Frequency noise stabilisation of a 1550 nm external cavity diode laser with hybrid feedback for next generation gravitational wave interferometry

Longer wavelength lasers will be needed for future gravitational wave detectors that use cryogenic cooling of silicon based test-mass optics. Diode lasers with a 1550 nm wavelength output are potential seed light sources for such a detector, however diode laser devices have a different spectral profile and higher frequency noise than the solid state lasers used in current detectors. We present a frequency stabilisation system for a 1550 nm external cavity diode laser capable of reducing the laser frequency noise to a level of 0.1HzHz up to 1 kHz with a unity gain frequency of 535 kHz using a hybrid analogue-digital servo with in-loop cancellation of resonant features. In addition, a method of high speed digital filter optimisation and automated design is demonstrated.