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Stabilized high-power laser system for the gravitational wave detector advanced LIGO

P. Kwee, C. Bogan, K. Danzmann, M. Frede, H. Kim, P. King, J. Pöld, O. Puncken, R. L. Savage, F. Seifert, P. Wessels, L. Winkelmann, and B. Willke

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Abstract

An ultra-stable, high-power cw Nd:YAG laser system, developed for the ground-based gravitational wave detector Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory), was comprehensively characterized. Laser power, frequency, beam pointing and beam quality were simultaneously stabilized using different active and passive schemes. The output beam, the performance of the stabilization, and the cross-coupling between different stabilization feedback control loops were characterized and found to fulfill most design requirements. The employed stabilization schemes and the achieved performance are of relevance to many high-precision optical experiments.

© 2012 Optical Society of America

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Figures (8)
Fig. 1
Fig. 1 Pre-stabilized laser system of Advanced LIGO. The three-staged laser (NPRO, medium power amplifier, high power oscillator) and the stabilization scheme (pre-mode-cleaner, power and frequency stabilization) are shown. The input-mode-cleaner is not part of the PSL but closely related. NPRO, non-planar ring oscillator; EOM, electro-optic modulator; FI, Faraday isolator; AOM, acousto-optic modulator.

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Fig. 2
Fig. 2 Detailed schematic of the power noise sensor setup for the first power stabilization loop. This setup corresponds to PD2 in the overview in Fig. 1. λ/2, waveplate; PBS, polarizing beam splitter; BD, glass filters used as beam dump; PD, single element photodetector; QPD, quadrant photodetector.

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Fig. 3
Fig. 3 Cross coupling transfer functions (a...n) were measured between four different control loops. Measurements between the error signal (E), the control signal (C), and in the case of the power stabilization loop the out-of-loop signal (OOL) were performed. Upper bounds and approximate values of the in general frequency dependent transfer functions are given.

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Fig. 4
Fig. 4 Modescan performed with a DBB upstream and downstream of the PMC. The PMC suppressed higher order transverse modes and increased the power fraction in the TEM00 mode from 97.2% to 99.5%. A CCD image of the TEM40 in transmission of the PMC is shown.

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Fig. 5
Fig. 5 Beam pointing fluctuations measured with a DBB upstream and downstream of the PMC. All four degrees of freedom, shift and tilt in horizontal and vertical directions, were measured at both locations. The PMC reduced the pointing fluctuations below the design requirements. For reference, one measurement upstream of the PMC was projected downstream of the PMC using the expected pointing suppression factor of the PMC.

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Fig. 6
Fig. 6 Relative power noise at radio frequencies measured upstream and downstream of the PMC. The filtering of the PMC reduced the power noise below the design requirements. Both measurements were limited by shot noise (solid lines) for frequencies above about 10 MHz. The peak at 35.5 MHz is caused by phase modulation sidebands required for injection and PMC locking.

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Fig. 7
Fig. 7 Relative power noise in the detection band. The first power stabilization loop reduced the free running power fluctuations by several orders of magnitude. The out-of-loop measured power noise fulfills its requirements for frequencies above 60 Hz. The design requirements of the second loop are shown only for reference. A projection of the pointing noise showed that the out-of-loop measurement was not limited at low frequencies by this noise source.

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Fig. 8
Fig. 8 Frequency noise measured downstream of the PMC with and without frequency stabilization. The measured in-loop frequency noise was consistent with the design requirements. To deduce an upper bound for the actual frequency noise an out-of-loop measurement with the DBB was performed with engaged frequency stabilization.

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