4. Discussion

3.5. Intensity noise and CEP stabilization

102 High Power Laser Systems

average and 60 MW peak power.

Since power-fluctuations can be enhanced in nonlinear processes like SPM, high-harmonic generation, difference frequency generation or optical parametric amplification it is necessary that the pulses generated from the oscillator are as noiseless as possible. It is not obvious that high-power oscillators, especially those with long cavities, can be as silent as usual low power bulk KLM oscillators. Concerns were also raised by some that high-power KLM oscillators would also suffer from instabilities since the initiation of mode-locking requires a setup close to the cavity stability limit. State of the art KLM thin-disk oscillators deliver more than 100 W average power and, thus, are by two orders of magnitude more powerful than usual KLM bulk oscillators. For instance, the oscillator described in the previous section [8] delivers 150 W

The intensity noise of this oscillator was characterized (see Figure 8). Measurement of the laser output with an RF-signal analyzer reveals that nearly all of the noise is generated in the lowfrequency band below 10 kHz. This can be attributed to both technical noise such as watercooling, pump-source or air-turbulences as well as gain-relaxation dynamics. The latter lies in the lower kHz range due to the approximately 1 ms upper state lifetime of Yb:YAG. An intensity noise of 0.3% rms deviation in the 1 Hz–1 MHz window is typical for KLM thin-disk oscillators. At slightly reduced power levels even better values can be measured on a daily basis. These intensity-noise values are comparable to commercial Ti:Sapphire oscillators and promise good

Many advanced scientific applications such as high precision spectroscopy in the XUV-VUV range or attosecond pulse generation require the lasers to be CEP stabilized. Obviously, the demonstration of CEP stabilization of femtosecond thin-disk oscillators is an important step towards enabling these applications with a compact, transportable thin-disk oscillator source. Moreover, a low intensity noise is a critically important prerequisite for the CEP stabilization demonstration. The first carrier envelope offset frequency stabilization of a femtosecond thindisk oscillator was demonstrated with a KLM thin-disk oscillator providing a remarkably high average power of 45 W and a peak power over 4 MW [58]. A similar demonstration followed with a SESAM mode-locked thin-disk oscillator, providing a moderate output power of only

Figure 8. (a) RF signal around the repetition rate of the oscillator with 60 MW output peak power. Small sidebands are

visible more than 75 dB below the signal and (b) low-frequency noise performance.

results for CEP stabilization of high peak and average power thin-disk oscillators.
