Optical pumping of laser materials is an effective way to create a population inversion necessary for laser operation. However, a fraction of the pump energy is always transfered as heat into the laser material, which is mainly caused by the quantum defect. For Yb3+-doped materials, the small energy difference between the pump level and the laser level and the pumping with narrowband high-power laser diodes result in a quantum defect of approx. 9%, which is significantly lower compared to other dopants e.g. Ti3+ (33%) or Nd3+ (24%). Due to the low heat introduction, high optical-to-optical efficiency and high repetition rate laser systems based on diode-pumping are well-suited for a number of applications. Here, however, laser beam quality is of crucial importance. Phase distortions and beam profile modulations can lead to optical damages as well as a significant reduction of the focal spot intensity.
Pump-induced phase aberrations are the main cause for phase distortions of the amplified laser beam. The heat transferred to the material causes a change of the refractive index (dn/dT), thermal expansion and stress within the laser material, eventually leading to spatial phase aberrations (also called ‘thermal lens’). However, the spatially dependent distribution of the population inversion itself also leads to spatial phase aberrations. Since electron excitation directly leads to a change in the charge distribution of the laser active ions, the dynamic response of the material to external fields changes. These electronic phase aberrations (also called ‘population lens’) are described by a change in the polarizability of the material. Due to the low quantum defect of Yb3+-doped materials, this effect becomes more important.
We show the first comprehensive spatio-temporal characterization of the pump-induced phase aberration including both effects. A high-resolution interference measurement was carried out with time steps of 50µs for times during the pump period and the cooling period between subsequent pump pulses. We found that both phase effects significantly contribute to the overall phase distortions. Since the temporal characteristic of the electronic phase depends on the fluorescence lifetime and the thermal phase on the thermal diffusivity, both phase effects could be distinguished by their different lifetimes. The measurements were carried out for Yb:YAG, Yb:CaF2 and Yb:glass, and are in excellent agreement to our detailed, COMSOL-based, spatio-temporal phase simulations. Since Yb:CaF2 and Yb:glass provide a negative dn/dT, the electronic phase change becomes even more important and, in case of Yb:CaF2, almost completely compensates the thermal phase imprint of a pump pulse during the time frame of laser pulse amplification.
We present a novel approach for the construction of a high energy, high power burst mode laser system, based on diode pumped cryogenically cooled Yb:CaF<sub>2</sub>. The system consists of a frontend producing pulses of 300 fs duration with 1 MHz. Bursts of 1000 subsequent pulses are cut from the continuous train by an electro optical modulator. Afterwards the duration of the individual pulses is stretched to 50 ps.
The amplifier system consists of two amplifiers. Both amplifiers utilize mirror based relay imaging schemes to allow for a sufficient number of extraction passes for achieving efficient energy extraction. The goal parameters of the system are to achieve a total energy of 5J per burst with a repetition rate of 10Hz.
Amplification results for the first of two amplifiers are demonstrated. A total output energy of 480 mJ was achieved
corresponding to an optical to optical efficiency from absorbed pump energy to extracted energy of more than 17%.
Single pulse energies of up to 7.5mJ are generated when changing to less pulses per burst.
To achieve a constant energy from pulse to pulse during the burst we present a technique based on the modulation of the laser diode current during one pulse. With this technique the gain variation during the burst was than 5% peak to peak.
We present temperature dependent gain measurements with different Ytterbium doped laser media, such as Yb:YAG, Yb:FP15-glass and Yb:CaF<sub>2</sub> in a multi-pass amplifier setup. The temperature of these materials was adjusted arbitrarily between 100K and 300K, while heat removal was realized by transverse cooling. In order to obtain a good beam profile throughout the amplification process, we used an all-mirror based relay imaging setup consisting of a telescope accomplishing a 4f-imaging with a plane mirror in each image plane. The amplification beam is then coupled into the cavity and doing several round trips wandering over the surface of the spherical mirrors. Hence the laser material is placed in one of the image planes, the beam quality of the amplifier was ruled by the intensity profile of the pumping laser diodes consisting of two stacks with 2.5kW peak output power each. Due to the given damage threshold fluence, the output energy of the amplifier was limited to about 1J at a beam diameter of 4.5 mm (FWHM). The seed pulses with a duration of 6 ns were generated in a Yb:FP15-glass cavity dumped oscillator with further amplification up to the 100mJ level by a room temperature Yb:YAG multi pass amplifier. The 1 Hz repetition rate of the system was limited by the repetition rate of the front-end. With Yb:YAG for instance an output energy of 1.1 J with an record high optical to optical efficiency of more than 35% was achieved, which was further increased to 45% for 500 mJ output energy.
For laser performance simulations, optical properties of applied active materials have to be exactly known. Here we
report on temperature dependent emission and absorption cross section measurements for Yb:YAG, Yb:CaF<sub>2</sub> and
Yb:FP15-glass. The temperature of the samples was aligned in steps of 20 K between 100 K and room temperature with
a liquid nitrogen driven cryostat. Absorption spectra were obtained with a fiber coupled white light source and
fluorescence spectra by excitation with a fiber coupled 10W laser diode at 970 nm. All spectral measurements were
performed with a scanning spectrum analyzer, providing a spectral resolution down to 0.05 nm. By applying the
McCumber relation in combination with the Fuchtbauer-Ladenburg method, we were able to obtain a valid emission
cross section over the whole range of interest from the measured data.
We report on a novel 10 mJ-level diode-pumped Yb:KYW amplifier at 1040 nm, which generates picosecond pulses at a
repetition rate of 10 Hz. It will be used in the front end of a petawatt laser system for pumping an optical parametric
amplifier (OPA) for contrast enhancement. For synchronization purposes the amplifier is seeded by pulses that are
derived from the femtosecond oscillator. After stretching by a volume Bragg grating and amplification in a double stage
fiber amplifier the pulses are injected into the Yb:KYW regenerative cavity. Finally, the pulses are compressed to 1 ps
before preparing pump pulses for the OPA by second harmonic conversion.