using a saturable absorber mirror (SESAM) is the most common method for obtaining ultra-short pulses with high average powers in TDL. Up to 275 W of average power with pulse-durations of 583 fs [3] and up to 80 μJ of pulse energy with pulse-durations of 1.07 ps [4] have been demonstrated with SESAM mode-locked Yb:YAG TDL oscillators at low air pressure (0.5 - 1 mbar) in order to limit nonlinear effects. Yb:YAG has also been used recently in a thin-disk multipass-amplifier providing an average power of up to 1.1 kW with 7.3-picosecond pulses [5].
However, the shortening of the pulse durations using Yb:YAG is limited by its gain-bandwidth and typically results in pulses longer than 500 fs as obtained with SESAM mode-locking. So far, only Kerr-lens mode-locking (KLM) has provided much shorter pulse-durations, using this material. Pulses with a duration of 330 fs were demonstrated at an average output power of 230 W in a KLM Yb:YAG thin-disk laser [6]. On the other hand, other Yb-doped laser crystals have been reported [7–10] to be very suitable for the generation of sub-500-femtosecond pulses by SESAM mode-locking thanks to their favorable spectroscopic properties. These are for example Yb:ScSiO5 (Yb:SSO), Yb:CaGdAlO4 (Yb:CALGO), Yb:Lu2O3 and Yb:LuScO3. With Yb:SSO, 27.8 W of average power with a pulse duration of 298 fs were obtained from a passively mode-locked thin-disk oscillator [7]. With Yb:CALGO slightly higher pulse energies of 1.3 μJ were reached at about the same average power and pulse duration [8]. At a longer pulse duration of 583 fs an Yb: Lu2O3 TDL with 63 W of average power and 0.8 μJ of pulse energy was demonstrated [9]. With the mixed sesquioxide material Yb:LuScO3 an average power of 23 W at a pulse duration of 235 fs has been reported [10].
Another promising crystal for the generation of high-power and high-energy ultra-short pulses is Yb:CaF2. This quasi-three level material combines a broad emission bandwidth with a suitable thermal conductivity of about 9.7 W/(m∙K) (undoped) [11]. The crystal is a well-known material which can be produced in large sizes as required for power scaling. A good overview of the spectroscopic and thermal properties of Yb:CaF2, as well as a track of the crystal’s development for laser applications can be found in [12]. Research efforts during the last years led to promising mode-locking results with several bulk material configurations [13–15]. Pulses as short as 48 fs have been obtained with an average output power of 2.7 W using Kerr-lens mode-locking [15]. To obtain this result, a 12 W fundamental mode fiber laser emitting at 979 nm was used for longitudinal pumping. In contrast to thin disk oscillators, power scaling possibilities are limited with this approach due to the longitudinal pumping and the rather complex pump source. A first demonstration of a continuous-wave Yb:CaF2 TDL in multimode and fundamental mode with average powers of 250 W and 13 W, respectively, was published in [16]. In the following we report on the continuation of this development and present the first realization of a passively mode-locked Yb:CaF2 TDL.
2. Resonator design to prevent Q-switching instabilities
The design of such a laser is challenging due to the material’s low emission cross-section, which makes Q-switching instabilities more likely to occur in mode-locked operation [17]. To avoid Q-switched mode-locking (QML), the intra-cavity pulse energy needs to be above the critical pulse energy [17]
This threshold depends on the gain saturation-fluence Fsat,L of the laser set-up and the saturation fluence of the saturable absorber Fsat,A, as well as on the area of the laser mode in the gain medium Aeff,L and on the SESAM Aeff,A. The critical pulse energy increases with higher modulation depth ΔR of the SESAM. Equation (1) is a good approximation for the behavior of SESAM-mode-locked lasers in the picosecond regime. For lasers with sub-picosecond pulse durations, exploiting soliton pulse shaping effects, the stability range can be shifted to significantly lower pulse energies [17]. The gain saturation-fluence of the laser set-up (depending on both, the resonator and the intrinsic saturation fluence of the laser crystal) is defined as
where (h ∙ ν) is the photon energy, m is the number of passes through the gain medium per roundtrip and σL,em, σL,abs are the emission and absorption cross-sections of the (three-level) laser medium at the laser wavelength. For a four-level laser medium there is no re-absorption at the laser wavelength, therefore σL,abs in (2) would be zero for that case. As can be concluded from Eqs. (1) and (2), high tendency for Q-switching can be expected for laser active media with low σL,em (and σL,abs). The emission (and absorption) cross-sections of some materials recently used for mode-locked TDLs as well as for the Yb:CaF2 are given as an orientation in Table 1.
Table 1. Emission and Absorption Cross-sections [10−20 cm2]
Due to the low emission cross-section of Yb:CaF2, the tendency towards Q-switching instabilities needs to be addressed with the resonator design. Generally, higher intra-cavity pulse energies help to prevent QML. Higher pulse energies can be achieved by operating at higher average power or lower repetition-rate. Smaller mode-size in the gain medium or on the SESAM, as well as more passes through the gain material per roundtrip also help to avoid Q-switching instabilities. The pulse energy and peak power need to be well below the damage threshold of the SESAM. In this work a resonator with eight passes through the Yb:CaF2 disk per roundtrip was used to reduce the risk of Q-switching instabilities. Furthermore, the associated increase of the cavity length reduces the repetition-rate and hence increases the pulse energy.
For the laser experiments a 250 µm thick, [111]-cut Yb:CaF2 disk with a wedge angle of 0.1° and with a nominal Yb-doping concentration of 4.5at.% was used. The disk has an anti-reflective (AR) coating for pump- and laser-wavelength on the front side and a highly-reflective (HR) coating for both wavelengths on the backside. The crystal was glued on a water-cooled copper heat sink which resulted in a radius of curvature (RoC) of 2.8 m in the horizontal (x-axis) and a RoC of 2.0 m in the vertical (y-axis) direction. Although the crystal disk cracked during previous cw-experiments it can still be used in the center region with a small pump spot diameter of approximately 1 mm.
The disk was pumped by means of a fiber-coupled laser diode with an output power of up to 100 W in a standard 24-passes pump module from the Institut für Strahlwerkzeuge (IFSW). The diameter of the homogeneous pump spot on the disk was 1 mm. To avoid damaging the disk further, the pump power was limited to 43 W, corresponding to a maximum pump power density of 5.5 kW/cm2 in this configuration. The central wavelength of the diode was adjusted to 980 nm (spectral width < 3 nm FWHM) for optimal pump absorption by controlling the diode’s temperature.
The resonator set-up for the mode-locking experiments is shown in Fig. 1.The total resonator length was 4.3 m which corresponds to a repetition rate of 35 MHz. The overall footprint of the cavity was 800 mm by 250 mm. The lengths of the paths numbered 1 to 10 in Fig. 1 are listed in Table 2.
Fig. 1 Schematic resonator set-up (not to scale), the used mirrors are a plane output coupler (OC) and plane Gires-Tournois-Interferometer type mirrors (GTIs); The HR-coated mirrors are concave with the radii of curvature (RoC) given in mm.
Table 2. Distances of Resonator Arms [mm]
The fundamental mode diameter (at 1/e2 of the Gaussian intensity distribution) at the location of the disk was calculated to be 0.7 mm and ensured single transverse mode operation. Soliton mode-locking was achieved using a standard SESAM (provided by BATOP GmbH) with the following parameters: Modulation depth ∆R of 0.6%, non-saturable losses of 0.4%, a saturation fluence of 70 µJ/cm2, a damage threshold of 3 mJ/cm2, and a relaxation time of 1 ps. The calculated mode diameter (again at 1/e2 of Gaussian intensity distribution) at the location of the SESAM was 0.6 mm. An uncoated 3 mm thick plate of fused silica (P1 in Fig. 1) was inserted into the cavity at Brewster’s angle to select linear polarization. Due to the cubic crystal structure of CaF2, there is no gain advantage for a particular state of polarization. Furthermore, a 6.35 mm thick AR coated plate of fused silica (P2 in Fig. 1), placed approximately 20 mm in front of the SESAM induced additional self-phase modulation (SPM) in mode-locked operation. The two GTIs account for a roundtrip group delay dispersion (GDD) of −4600 fs2.
3. Thin-disk laser in cw and mode-locked operation
The cavity was first characterized in cw-operation, using an HR end mirror instead of the SESAM. Up to 9.7 W of output power were extracted with an optical-to-optical efficiency of 22.6%. The same output coupler with a transmission of 5.2% was chosen for the subsequent mode-locking experiments.
Stable soliton mode-locking was observed from 5 W to 6.6 W of average output power. This corresponds to an optical efficiency of 15.3% (at 43 W of incident optical pump power). The mode-locking was started with a slight knock to the optical table and once running stayed stable for the rest of the experiment (a couple of hours). The autocorrelation trace of the output pulses and the corresponding spectrum at the maximum output power of 6.6 W are shown in Fig. 2(a) and Fig. 2(b), respectively. Pulse-durations τp of 445 fs and an emission spectral bandwidth Δλ of 2.6 nm (FWHM) at the central wavelength of 1033.4 nm were measured. The time-bandwidth product of 0.323 exceeds the theoretical transform-limited value for sech2 soliton mode-locked pulses by less than 3%.
Fig. 2 (a) Intensity autocorrelation trace, measured at 6.6 W; (b) Optical spectrum of the laser pulses at the same output power.
Single soliton pulse operation was confirmed using a fast photodiode together with sampling oscilloscope and a wide-range autocorrelation scan of 50 ps. The repetition rate was measured to be 34.7 MHz. The corresponding pulse-energy and peak-power are 0.19 µJ and 0.4 MW, respectively. The intra-cavity pulse energy of approximately 2.8 - 3.7 μJ is in good agreement with the stability criterion for QML as reported for sub-picosecond pulses in [17]. The cavity’s self-phase modulation factor γSPM should be around 10 mrad/MW to match the soliton-equation
taking into account only the GDD per roundtrip of the two GTIs (|D|), the intra-cavity pulse energy (EP), and the measured pulse duration (τp). This value of γSPM is in good agreement with the theoretical value, calculated with the mode-sizes and the non-linear coefficients of the elements in the resonator. The nonlinearities of the resonator air account for approximately one fifth of the total nonlinear roundtrip phase-shift. The nonlinearities of air were estimated with the B-Integral, assuming beam sizes according to our resonator-design calculations and constant peak power. To avoid Q-switching instabilities, the laser was not operated below 5 W of average output power. The beam quality was measured behind GTI2 (and in accordance with ISO 11146) to be near diffraction limited with an M2 of 1.09 and 1.12 along the two axes of symmetry, see Fig. 3.According to our calculations the laser beam is slightly astigmatic at the output coupler, which is introduced by different radii of curvature in the two axes of the disk.
Fig. 3 Beam caustic of M2-measurement at an output power of 6.6 W, showing only small astigmatism and M2 of 1.09 and 1.12 along the two axes of symmetry.
Further reducing the pulse duration was explored by decreasing the amount of negative GDD, but this led to mode-locking instabilities, which were revealed by fluctuations of the peak-power (observed using the fast photodiode) as well as by modulations of the optical output spectrum. This may indicate the onset of multi-pulsing as described in [22]. The autocorrelation signals of the output pulses indicated shorter pulses down to 228 fs, but the signal was noisy, probably due to the above mentioned power fluctuations. A more stable operation could not be achieved by using SESAMs with higher modulation-depth, due to the observed Q-switching instabilities. However, higher powers, higher efficiencies and shorter pulses are possible using crystals with better quality and larger clear apertures (enabling larger pump spot) and will be subject of future investigations on Yb:CaF2 in thin-disk oscillators and amplifiers.
4. Discussion
In conclusion the first demonstration of a passively mode-locked Yb:CaF2 thin-disk laser using a SESAM to start and stabilize soliton mode-locking has been presented. Close to transform-limited pulses with durations of 445 fs were demonstrated at an average output power of 6.6 W and an optical efficiency of 15.3%. To the best of our knowledge this is the highest average power achieved from a mode-locked Yb:CaF2 laser so far. At the repetition rate of 34.7 MHz this corresponds to an output pulse energy of 190 nJ and a peak-power of 0.4 MW. Table 3 gives an overview on the state of the art results with mode-locked Yb:CaF2 lasers compared to the results obtained within this work. Compared to [15] the average output power could be doubled and more importantly the pulse energy was increased by a factor of five. The average output power can be scaled up in future developments by using larger pump spots on the disks. Higher powers and shorter pulse-durations with this material, using SESAMs with higher modulation depth and very low saturation fluences (<35 µJ/cm2) to avoid Q-switching, are the subject of our future investigations.
Table 3. Overview on State of the Art Yb:CaF2 Mode-locked Lasers
Acknowledgments
The authors acknowledge their colleagues Michael Eckerle, Jan-Philipp Negel, Stefan Piehler, Frédéric Druon and Patrick Georges, for the fruitful discussions. This work was supported by the German Research Foundation (DFG) within the funding programme Open Access Publishing.
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