CW and mode-locked laser operation based on an Yb3+:Y2O3 ceramic thin disk is reported. In CW laser operation, an output power of 70 W with an optical-to-optical efficiency of 57.4% was achieved. A higher slope efficiency of 70% was also obtained with a 50-W pump laser diode with more suitable emission characteristics. In mode-locked laser operation, pulses as short as 547 fs with an average power of 7.4 W were obtained. To our knowledge, this is the first demonstration of a high power CW and mode-locked laser operation based on Yb3+:Y2O3 ceramic thin disks.
©2012 Optical Society of America
Highly efficient high power lasers based on Yb3+-doped materials have attracted considerable attention. The simple energy-level scheme of the Yb3+ ion (2F5/2 ↔ 2F7/2 inter-manifold transition) leads to a very small quantum defect and avoids undesirable processes such as excited-state absorption, cross relaxation, and concentration quenching  so that highly efficient high power laser operation with comparably low thermal load can be realized under laser diode (LD) pumping. Recently, high power lasers based on fiber and thin disk laser geometries have seen a significant development. For example, 10-kW single transverse mode continuous wave (CW) operation has been achieved with fiber laser geometry . A multi-transverse mode CW power of 5.4-kW has been achieved with single thin disk laser geometry . In addition, a 16 kW thin disk laser with a 4-disk system has become commercially available. Compared with the fiber laser geometry, the thin disk laser geometry has an advantage with respect to high power femtosecond laser operation. Fiber laser geometry suffers large nonlinearity due to a long interaction length and a small laser mode diameter inside the fiber. Therefore a complicated system consisting of a large mode area rod type photonic crystal fiber, a chirped pulse amplifier, and an external pulse compression technique is required to achieve high average power femtosecond pulse operation [4–6]. On the other hand, the thin disk laser geometry can suppress excess nonlinearity inside the cavity because of its thin active material length and large laser mode diameter. As high as 30.6-μJ pulse energy has been directly obtained from an active multi-pass Yb:YAG thin disk mode-locked laser . As the sesquioxides are better suited for high power ultrashort pulses due to their higher thermal conductivities and the broader emission bandwidths, 141 W average power operation has also been achieved with an Yb:Lu2O3 mode-locked thin disk laser . Nowadays the thin disk laser operations based on various kinds of gain materials have also been investigated [e.g. 9]. In addition, very recently, Kerr-lens mode locked Yb:YAG thin disk laser operation  and the first sub-100-fs thin disk laser operation  with optimized SESAMs  have also been achieved.
In this paper, we report CW and mode-locked Yb3+:Y2O3 ceramic thin disk laser operation. In case of the CW laser operation, an output power of 70 W with an optical-to-optical efficiency of 57.4% was achieved with a simple linear cavity and a 140-W pump LD. A slope efficiency of 70% was also obtained using a 50-W pump LD with more suitable emission characteristics allowing for higher efficiencies. In case of the mode-locked laseroperation, pulses as short as 547 fs with an average power of 7.4 W were obtained with a SESAM of 0.4% modulation depth. To our knowledge, this is the first report of a high power CW and mode-locked Yb3+:Y2O3 ceramic thin disk laser.
2. Properties of Yb3+:Y2O3 ceramic
Yb3+-doped cubic sesquioxides (RE2O3: RE = Y, Lu or Sc) are recognized as excellent gain media for high power femtosecond lasers, since they have high thermal conductivities, an isotropic structure, and broader amplification bandwidths than that of Yb:YAG. Among the Yb3+-doped sesquioxides, Yb3+:Y2O3 has the broadest gain bandwidth (Fig. 1 ). The FWHM of Yb3+:Y2O3’s emission spectrum at the wavelength of 1031 nm is 15 nm, corresponding to a ~74 fs pulse duration by assuming a transform-limited sech2 pulse shape. In addition, undoped Y2O3 has a higher thermal conductivity than Lu2O3. Therefore high thermal conductivity still can be expected for a low Yb3+-doping level. The measured thermal conductivities of undoped Y2O3 and 2% Yb-doped Y2O3 ceramics are 12.6 ± 0.2 W/mK and 7.9 ± 0.1 W/mK, respectively. However, Y2O3 has a melting point of ~2430 °C and undergoes a phase transition near 2280 °C, which had limited the growth of large crystals with high optical quality, needed for applications as a laser material. In this experiment, we used Yb3+:Y2O3 ceramic material fabricated with nanocrystalline and vacuum sintering technologies, which enable us to fabricate laser quality Yb3+:Y2O3 ceramics. These provide the additional advantage of a 2.5 times higher fracture toughness than that of Y2O3 single crystals , which could allow for very high pump intensities. High power laser operation based on quasi CW pumped Y2O3 ceramic has been reported . Low power SESAM mode-locked laser operation of 188-fs pulse duration  and Kerr-lens mode-locked laser operation of 68-fs pulse duration with Yb3+:Y2O3 ceramics  have already been reported. In thin disk geometry this material is an interesting candidate for high power femtosecond lasers.
For the CW laser experiments, we used a 2% Yb-doped Y2O3 ceramic thin disk. The disk has a thickness of 400 μm (at center) and a wedge of 30’. It was bonded on a copper heat sink with InSn solder. As pump sources, a 50-W fiber coupled LD (JENOPTIC, Ø = 600 μm, NA = 0.22, λ = 975 nm, Δλ = ~2.5 nm, Peltier cooling) or a 140-W fiber coupled LD (JENOPTIC, Ø = 600 μm, NA = 0.22, λ = 975 nm, Δλ = ~3 nm, water cooling) were used with a commercial 24-pass thin disk pump unit. The pump spot diameter was ~1.2 mm. The cavity was a simple linear cavity composed of a flat highly reflective mirror coating on the backside of the Yb3+:Y2O3 thin disk and an output coupler (OC, Radius of curvature (ROC) of 100 mm) (Fig. 2 ). With the 50-W pump source, we tested five OCs of different transmittance to find the optimum output-coupling efficiency. The maximum slope efficiency of 70% at the wavelength of 1031 nm was obtained with a 3.3% transmittance OC (Fig. 3(a) ). It is noteworthy that we did not use absorbed pump power to calculate the efficiency. The efficiency was calculated with respect to the incident pump power directly at the fiber end. The efficiency is slightly lower than that of Yb3+:Lu2O3 and Yb3+:Sc2O3 single crystals (typically ~80%) . However, we do not think that the lower efficiency is an intrinsicproperty of the Yb3+:Y2O3 laser ceramic. More likely differences in the backside coating consisting of the HR coating itself and a metal coating on our crystal compared to the coating system presented in  and/or scattering losses are responsible for the lower efficiency. With the 140-W pump source, a maximum output power of 70 W was obtained at an emission wavelength of 1031 nm with a maximum pump power of 140 W (Fig. 3(b)). The 140-W LD emission contains some of the cladding mode and it suffers losses at the in-coupling optics. The actual maximum incident pump power was 122 W and the optical-to-optical efficiency and slope efficiency against actual incident pump power were 57.4% and 68.2%, respectively. The measured beam quality value M2 was ~30, because the TEM00 laser mode diameter is about 10 times smaller than the pumped area in this configuration. No thermal problems were observed in the experiment and the maximum output power was limited only by the available pump power. Therefore even higher output power could be expected with higher available pump powers.
We also demonstrated the Yb3+:Y2O3 ceramic thin disk laser in a mode-locked configuration. The schematic picture of the mode-locked laser cavity is depicted in Fig. 4(a) . The calculated laser mode diameters are also shown in Fig. 4(b) and 4(c). In the calculation, we assumed ROC of 2000 mm (convex) at the thin disk as a result of a bonding and thermally induced stress at a pump power of less than 100 W . We used a 2% Yb-doped Yb3+:Y2O3 ceramic thin disk with a thickness of 300 μm and wedge of 30’. As a pump source, a 100-W fiber coupled LD (LIMO GmbH, Ø = 200 μm, NA = 0.22, λ = 975 nm, Δλ = ~5 nm) was used with a homemade 16-pass thin disk pump unit (with a parabolic mirror of 150 mm ROC). The pump spot diameter was calculated to be ~1.0 mm. The output coupler has a transmittance of 5%. As dispersion compensation elements, we used two or four (Fig. 4 (a)) chirped mirrors (CPM, Layertec Inc., −550 fs2). A semiconductor saturable absorber mirror (SESAM, BATOP, 0.4% modulation depth, 90 μJ/cm2 saturation fluence, 0.3% nonsaturable absorption, air cooling) was used for passive mode locking. The estimated total loss of the mirrors (except for the output coupler transmission) in the cavity is less than 1%. The thin disk was not put into the focusing point of the cavity to achieve a better mode matching with the pump laser mode (Fig. 4(b)). In this cavity we achieved 10 W of CW output power with a pump power of 60 W using a HR mirror instead of the SESAM. Compared to the aforementioned multi-mode
CW laser operation, the laser efficiency was strongly degraded, which was probably mainly caused by the TEM00 operation with worse mode matching under the multi-mode LD pumping and the less efficient homemade multi-pass pump module. The distortions in the thin disk which could have been induced during the bonding process also could lead to higher losses in TEM00 operation than in multi-mode laser operation. The laser was linearly polarized in a horizontal direction without any polarization selective elements in the cavity, probably caused by the small incident angles at the cavity mirrors and thin disk. By replacing the HR mirror with the SESAM, mode-locked operation was obtained. Pulses as short as 587 fs with a spectral bandwidth of 2.6 nm were obtained (Fig. 5(a) ). The time bandwidth product was 0.43 (ideal 0.315). Corresponding to the cavity length of 3.33 m, the repetition rate was 45 MHz (Fig. 5(b), 5(c)). The estimated fluence on the SESAM is ~360 μJ/cm2 (assuming a laser mode radius of 0.3 mm) which is 4 times higher than its saturation fluence. Near the threshold pump power of the mode-locking (Fig. 6(a) ), we slightly moved the position of SESAM to initiate the mode-locking. However, the mode-locking was self-starting at higher pump power. Above 2.3-W average power, harmonic mode locking with twice the repetition rate was observed (90 MHz, Fig. 6). To suppress harmonic mode locking, we inserted additional two chirped mirrors (Fig. 4(a) inset, 2 × −550 fs2) into the cavity.
We also changed the distances between the folding mirrors to increase the laser mode areas in the gain medium and on the SESAM (Fig. 4(c). The distances between folding mirrors were slightly decreased). Pulses as short as 547 fs at an average output power of 7.4 W were obtained at a pump power of 63 W (Fig. 7 ). The spectral bandwidth and the time bandwidth product were 2.5 nm and 0.386, respectively. The slope efficiency was 17.2%. The estimated fluence on the SESAM is ~660 μJ/cm2 (assuming a laser mode radius of 0.4 mm) which is ~7 times higher than its saturation fluence. Above 63-W pump power, the mode-locked laser operation became unstable due to the large heat load of the SESAM caused by the high nonsaturable losses and the insufficient air cooling. Both effects also limited the maximum average output power. In further experiments using a SESAM with a higher modulation depth of 1.4% we observed SESAM damage due to Q-switching instabilities.
In conclusion, CW and mode-locked Yb3+:Y2O3 ceramic thin disk laser operation was achieved. In the case of CW laser operation, an output power of 70 W with an optical-to-optical efficiency of 57.4% was achieved with a simple two-mirror cavity. A slope efficiency of 70% was also obtained with a 50-W pump LD. In the case of mode-locked laser operation, pulses as short as 547 fs with an average power of 7.4 W were obtained with a SESAM of 0.4% modulation depth. This is a step forward in the field of high average power ultrashort pulse ceramic thin disk lasers. We believe that much higher efficiency will be possible at higher average power levels and shorter pulse durations based on Yb3+:Y2O3 ceramic thin disks can be achieved by further optimization. In addition the ceramic technology is also suitable for fabricating a composite material, in which a very thin gain length material bonded with undoped ceramics will be obtained that is easy to handle and enables us to control the Kerr nonlinearity without increasing the heat load.
This work was partially supported by the JSPS Institutional Program for Young Researcher Overseas Visits. Christian Kränkel acknowledges support by the Joachim Herz Stiftung.
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