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Laser System

The AEI 10m Prototype Interferometer will use a 35W solid-state laser as its light source. In this laser system the light the of a 2W Nd:YAG seed laser (Mephisto build by Innolight following a non-planar ring oscillator (NPRO) design) passes four Nd:YVO4 crystals in an amplifier configuration (see Fig.1).

Laser system
Fig. 1: Schematic of the 35W laser system

Each crystal is pumped by a 400μm diameter, NA:0.22 fiber-coupled laser diode with a nominal power of 45W. To increase the lifetime of these diodes the pump power is set to 33W during normal operation. The laser crystal with a 3mm×3mm cross section consists of a 2mm un-doped and a 8mm long doped Nd:YVO4 region (0.3 at.%). The un-doped region was bonded to the crystal to reduce the thermal stress at the surface where the pump beam enters.

To separate the pump light from the laser light, a dichroic 45 degree mirror with an anti-reflection coating for the pump-light wavelength and a high-reflection coating for the laser wavelength was implemented. Efficient cooling of the laser crystal was realized by wrapping it into a 500μm thick indium foil and by mounting this assembly in a water-cooled copper block. After passing the four laser heads the NPRO 2W beam is amplified to a power level of 35W. The wavelength matching between the Nd:YAG and the Nd:YVO4 system can be done by temperature tuning of the NPRO. As a result of the broad emission profile of Nd:YVO4 of approximately 0.9 nm (FWHM) an efficient amplification was achieved over a large NPRO temperature range of 25 - 40 °C. The pump spot diameter in each crystal was optimized to achieve an high extraction efficiency without degrading the spatial beam profile. The spatial beam quality of the NPRO (97% TEM00 mode content) is only slightly degraded by the amplification process, and 95% of the power of the 35W front end was measured to be in the TEM00 mode. This small degradation can be attributed to the 3rd and 4th amplification stage as after the first two stages a TEM00 mode content of 97%could be measured.

The frequency fluctuations of the 35W laser are at the same level as those of the 2W NPRO seed. The relative power noise (RPN) of the 35W front end is identical to the NPRO RPN at frequencies below 50Hz and exceeds it by a factor of up to 5 for frequencies in the audio band. Several neested feed-back control loops will be used to stabilize the power as well as the frequency of the laser to levels appropriate for the main interferometer experiments.
 

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