Power scaling and quantum noise measurements in Nd:YAG slab laser amplifiers

High power, TEM00 mode, low-noise lasers are required for the Laser Interferometer Gravitational Wave Observatory (LIGO). Since detector shot noise scales inversely with laser power, there is a strong incentive to scale the existing LIGO laser source to the 200-W level and beyond for improved sensitivity to gravitational waves. Master-Oscillator-Power-Amplifier (MOPA) systems are well suited for power scaling while preserving the spatial and temporal coherence of the master oscillator. The key to efficient extraction from amplifiers is to operate them in the saturated regime, where a significant fraction of the population inversion contributes to coherent amplification. A major benefit of operating an amplifier in this regime is that the quantum noise added due to amplification is reduced.; The quantum-noise behavior of saturated amplifiers was demonstrated by saturating the gain of a 100-W-class zigzag end-pumped slab amplifier using a high-power beam and measuring the power noise detected by a single-spatial-mode probe beam traversing the same optical path through the amplifier. Quantum-noise measurements on a saturated amplifier showed that gain saturation results in high extraction efficiency while lowering the quantum noise towards the shot-noise limit set by losses in the optical amplifier. These are the first quantitative measurements of the quantum noise of a free-space Nd:YAG optical amplifier as it transitions from the linear regime into the heavily saturated regime.; Experimental results are presented of a highly efficient end-pumped Nd:YAG zigzag-slab power amplifier which has been scaled to the 100-W class with almost 90% power in the fundamental TEM00 mode. This approach can be power scaled to several hundred watts of continuous-wave laser power. Parasitic oscillations and amplified spontaneous emission, key obstacles in building high-power amplifiers, are addressed with a novel technique based on simple coatings, which transmit parasitic modes out of the slab while trapping the pump light. This enables fabrication of an efficient, high-gain TEM 00-mode and power-scalable slab laser for Advanced LIGO and other applications requiring a high-power TEM00-mode laser source.