Advanced LIGO (aLIGO) requires multiple frequency sidebands to disentangle all of the main interferometer's length signals. This paper presents the results of a risk reduction experiment to produce ...two sets of frequency sidebands in parallel, avoiding mixed 'sidebands on sidebands'. Two phase modulation frequencies are applied to separate Electro-Optic Modulators (EOMs), with one EOM in each of the two arms of a Mach-Zehnder interferometer. In this system the Mach-Zehnder's arm lengths are stabilized to reduce relative intensity noise in the recombined carrier beam by feeding a corrective control signal back to the Rubidium Titanyl Phosphate (RTP) EOM crystals to drive the optical path length difference to zero. This setup's use of the RTP crystals as length actuators provides enough bandwidth in the feedback to meet arm length stability requirements for aLIGO.
We have shown that pump light intensity stabilisation of a single-mode laser diode pumped Nd:YAG non-planar ring oscillator (NPRO) results in significant intensity noise reduction of the NPRO, as ...well as frequency noise suppression in the same order of magnitude. This effect does not occur in conventional laser diode array pumped NPROs due to mode beating effects originating in the multi-mode pump. As opposed to individual intensity and frequency stabilisation, pump light stabilisation contributes a simplified stabilisation scheme for single-mode laser diode pumped NPROs for high precision applications.
We report on 50% conversion efficiency for frequency doubling of a 854 nm GaAlAs diode laser using a potassium niobate crystal in an external ring cavity. Frequency stabilization of the diode laser ...is achieved by direct optical feedback from the doubling cavity or from a grating in Littrow configuration. We determine 200 kHz linewidth from the measured spectral density of the frequency noise and from a beat signal measurement of two independent diode lasers. The blue output power of 7.8 mW shows rms-fluctuations of less than 0.6% in 1 MHz bandwidth.
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