PDF《of 220 W》

35 W ampli?er system, was realized. In the following sections, the optical and me- chanical setup of the high-power oscillator and the system performance are presented.

2 Laser concept The laser development leading to the laser system for ad- vanced LIGO described in this paper has been carried out

530 L. Winkelmann et al. over several stages. The fundamentals have been derived by Frede et al. in preliminary experiments. With a ?rst injection-locked two head laser system, a single-frequency output power of

87 W could be achieved [6]. The feasibility of power scaling by doubling the number of laser crystals was demonstrated in a ?rst four head prototype laser with a diffraction limited output beam and an output power of

213 W, but without injection-locking [7]. In the migration from this prototype laser to the ?nal injection-locked laser system presented here, several improvements, which will be described in the following sections, have been realized in order to make the system more robust and reliable for long term operation in terms of beam quality, output power and injection-locking behavior. For this laser system, an identical version of the

35 W ampli?er, which is already operated at two LIGO sites, was used as seed laser. As sketched in the system schematic of the laser in Fig. 1, a

2 W non-planar ring oscillator (NPRO, manufactured by InnoLight) is ampli?ed in a single pass through four longitudinally pumped Nd:YVO4 crystals (de- tails can be found in [4]). The single frequency beam from the ampli?er is then fed into the main oscillator, which con- sists of four end-pumped Nd:YAG crystals arranged in a ring resonator. This resonator has got an asymmetric structure for selective higher order mode discrimination and features a piezo-actuated mirror for active length control. Both sys- tems are optically decoupled by a Faraday isolator speci?- cally designed for high power levels [8]. To injection-lock the high-power oscillator following the PoundCDreverCHall scheme [9], an EOM was placed between NPRO and power ampli?er. In the following section, the optical con?guration of the high-power oscillator will be presented in detail. Fig.

1 Schematic setup of the injection-locked laser system for ad- vanced LIGO 2.1 Optical setup To achieve high linearly polarized output power levels and good beam quality, a longitudinally pumped laser design was chosen. The presented laser features four Nd:YAG crys- tals for a reduction of heat load in each active media com- pared to a single crystal design. Each laser crystal is pumped by seven ?ber coupled diode lasers with a combined maxi- mum output power of

315 W (7 *

45 W). The seven at- tached

400 ?m multimode ?bers are combined into a single ?ber bundle with polished bare-end tips. To avoid hot spots due to the non-uniformity of the pump light distribution, the light from the ?ber bundle is homogenized by propagating through a

100 mm long and

2 mm in diameter fused silica rod. Due to total internal re?ection inside the glass rod, the pump light is mixed, resulting in a parabolic intensity pro?le measured at the exit pupil. As shown in Fig. 2, the tip of the glass rod is imaged with two lenses through the end facet of the laser crystal forming a focus with the imaging plane approx.

10 mm inside the rod. Two undoped

7 mm long end-caps at both ends of the laser crystal prevent bulging of the end facets due to ther- mal stress and enable ef?cient water cooling of the pumped region. The total length of the laser crystal is

54 mm and it has a diameter of