O. H. Heckl

SESAMs for High-Power Oscillators: Design Guidelines and Damage Thresholds

We present for the first time to the best of our knowledge a systematic study of lifetime and damage of semiconductor saturable absorber mirrors (SESAMs) designed for operation in high-power oscillators. We characterize and compare nonlinear reflectivity and inverse saturable absorption (ISA) parameters as well as damage threshold and lifetime of different representative SESAMs under test using a nonlinear reflectivity measurement setup at unprecedented high fluence levels. We investigate the catastrophic damage that occurs at very high fluences by demonstrating a dependence of the damage threshold on the ISA parameter F2 and the maximum reflectivity fluence F0. We can clearly demonstrate that the damage fluence Fd scales proportionally to √F2 for all SESAMs. In the case of SESAMs with the same absorber where the product Fsat .ΔR is constant, the damage fluence Fd scales proportionally to F0. Therefore, damage occurs due to heating of the lattice by the energy absorbed due to the ISA process and is not related to the quantum well (QW) absorbers. Furthermore, we present guidelines on how to design samples with high saturation fluences, reduced induced absorption, and high damage thresholds. Using multiple QWs and a suitable di-electric topsection, we achieved SESAMs with saturation fluences >;200 μj/cm2, nonsaturable losses <;0.1%, and reduced ISA. Our best sample could not be damaged at a maximum available fluence of 0.21 J/cm2 and a peak intensity of 370 GW/cm2. These SESAMs will be suitable for future high-power femtosecond oscillators in the kilowatt average output power regime, which is very interesting for attosecond science and industrial material processing applications.

Beam delivery and pulse compression to sub-50 fs of a modelocked thin-disk laser in a gas-filled Kagome-type HC-PCF fiber.

We present two experiments confirming that hypocycloid Kagome-type hollow-core photonic crystal fibers (HC-PCFs) are excellent candidates for beam delivery of MW peak powers and pulse compression down to the sub-50 fs regime. We demonstrate temporal pulse compression of a 1030-nm Yb:YAG thin disk laser providing 860 fs, 1.9 µJ pulses at 3.9 MHz. Using a single-pass grating pulse compressor, we obtained a pulse duration of 48 fs (FWHM), a spectral bandwidth of 58 nm, and an average output power of 4.2 W with an overall power efficiency into the final polarized compressed pulse of 56%. The pulse energy was 1.1 µJ. This corresponds to a peak power of more than 10 MW and a compression factor of 18 taking into account the exact temporal pulse profile measured with a SHG FROG. The compressed pulses were close to the transform limit of 44 fs. Moreover, we present transmission of up to 97 µJ pulses at 10.5 ps through 10-cm long fiber, corresponding to more than twice the critical peak power for self-focusing in silica.

Frontiers in passively mode-locked high-power thin disk laser oscillators.

Semiconductor saturable absorber mirror (SESAM) mode-locked thin disk lasers define the state-of-the-art performance for high average power and high pulse energy femtosecond laser oscillators. To date pulse energies above 30 µJ and average powers above 140 W have been demonstrated. In this paper we review the achievements of mode-locked thin disk lasers in terms of average power and pulse energy. Stable mode locking requires single transverse mode operation even at the highest average power, which is challenging and therefore addressed in more detail. We then summarize our expectations on the main challenges and limitiations for the next generation of mode-locked thin disk laser oscillators with an average power above 500 W and pulse energies in excess of 100 µJ.

Continuous-wave and modelocked Yb:YCOB thin disk laser: first demonstration and future prospects

Yb:YCOB is a very attractive material for femtosecond pulse generation given its broad emission bandwidth. We demonstrate continuous-wave power scaling in the thin disk geometry to the 100-W level with a 40% optical-to-optical efficiency in multi-mode operation. Furthermore, we present initial modelocking results in the thin disk geometry, achieving pulse durations as short as 270 fs. The modelocked average power is, however, limited to less than 5 W because of transverse mode degradation. This is caused by anisotropic thermal aberrations in the 15% Yb-doped thin disks which were 300 to 400 µm thick. This result confirms the potential of Yb:YCOB to generate short femtosecond pulses in the thin disk geometry but also makes clear that significantly thinner disks are required to overcome the thermal limitations for high power operation.

Femtosecond Yb:Lu(2)O(3) thin disk laser with 63 W of average power.

We present successful power-scaling of an Yb:Lu(2)O(3) thin disk laser to record high-power levels both in cw and mode-locked operation. In a simple multimode resonator we achieved 149 W of output power in cw operation with 73% optical-to-optical efficiency (eta(opt)). Building an 81 MHz fundamental transverse mode resonator with dispersion compensation and a semiconductor saturable absorber mirror (SESAM) for passive mode locking we achieved 63 W of average power in 535 fs pulses (eta(opt)=35%). The output beam is nearly diffraction limited (M(2)<1.2). The 0.78 microJ pulses with a peak power of 1.28 MW had a central wavelength of 1034 nm and were close to the Fourier transform limit. With an SESAM with a larger modulation depth we obtained pulses as short as 329 fs at 40 W average power corresponding to a pulse energy of 0.49 microJ and a peak power of 1.32 MW.

Temporal pulse compression in a xenon-filled Kagome-type hollow-core photonic crystal fiber at high average power

We spectrally broaden 1-ps pulses from a 14.3-W, 10.6-MHz thin-disk laser in a 30.5-cm long Xe-filled HC-PCF. After compression, we achieve 9.1-W in 470-fs pulses (63% overall efficiency). No depolarization or damage is observed.

Pulse compression of a high-power thin disk laser using rod-type fiber amplifiers

We report on two pulse compressors for a high-power thin disk laser oscillator using rod-type fiber amplifiers. Both systems are seeded by a standard SESAM modelocked thin disk laser that delivers 16 W of average power at a repetition rate of 10.6 MHz with a pulse energy of 1.5 μJ and a pulse duration of 1 ps. We discuss two results with different fiber parameters with different trade-offs in pulse duration, average power, damage and complexity. The first amplifier setup consists of a Yb-doped fiber amplifier with a 2200 μm2 core area and a length of 55 cm, resulting in a compressed average power of 55 W with 98-fs pulses at a repetition rate of 10.6 MHz. The second system uses a shorter 36-cm fiber with a larger core area of 4500 μm2. In a stretcher-free configuration we obtained 34 W of compressed average power and 65-fs pulses. In both cases peak powers of > 30 MW were demonstrated at several μJ pulse energies. The power scaling limitations due to damage and self-focusing are discussed.

227-fs pulses from a mode-locked Yb:LuScO 3 thin disk laser

The highest average output power and pulse energy from mode-locked laser oscillators are currently achieved using semiconductor saturable absorber mirror (SESAM) [1] mode-locked thin disk lasers (TDL) [2]. Up to now the shortest pulses of 240 fs at 22 W average output power in this configuration were demonstrated with the monoclinic Yb:KYW [3], which suffers from strong anisotropy in its thermal properties, which prevents further power scaling. Using the cubic sesquioxide Yb:Lu 2 O 3 as gain medium, pulses as short as 370 fs could be obtained with a pump power limited output power of 20.5 W at 1034 nm [4]. The new mixed sesquioxide Yb:LuScO 3 [5] provides a nearly doubled gain bandwidth compared to Yb:Lu 2 O 3 of more than 20 nm and supports the generation of significantly shorter pulses.

63-W average power from femtosecond Yb:Lu 2 O 3 thin disk laser

Thin disk lasers are ideally suited for high-power laser applications both in continuous-wave (cw) and pulsed operation. Several kilowatts of cw output power and femtoscond pulses with energies of more than 10 µJ have been achieved with Yb:YAG [1–3], which is the current standard material used in nearly all thin disk lasers. Since several years, new Yb-doped thin disk gain materials are developed, which target higher efficiency and shorter pulse durations. Yb:Lu2O3 is a very promising material, as its thermal conductivity remains high even for high Yb-doping concentrations. Furthermore, it has a lower quantum defect and larger emission bandwidth. Over 30 W of output power at more than 80% slope efficiency and 72% optical-to-optical efficiency have already been reported with this material [4]. Initial SESAM mode-locking experiments resulted in an average output power of 24 W and a minimal pulse duration of 370 fs [5], which is significantly shorter than the typical ≫700 fs achieved with Yb:YAG thin disk lasers.

Ultrafast thin disk lasers: Towards intralaser extreme nonlinear optics

Ultrafast thin disk lasers generate higher power levels than any other femtosecond oscillator technology. We review the current state of the art and give an outlook towards new applications such as intralaser extreme nonlinear optics.